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Polycyclic aromatic hydrocarbons (PAHs) exhibit intriguing characteristics that position them as promising candidates for advancements in organic semiconductor technologies. Notably, tetracene finds substantial utility in Electronics due to its application in organic light-emitting diodes (OLEDs) and organic field-effect transistors (OFETs). The strategic introduction of an isoelectronic boron-nitrogen (B,N) pair to replace a carbon-carbon pair in acenes introduces changes in the electronic structure, allowing for the controlled modulation of diradical characteristics. Consequently, this B,N substitution enables precise adjustments in chemical, optical, and electronic attributes. In this work, we undertook a systematic exploration of thermally activated delayed fluorescence (TADF) phenomena within a set of 77 B,N-substituted derivatives of tetracene. The primary objective was to identify and select prospective molecules for the fabrication of OLEDs. Employing multiconfigurational methods of computational quantum chemistry, we conducted an extensive investigation to unravel the potential candidates. As a result, we identified molecules that might exhibit the sought-after TADF behavior. Descriptors such as excitation energies, harmonic oscillator model of aromaticity (HOMA) and fractional occupation number weighted density (FOD) were assessed and indicated five candidates with stability comparable to that of pristine tetracene. This research not only contributes to a deeper understanding of the influence of B,N substitution on acene derivatives but also opens doors for the development of organic electronics by harnessing the properties of these selected molecules.

The diradical benzyne isomers are excellent prototypes for evaluating the ability of an electronic structure method to describe static and dynamic correlation. The benzyne isomers are also interesting molecules with which to study the fundamentals of through-space and through-bond diradical coupling that is important in so many electronic device applications. In the current study, we utilize the multireference methods MC-SCF, MR-CISD, MR-CISD+Q, and MR-AQCC with an (8,8) complete active space that includes the σ, σ*, π and π* orbitals, to characterize the electronic structure of ortho-, meta- and para -benzyne. We also determine the adiabatic and vertical singlet-triplet splittings for these isomers. MR-AQCC and MR-CISD+Q produced energy gaps in good agreement with previously obtained experimental values. Geometries, orbital energies and unpaired electron densities show significant through-space coupling in the o - and m -benzynes, while p -benzyne shows through-bond coupling, explaining the dramatically different singlet-triplet gaps between the three isomers.

We present a complete simplest-level electron nuclear dynamics (SLEND) investigation of H + + C 2 H 2 at collision energies E Lab = 30, 200, and 450 eV. This reaction is relevant in astrophysics and provides a computationally feasible prototype for proton cancer therapy reactions. SLEND is a time-dependent, variational, direct, and nonadiabatic method that adopts a classical-mechanics description for the nuclei and a Thouless single-determinantal wave function for the electrons. We perform this study with our code PACE, which incorporates the One Electron Direct/Electron Repulsion Direct (OED/ERD) atomic integrals package developed by the Bartlett group. Current SLEND simulations with the 6-31G** basis set involves 2,646 trajectory calculations from 9 nonredundant, symmetry-inequivalent projectile-target orientations. For H + + C 2 H 2 at E Lab = 30 eV, SLEND/6-31G** simulations predict one simple scattering process, and three reactive ones: C 2 H 2 hydrogen substitution, C 2 H 2 fragmentation into two CH moieties, and C 2 H 2 fragmentation into CHC and H moieties, respectively. We reveal and analyze the mechanisms of these processes through computer animations; this valuable chemical information is inaccessible by experiments. The SLEND/6-31G** scattering angle functions exhibit primary and secondary rainbow scattering features that vary with the projectile-target orientations and collision energies. SLEND/6-31G** predicts 1-electron-transfer (1-ET) integral cross sections at E Lab = 30, 200, and 450 eV in good agreement with their experimental counterparts. SLEND/6-31-G** predicts 1-ET differential cross sections (DCSs) at E Lab = 30 eV that agree well with their experimental counterparts over all the measured scattering angles. In addition, SLEND/6-31G** predicts 0-ET DCSs at E Lab = 30 eV that agree well with their experimental counterparts at low scattering angles, but less satisfactorily at higher ones. Remarkably, both the 0- and 1-ET DCSs from SLEND/6-31G** exhibit distinct primary rainbow scattering signatures in excellent agreement with their experimentally inferred counterparts. Furthermore, both SLEND/6-31G** and the experiment indicate that the primary rainbow scattering angles from the 0- and 1-ET DCSs are identical (an unusual fact in proton-molecule collisions). Through these rainbow scattering predictions, SLEND has also validated a procedure to extract primary rainbow angles from structureless DCSs. We analyze the obtained theoretical results in comparison with available experimental data and discuss forthcoming developments in the SLEND method.

Nonadiabatic interaction of adsorbate nuclear motion with the continuum of electronic states is known to affect the dynamics of chemical reactions at metal surfaces. A large body of work has probed the fundamental mechanisms of such interactions for atomic and diatomic molecules at surfaces. In polyatomic molecules, the possibility of mode-specific damping of vibrational motion due to the effects of electronic friction raises the question of whether such interactions could profoundly affect the outcome of chemistry at surfaces by selectively removing energy from a particular intramolecular adsorbate mode. However, to date, there have not been any fundamental experiments demonstrating nonadiabatic electron-vibration coupling in a polyatomic molecule at a surface. In this work, we scatter excited metastable formaldehyde and formaldehyde-d 2 from a low work function surface and detect ejected exoelectrons that accompany molecular relaxation. The exoelectron ejection efficiency exhibits a strong dependence on the vibrational mode that is excited: out-of-plane bending excitation (ν 4 ) leads to significantly more exoelectrons than does CO stretching excitation (ν 2 ). The results provide clear evidence for mode-specific energy transfer from vibration to surface electrons.

High-energy-density aluminum nanoparticles (AlNPs) upon thermal annealing followed by superquenching result in elevated stress levels in the metallic core and reduced surface energy at the core-shell interface. Isomer-selective vacuum ultraviolet-based photoionization mass spectrometry coupled to a high-temperature chemical microreactor reveals that these stress-altered AlNPs (SA-AlNPs) exhibit distinctive temperature-dependent reactivities toward catalytic decomposition of the hydrocarbon jet fuel exo -tetrahydrodicyclopentadiene (JP-10, C 10 H 16 ) compared to untreated AlNPs (UN-AlNPs). SA-AlNPs show a delayed initiation of the decomposition for JP-10 by 200 K relative to the UN-AlNPs; however, the full decomposition is achieved at a 100 K lower temperature. Furthermore, there are fewer oxygenated products that are generated from the alumina surface-induced heterogeneous oxidation process and a larger fraction of closed- and open-shell hydrocarbons. Chemical insight bridging the reactivity order of SA-AlNPs at low and high temperatures, simultaneously, is obtained via a detailed examination of the product branching ratios obtained in this study.

The oxidation of gas-phase exo -tetrahydrodicyclopentadiene (JP-10, C 10 H 16 ) over aluminum nanoparticles (AlNP) has been explored between a temperature range of 300 and 1250 K with a novel chemical microreactor. The results are compared with those obtained from chemical microreactor studies of helium-seeded JP-10 and of helium-oxygen-seeded JP-10 without AlNP to gauge the effects of molecular oxygen and AlNP, respectively. Vacuum ultraviolet (VUV) photoionization mass spectrometry reveals that oxidative decomposition of JP-10 in the presence of AlNP is lowered by 350 and 200 K with and without AlNP, respectively, in comparison with pyrolysis of the fuel. Overall, 63 nascent gas-phase products are identified through photoionization efficiency (PIE) curves; these can be categorized as oxygenated molecules and their radicals as well as closed-shell hydrocarbons along with hydrocarbon radicals. Quantitative branching ratios of the products reveal diminishing yields of oxidized species and enhanced branching ratios of hydrocarbon species with the increase in temperature. While in the low-temperature regime (300-1000 K), AlNP solely acts as an efficient heat transfer medium, in the higher-temperature regime (1000-1250 K), chemical reactivity is triggered, facilitating the primary decomposition of the parent JP-10 molecule. This enhanced reactivity of AlNP could plausibly be linked to the exposed reactive surface of the aluminum (Al) core generated upon the rupture of the alumina shell material above the melting point of the metal (Al).

The complete active space self-consistent field (CASSCF) method is a cornerstone in modern excited-state quantum chemistry providing the starting point for most common multireference computations. However, CASSCF, when used with a minimal active space, can produce significant errors (>2 eV) even for the excitation energies of simple hydrocarbons if the states of interest possess ionic character. After illustrating this problem in some detail, we present a diagnostic for ionic character, denoted as Q   a t , that is readily computed from the transition density. A set of 11 molecules is considered to study errors in vertical excitation energies. State-averaged CASSCF obtains a mean absolute error (MAE) of 0.87 eV for the 34 singlet states considered. We highlight a strong correlation between the obtained errors and the Q   a t diagnostic, illustrating its power to predict problematic cases. Conversely, using multireference configuration interaction with single and double excitations and Pople's size extensivity correction (MR-CISD+P), excellent results are obtained with an MAE of 0.11 eV. Furthermore, correlations with the Q   a t diagnostic disappear. In summary, we hope that the presented diagnostic will facilitate reliable and user-friendly multireference computations on conjugated organic molecules.

The phenalene (triangulene) and olympicene molecules belong to the polycyclic aromatic hydrocarbon class, which have attracted substantial technological interest due to their unique electronic properties. Electronic structure calculations serve as a valuable tool in investigating the stability and reactivity of these molecular systems. In the present work, the multireference calculations, namely, the complete active space second-order perturbation theory and multireference averaged quadratic coupled cluster (MR-AQCC), were employed to study the reactivity and stability of phenalene and olympicene isomers, as well as their modified structures where the sp 3 -carbon at the borders were removed. The harmonic oscillator model of aromaticity (HOMA) and the nucleus-independent chemical shift as geometric and magnetic indexes calculated with density functional theory were utilized to assess the aromaticity of the studied molecules. These indexes were compared with properties such as the excitation energy and natural orbitals occupation. The reactivity analyzed using the HOMA index combined with MR-AQCC revealed the radical character of certain structures as well as the weakening of their aromaticity. Moreover, the results suggest that the removal of sp 3 -carbon atoms and the addition of hydrogen atoms did not alter the π network and the excitation energies of the phenalene molecules.

The nonplanar character of graphene with a single carbon vacancy (SV) defect is investigated utilizing a pyrene-SV model system by way of complete-active-space self-consistent field theory (CASSCF) and multireference configuration interaction singles and doubles (MR-CISD) calculations. Planar structures were optimized with both methods, showing the 3 B 1 state to be the ground state with three energetically close states within an energy range of 1 eV. These planar structures constitute saddle points. However, following the out-of-plane imaginary frequency yields more stable (by 0.22 to 0.53 eV) but nonplanar structures of C s symmetry. Of these, the 1 A' structure is the lowest in energy and is strongly deformed into an L shape. Following a further out-of-plane imaginary frequency in the nonplanar structures leads to the most stable but most deformed singlet structure of C 1 symmetry. In this structure, a bond is formed between the carbon atom with the dangling bond and a carbon of the cyclopentadienyl ring. This bond stabilizes the structure by more than 3 eV compared to the planar 3 B 1 structure. Higher excited states were calculated at the MR-CISD level, showing a grouping of four states low in energy and higher states starting around 3 eV.

The ongoing shift toward clean, sustainable energy is a primary driving force behind hydrogen fuel research. Safe and effective storage of hydrogen is a major challenge (particularly for mobile applications) and requires a detailed understanding of the atomic level interactions of hydrogen with its host materials. The light mass of hydrogen, however, implies that quantum effects are important, so a quantum dynamical treatment is required to properly account for these effects in computational simulations. As one such example, we describe herein the hydrogen exchange dynamics between a hydride and a dihydrogen ligand in the [FeH(H 2 )(PH 3 ) 4 ] + model complex. A global three-dimensional (3D) potential energy surface (PES) was constructed by fitting to and interpolating from a discrete set of grid points computed using density functional theory; exact quantum dynamical calculations were then carried out on the 3D PES using discrete variable representation basis sets. Energy levels and their quantum tunneling splittings were computed up to 3000 cm -1 above the ground state. Within that energy range, all three fundamentals have been identified using wave function plots, as well as the first three overtones of the exchange (reaction coordinate) motion and several of its combination bands. From the tunneling splittings, the Boltzmann-averaged tunneling rates were computed. The Arrhenius plot of the total exchange rate shows a clear transition around 150 K, below which the activation energy is essentially zero and above which it is less than half of the electronic structure barrier. This indicates that exchange rates are governed by quantum tunneling throughout the relevant temperature range with the low-temperature regime dominated by a single quantum (ground) state. This work is the first-ever fully quantum dynamical study to investigate the hydrogen exchange dynamics between hydride and dihydrogen ligands coordinated to a transition-metal complex.

A number of conjugated molecules are designed with extremely long single C-C bonds beyond 2.0 Å. Some of the investigated molecules are based on analogues to the recently discovered molecule by Kubo et al. These bonds are analyzed by a variety of indices in addition to their equilibrium bond length including the Wiberg bond index, bond dissociation energy (BDE), and measures of diradicaloid character. All unrestricted DFT calculations indicate no diradical character supported by high-level multireference calculations. Finally, N FOD was computed through fractional orbital density (FOD) calculations and used to compare relative differences of diradicaloid character across twisted molecules without central C-C bonding and those with extremely elongated C-C bonds using a comparison with the C-C bond breaking in ethane. No example of direct C-C bonds beyond 2.4 Å are seen in the computational modeling; however, extremely stretched C-C bonds in the vicinity of 2.2 Å are predicted to be achievable with a BDE of 15-25 kcal mol -1 .

3-Hydroxyflavon (3-HF) represents an interesting paradigmatic compound to study excited-state intramolecular proton transfer (ESIPT) and intermolecular (ESInterPT) processes to explain the experimentally observed dual fluorescence in solvents containing protic contamination (water) as opposed to single fluorescence in highly purified nonpolar solvents. In this work, adiabatic on-the-fly molecular dynamics simulations have been performed for isolated 3-HF in an aqueous solution using a polarizable continuum model and including explicit water molecules to represent adequately hydrogen bonding. For the calculation of the excited state, time-dependent density functional theory and the Becke-3-Lee-Yang-Parr (B3LYP) functional have been used. For the isolated 3-HF, ultrafast ESIPT from the enol group to the neighboring keto group has been observed. The calculated PT time of 48 fs agrees well with the experimental value of 39 fs. Addition of one water molecule quenches this ESIPT process but shows an intermolecular concerted or stepwise tautomerization process via the bridging water molecule. Adding a second or more water molecules inhibits this ESInterPT process to a large degree. Most of the trajectories do not show any PT, preserving the initial excited-state enol structure, which is the origin of the violet-blue fluorescence appearing in the solvents contaminated with protic components.

A preexisting chemisorbed defect is well-known to increase the reactivity of graphene which is normally chemically inert. Specifically, the presence of chemisorbed hydrogen atoms forming an sp 3 -hybridized C-H bond is known to increase the reactivity of neighboring carbon atoms toward additional hydrogenation with wide-ranging applications from materials science to astrochemistry. In this work, static DFT and DFT-based direct dynamics simulations are used to characterize the reactivity of a graphene sheet around an existing C-H bond defect. The spin density landscape shows how to guide subsequent H atom additions, always bonding most strongly to the carbon atom with greatest spin density. Molecular dynamics of an impinging H atom under thermal conditions with defect graphene was used to determine the statistics of probable reactions. The most frequent outcome is inelastic scattering (48%) and then Eley-Rideal (ER) abstraction of the chemisorbed H atom as vibrationally hot H 2 (40%), while the least likely, but probably most interesting, result is formation of a novel C-H bond (12%). The C-H bonds always form in the β sublattice. The carbon atom in the para position shows to be most reactive toward the incoming H atom, followed by the ortho carbon, in agreement with the spin density computed in the static calculations. Globally, the graphene energy surface is repulsive, but the defects create local channels into this energy surface through which reactants can move locally through and react with the activated surface without a barrier.

Direct dynamics simulations with the M06/6-311++G(d,p) level of theory were performed to study the 3 CH 2 + 3 O 2 reaction at 1000 K temperature on the ground state singlet surface. The reaction is complex with formation of many different product channels in highly exothermic reactions. CO, CO 2 , H 2 O, OH, H 2 , O, H, and HCO are the products formed from the reaction. The total simulation rate constant for the reaction at 1000 K is (1.2 ± 0.3) × 10 -12 cm 3 molecule -1 s -1 , while the simulation rate constant at 300 K is (0.96 ± 0.28) × 10 -12 cm 3 molecule -1 s -1 . The simulated product yields show that CO is the dominant product and the CO:CO 2 ratio is 5.3:1, in good comparison with the experimental ratio of 4.3:1 at 1000 K. On comparing the product yields for the 300 and 1000 K simulations, we observed that, except for CO and H 2 O, the yields of the other products at 1000 K are lower at 300 K, showing a negative temperature dependence.

Functionalization of quantum carbon dots (QCDs) and graphene quantum dots (GQDs) is a popular way to tune their optical spectra increasing their potential applicability in material science and biorelated disciplines. Based on the experimental observation, functionalization by fluorine atoms induces substantial shifts in absorption and emission spectra and an intensity increase. Understanding of the effects due to fluorine functionalization at the atomic scale level is still challenging due to the complex structure of fluorinated QCDs. In this work, the effect of covalent edge-fluorination and fluorine anion doping on absorption and emission spectra of prototypical polycyclic aromatic hydrocarbons pyrene and circum-pyrene has been investigated. The ways to achieve efficient red-shifts in the UV spectra and obtaining reasonable intensities stood in the focus of the work. High-level quantum chemical methods based on density functional theory/multireference configuration interaction (DFT/MRCI) and single-reference second-order algebraic diagrammatic construction (ADC(2)) and density functional theory (DFT) using the CAM-B3LYP functional have been used for this purpose. The calculations show that doping with the fluoride anion can have significant effects on the electronic spectrum. However, the effect of the fluoride ion is strongly dependent on its position with respect to the QCD. The localization above the GQDs causes large red-shifts to both the absorption and emission of spectra of GQDs, while in-plane localization leads to only negligible shifts and a tendency to dissociation after electronic excitation. Thus, large red-shifts, observed in complexes with F - , are obtained due to the introduction of new excited states with large CT character not yet been considered previously in this context, although they have the potential to significantly influence the photophysics of quantum dots. Doping by edge fluorination redshifts the spectra only slightly. This study provides insights on fluorine-doped GQDs, which is conducive to promoting its rational design and controllable synthesis.

The dynamics of the HBr + + CO 2 → HOCO + + Br reaction was recently investigated with guided ion beam experiments under various excitations (collision energy of the reactants, rotational and spin-orbital states of HBr + , etc.), and their impacts were probed through the change of the cross section of the reaction. The potential energy profile of this reaction has also been accurately characterized by high-level ab initio methods such as CCSD(T)/CBS, and the UMP2/cc-pVDZ/lanl08d has been identified as an ideal method to study its dynamics. This manuscript reports the first ab initio molecular dynamics simulations of this reaction at two different collision energies, 8.1 kcal/mol and 19.6 kcal/mol. The cross sections measured from the simulations agree very well with the experiments measured with HBr + in the 2 ∏ 1/2 state. The simulations reveal three distinct mechanisms at both collision energies: direct rebound (DR), direct stripping (DS), and indirect (Ind) mechanisms. DS and Ind make up 97% of the total reaction. The dynamics of this reaction is also compared with nucleophilic substitution (S N 2) reactions of X - + CH 3 Y → CH 3 X + Y - type. In summary, this research has revealed interesting dynamics of the HBr + + CO 2 → HOCO + + Br reaction at different collision energies and has laid a solid foundation for using this reaction to probe the impact of rotational excitation of ion-molecule reactions in general.

To address the possible role of pyrene dimers in soot, chemical dynamics simulations are reported to provide atomistic details for the process of collisional association of pyrene dimers and ensuing decomposition of pyrene dimers. The simulations are performed at 600, 900, 1200, 1600, and 2000 K temperatures ( T ) with different collisional impact parameters ( b ; 0-18 Å) using the all-atom optimized potentials for liquid simulations intermolecular force field. Corresponding to each b, ensembles of 1000 trajectories are computed up to a maximum time of 110 ps at each T . Microcanonical association rate constants for the pyrene-dimerization processes decrease with an increase in T . The ensuing dissociation of the pyrene dimers is statistical and could be well represented by the Rice-Ramsperger-Kassel-Marcus theory of unimolecular dissociation. Fits of the dissociation rate constants versus the harmonic Rice-Ramsperger-Kassel equation revealed that partial energy randomization occurs among the inter- and intramolecular vibrational modes during the dissociation of pyrene dimers, whereas rotational and translational modes play a significant role. Based on the low probability of association and short lifetime at 1600 (∼13.3 ps) and 2000 (∼12.8 ps) K, it is concluded that pyrene dimers are unlikely to play any major role in soot nucleation processes.

Quasi-classical direct dynamics simulations, performed with the B3LYP-D3/cc-pVDZ electronic structure theory, are reported for vibrational relaxation of the three NH stretches of the -NH 3 + group of protonated tryptophan (TrpH + ), excited to the n = 1 local mode states. The intramolecular vibrational energy relaxation (IVR) rates determined for these states, from the simulations, are in good agreement with the experiment. In accordance with the experiment, IVR for the free NH stretch is slowest, with faster IVR for the remaining two NH stretches which have intermolecular couplings with an O atom and a benzenoid ring. For the free NH and the NH coupled to the benzenoid ring, there are beats (i.e., recurrences) in their relaxations versus time. For the free NH stretch, 50% of the population remained in n = 1 when the trajectories were terminated at 0.4 ps. IVR for the free NH stretch is substantially slower than for the CH stretch in benzene. The agreement found in this study between quasi-classical direct dynamics simulations and experiments indicates the possible applicability of this simulation method to larger biological molecules. Because IVR can drive or inhibit reactions, calculations of IVR time scales are of interest, for example, in unimolecular reactions, mode-specific chemistry, and many photochemical processes.

Surface hopping quantum mechanical/molecular dynamics simulations have been performed for the tetracyanoethylene-anthracene complex to investigate the evolution of charge-transfer (CT) states after excitation into a locally excited (LE) state of anthracene. The scaled opposite-spin (SOS) second-order algebraic diagrammatic construction (SOS-ADC(2)) has been used to achieve a balanced description of LE and CT states. The calculations have been performed for two media, the gas phase and water (described by molecular mechanics, MM). The two dynamics variants show strongly different behaviors. Even though in both cases the conversion from the LE state to lower-lying CT states occurs with 100 fs, in the gas phase, the system remains in the excited state for longer than 2 ps, while in water, it returns to the ground state within 0.5 ps. Moreover, while in the gas phase the original neutral equilibrium structure should be recovered, in water, the ion-pair (IPr) CT state is strongly stabilized, creating a new competing ground-state isomer. Thus, we predict that the ground state of the complex in water should be composed of two species, the original neutral state and an IPr state. The existence of an IPr ground state in strongly polar environments opens interesting possibilities for the design of efficient charge-separating organic donor-acceptor interfaces.

The 3 CH 2 + 3 O 2 reaction has a quite complex ground state singlet potential energy surface (PES). There are multiple minima and transition states before forming the 10 possible reaction products. A previous direct chemical dynamics simulation at the UM06/6-311++G(d,p) level of theory ( J. Phys. Chem. A 2019, 123, 4360-4369) found that reaction on this PES is predominantly direct without trapping in the potential minima. The first minima 3 CH 2 + 3 O 2 encounters is that for the 1 CH 2 O 2 Criegee intermediate and statistical theory assumes the reactive system is trapped in this intermediate with a lifetime given by Rice-Ramsperger-Kassel-Marcus (RRKM) theory. In the work presented here, a direct dynamics simulation is performed with the above UM06 theory, with the trajectories initialized in the 1 CH 2 O 2 intermediate with a random distribution of vibrational energy as assumed by RRKM theory. There are substantial differences between the dynamics for 1 CH 2 O 2 dissociation and 3 CH 2 + 3 O 2 reaction. For the former there are four product channels, while for the latter there are seven in agreement with experiment. Product energy partitioning for the two simulations are in overall good agreement for the CO 2 + H 2 and CO + H 2 O product channels, but in significant disagreement for the HCO + OH product channel. Though 1 CH 2 O 2 is excited randomly in accord with RRKM theory, its dissociation probability is biexponential and not exponential as assumed by RRKM. In addition, the 1 CH 2 O 2 dissociation dynamics follow non-intrinsic reaction coordinate (non-IRC) pathways. An important finding is that the nonstatistical dynamics for the 3 CH 2 + 3 O 2 reaction give results in agreement with experiment.

Addition of high-energy-density materials such as aluminum (Al) microparticles or nanoparticles to liquid propellants potentially improves performance of the fuel. We report on the effects of untreated, prestressed, and superquenched aluminum particles with diameters of 100 nm, 250 nm, 500 nm, 1.6 μm, and 8.8 μm on the combustion of JP-10 droplets acoustically levitated in an oxygen-argon atmosphere. Ignition was initiated by a carbon dioxide laser, and the resulting oxidation processes were traced by Raman, Fourier-transform infrared (FTIR), and ultraviolet-visible (UV-vis) spectroscopies together with high-speed optical and IR thermal-imaging cameras. The UV-vis emission spectra reveal that the key reactive radical intermediates hydroxyl (OH), methylidyne (CH), dicarbon (C 2 ), aluminum monoxide (AlO), and aluminum monohydride (AlH) were formed in addition to atomic aluminum (Al) and the final oxidation products of JP-10, namely, water (H 2 O) and carbon dioxide (CO 2 ). The Al particles facilitated ignition of the JP-10 droplets and produced higher temperatures in the combustion process of up to typically 2600 K. The effect of the Al particles on the ignition and maximum flame temperatures increased as the diameters reduced. The different stress treatments did not produce observable changes for the ignition or combustion of the droplets, which indicates that the liquid propellant was not significantly affected by manipulating the mechanical properties of the fuel particle additive. The initiation and enhancement of the combustion were a consequence of forming highly reactive atomic oxygen (O) and aluminum monoxide (AlO) radicals in the reaction of aluminum atoms with molecular oxygen in the gas phase. These radicals initiate the degradation of JP-10 via atomic hydrogen abstraction forming the hydroxyl (OH) and aluminum hydroxide (AlOH) radicals in reactions which are mainly exothermic by up to 68 kJ mol -1 . In contrast, hydrogen abstractions from JP-10 by molecular oxygen or atomic aluminum are strongly endothermic by up to 236 kJ mol -1 , thus making these reactions less competitive. The generation of C 10 H 15 hydrocarbon radicals from the JP-10 initiates successive oxidations and chain reactions with molecular oxygen leading eventually to carbon dioxide and water. These combined experimental results provide insight into how aluminum particles facilitate the oxidation and reaction mechanisms of JP-10 droplets.

The potential energy curves (PECs) for the interaction of 3 CH 2 with 3 O 2 in singlet and triplet potential energy surfaces (PESs) leading to singlet and triplet Criegee intermediates (CH 2 OO) are studied using electronic structure calculations. The bonding mechanism is interpreted by analyzing the ground state multireference configuration interaction (MRCI) wave function of the reacting species and at all points along the PES. The interaction of 3 CH 2 with 3 O 2 on the singlet surface leads to a flat long-range attractive PEC lacking any maxima or minima along the curve. The triplet surface stems into a maximum along the curve resulting in a transition state with an energy barrier of 5.3 kcal/mol at CASSCF(4,4)/cc-pVTZ level. The resulting 3 CH 2 OO is less stable than the 1 CH 2 OO. In this study, the biradical character (β) is used as a measure to understand the difference in the topology of the singlet and triplet PECs and the relation of the biradical nature of the species with their structures. The 3 CH 2 OO has a larger biradical character than 1 CH 2 OO, and because of the larger bond order of 1 CH 2 OO, the C-O covalent bond becomes harder to break, thereby stabilizing 1 CH 2 OO. Thus, this study provides insights into the shape of the PEC obtained from the reaction between 3 CH 2 and 3 O 2 in terms of their bonding nature and from the shape of the curves, the temperature dependence or independence of the rate of the reaction is discussed.

Diuron is a herbicide that has been classified as an environmental pollutant because of its harmful effects on living beings and environment. In the present work, the OH-initiated oxidation reaction of diuron is investigated by performing electronic structure calculations based on density functional theory (DFT) methods, M06-2X, ωB97X-D, and MPWB1K using the 6-31G(d,p) basis set. The CBS-QB3 method is used to validate the results obtained from the DFT methods. All possible initial hydrogen and chlorine atom abstraction reaction pathways involved in the oxidation of diuron were studied, and the favorable reaction pathways were found by analyzing the potential energy surface and thermochemistry of the reactions. The results obtained from the present work show that hydrogen atom abstraction from methyl and amine groups of diuron are energetically favorable, which leads to the formation of diuron radical intermediate and water molecule. The rate constant is calculated for most favorable reaction pathways by using canonical variation transition state theory (CVT) with small curvature tunnelling (SCT) correction over the temperature range 298-1000 K. The atmospheric lifetime of diuron is found to be around 15 days. The subsequent reaction of most favorable diuron radical intermediate with other atmospheric reactive species, such as O 2 , H 2 O, HO 2 , and NO x ( x = 1, 2) radicals are studied. The time-dependent DFT calculation is performed to study the photolysis of diuron and favorable diuron radical intermediates. This study provides thermochemical and kinetic data for the oxidation of diuron initiated by OH radical through H atom abstraction reaction.

Abnormalities in zinc metabolism have been linked to many diseases, including different kinds of cancers and neurological diseases. The present study investigates the fragmentation pathways of a zinc chaperon using a model peptide with the sequence acetyl-His 1 -Cys 2 -Gly 3 -Pro 4 -Tyr 5 -His 6 -Cys 7 (analog methanobactin peptide-5, amb 5 ). DFT/M05-2X and B3LYP geometry optimizations of [amb 5 -3H+Zn(II)] - predicted three lowest energy conformers with different chelating motifs. Direct dynamics simulations, using the PM7 semiempirical electronic structure method, were performed for these conformers, labeled a , b , and c , to obtain their fragmentation pathways at different temperatures in the range 1600-2250 K. The simulation results were compared with negative ion mode mass spectrometry experiments. For conformer a , the number of primary dissociation pathways are 11, 14, 24, 70, and 71 at 1600, 1750, 1875, 2000, and 2250 K, respectively. However, there are only 6, 10, 13, 14, and 19 pathways corresponding to these temperatures that have a probability of 2% or more. For conformer b , there are 67 pathways at 2000 K and 71 pathways at 2250 K. For conformer c , 17 pathways were observed at 2000 K. For conformer a , for two of the most common pathways involving C-C bond dissociation, Arrhenius parameters were calculated. The frequency factors and activation energies are smaller than those for C-C homolytic dissociation in alkanes due to increased stability of the product ions as a result of hydrogen bonding. The activation energies agree with the PM7 barriers for the C-C dissociations. Comparison of the simulation and experimental fragmentation ion yields shows the simulations predict double or triple cleavages of the backbone with Zn(II) retaining its binding sites, whereas the experiment exhibits single cleavages of the backbone accompanied by cleavage of two of the Zn(II) binding sites, resulting in b - and y -type ions.

Lock-arm supramolecular ordering cocrystals formed by π-stacked materials constitute an interesting class of materials, which exhibits ferroelectric behavior at room temperature. To characterize the charge transfer in excited states, two complexes with π-stacked donors and acceptors, the 1,5-naphthalene diol (NDI) donor and pyromellitic diimide with diethylene glycol arms (PDIA) acceptor, 5-amino-1-naphthol (AMN) donor and PDIA acceptor, were investigated. The electronic excitations were calculated using the scaled opposite-spin variant of ADC(2), time-dependent density functional theory (TD-DFT) using a long-range corrected (LC) functional (ωB97xD), and the TD-LC approach within density-functional-based tight binding (TD-LC-DFTB). Face-to-face mixed stacks and edge-to-face crossed stacks up to hexamers were investigated. The calculations show that the ground state of the complexes does not possess significant CT character. On the other hand, the lowest excited state (S 1 ) shows in all clusters a strong charge transfer. In several cases, the second excited state and also higher excited singlet states possess significant CT character. The orbitals involved in the excitation are mostly well localized and located on adjacent donor/acceptor pairs. Comparing different stacking directions, the vertical excitation energies for the NDI-PDIA crossed stacks are larger than those for the mixed stacks by 0.2-0.4 eV. In the case of the AMN-PDIA system, the energy differences are smaller (∼0.1 eV) with mostly the same energetic ordering as for the NDI-PDIA case. Strong red shifts in vertical fluorescence emission transitions have been computed, which could even lead to intersection between ground and first excited states, resulting in ultrafast radiationless decay and fluorescence quenching.

In a previous work [ Lakshmanan , S. ; J. Phys. Chem. A 2018 , 122 , 4808 - 4818 ], direct dynamics simulations at the M06/6-311++G(d,p) level of theory were reported for 3 CH 2 (X 3 B 1 ) + 3 O 2 (X 3 ∑ g - ) reaction on its ground-state singlet potential energy surface (PES) at 300 K. However, further analyses revealed the simulations are unstable for the 3 CH 2 (X 3 B 1 ) + 3 O 2 (X 3 ∑ g - ) reactants on the ground-state singlet surface and the trajectories reverted to an excited-state singlet surface for the 1 CH 2 (ã 1 A 1 ) + 1 O 2 (b 1 ∑ g + ) reactants. Thus, the dynamics reported previously are for this excited-state singlet PES. The PESs for the 3 CH 2 (X 3 B 1 ) + 3 O 2 (X 3 ∑ g - ) and 1 CH 2 (ã 1 A 1 ) + 1 O 2 (b 1 ∑ g + ) reactants are quite similar, and this provided a means to perform simulations for the 3 CH 2 (X 3 B 1 ) + 3 O 2 (X 3 ∑ g - ) reactants on the ground-state singlet PES at 300 K, which are reported here. The reaction dynamics are quite complex with seven different reaction pathways and nine different products. A consistent set of product yields have not been determined experimentally, but the simulation yields for the H atom, CO, and CO 2 are somewhat lower, higher, and lower respectively, than the recommended values. The yields for the remaining six products agree with experimental values. Product decomposition was included in determining the product yields. The simulation 3 CH 2 + 3 O 2 rate constant at 300 K is only 3.4 times smaller than the recommended value, which may be accommodated if the 3 CH 2 + 3 O 2 → 1 CH 2 O 2 potential energy curve is only 0.75 kcal/mol more attractive at the variational transition state for 3 CH 2 + 3 O 2 → 1 CH 2 O 2 association. The simulation kinetics and dynamics for the 3 CH 2 + 3 O 2 and 1 CH 2 + 1 O 2 reactions are quite similar. Their rate constants are statistically the same, an expected result, since their transition states leading to products have energies lower than that of the reactants and the attractive potential energy curves for 3 CH 2 + 3 O 2 → 1 CH 2 O 2 and 1 CH 2 + 1 O 2 → 1 CH 2 O 2 are nearly identical. The product yields for the 3 CH 2 + 3 O 2 and 1 CH 2 + 1 O 2 reactions are also nearly identical, only differing for the CO 2 yield. The reaction dynamics on both surfaces are predominantly direct, with negligible trapping in potential energy minima, which may be an important contributor to their nearly identical product yields.

Low-energy collision-induced dissociation (CID) of deprotonated l-cysteine S-sulfate, [cysS-SO 3 ] - , delivered in the gas phase by electrospray ionization, has been found to provide a means to form deprotonated l-cysteine sulfenic acid, which is a fleeting intermediate in biological media. The reaction mechanism underlying this process is the focus of the present contribution. At the same time, other novel species are formed, which were not observed in previous experiments. To understand fragmentation pathways of [cysS-SO 3 ] - , reactive chemical dynamics simulations coupled with a novel algorithm for automatic determination of intermediates and transition states were performed. This approach has allowed the identification of the mechanisms involved and explained the experimental fragmentation pathways. Chemical dynamics simulations have shown that a roaming-like mechanism can be at the origin of l-cysteine sulfenic acid.

Following work by Slater and Bunker, the unimolecular rate constant versus collision frequency, k uni (ω, E), is expressed as a convolution of unimolecular lifetime and collisional deactivation probabilities. This allows incorporation of nonexponential, intrinsically non-RRKM, populations of dissociating molecules versus time, N( t)/ N(0), in the expression for k uni (ω, E). Previous work using this approach is reviewed. In the work presented here, the biexponential f 1  exp(- k 1 t) + f 2  exp(- k 2 t) is used to represent N( t)/ N(0), where f 1 k 1 + f 2 k 2 equals the RRKM rate constant k( E) and f 1 + f 2 = 1. With these two constraints, there are two adjustable parameters in the biexponential N( t)/ N(0) to represent intrinsic non-RRKM dynamics. The rate constant k 1 is larger than k( E) and k 2 is smaller. This biexponential gives k uni (ω, E) rate constants that are lower than the RRKM prediction, except at the high and low pressure limits. The deviation from the RRKM prediction increases as f 1 is made smaller and k 1 made larger. Of considerable interest is the finding that, if the collision frequency ω for the RRKM plot of k uni (ω, E) versus ω is multiplied by an energy transfer efficiency factor β c , the RRKM k uni (ω, E) versus ω plot may be scaled to match those for the intrinsic non-RRKM, biexponential N( t)/ N(0), plots. This analysis identifies the importance of determining accurate collisional intermolecular energy transfer (IET) efficiencies.

Three diradical pyrazine isomers were characterized using highly correlated, multireference methods. The lowest lying singlet and triplet state geometries of 2,3-didehydropyrazine ( ortho), 2,5-didehydropyrazine ( para), and 2,6-didehydropyrazine ( meta) were determined. Two active reference spaces were utilized. The complete active space (CAS) (8,8) includes the σ and σ* orbitals on the dehydrocarbon atoms as well as the valence π and π* orbitals. The CAS (12,10) reference space includes two additional orbitals corresponding to the in-phase and out-of-phase nitrogen lone pair orbitals. Adiabatic and vertical gaps between the lowest lying singlet and triplet states, optimized geometries, canonicalized orbital energies, unpaired electron densities, and spin polarization effects were compared. We find that the singlet states of each diradical isomer contain two significantly weighted configurations, and the larger active space is necessary for the proper physical characterization of both the singlet and triplet states. The singlet-triplet splitting is very small for the 2,3-didehydropyrazine ( ortho) and 2,6-didehydropyrazine ( meta) isomers (+1.8 and -1.4 kcal/mol, respectively) and significant for the 2,5-didehydropyrazine ( para) isomer (+28.2 kcal/mol). Singlet geometries show through-space interactions between the dehydocarbon atoms in the 2,3-didehydropyrazine ( ortho) and 2,6-didehydropyrazine ( meta) isomers. An analysis of the effectively unpaired electrons suggests that the 2,5-didehydropyrazine ( para) isomer also displays through-bond coupling between the diradical electrons.

Aromaticity is a multivariable concept in organic chemistry that plays a central role for understanding the structure, stability, and reactivity of polycyclic aromatic hydrocarbons (PAHs). Several types of PAHs are characterized as singlet biradicaloid species and their chemical stability is intimately linked to the degree of aromatic character. In this study, theoretically designed routes to tune the biradical character (and thereby its chemical stability) of nitrogen-substituted octacenes have been investigated on the basis of the high-level multireference averaged quadratic coupled-cluster MR-AQCC method necessary for the appropriate description of polyradicaloid systems. The influence of nitrogen centers on the aromaticity of octacene is probed through structural (HOMA) and electron localization (ELF) indices by comparing the N- against NH-doping cases. These analyses reveal that the aromaticity and biradical character of octacene is only slightly affected by replacing one pair of CH groups with N atoms, i.e., by N-doping. However, a significant aromatic stabilization can be obtained when NH-doping is applied at the inner octacene rings; this is also accompanied by an overall decrease of the open-shell character, as evidenced by the gradual quenching of the unpaired electrons and increase in the singlet-triplet splittings when the NH doping groups are moved toward the center of the octacene molecule. Our findings aid in the rational design of new PAH compounds with balanced biradicaloid character and chemical stability which is important, e.g., for practical applications in organic solar cells based on the singlet-fission mechanism.

The atmospheric reaction of methyldichloroacetate (MDCA) with OH radical is studied using electronic structure calculations. Five different pathways were considered for the initial reactions, which results in the formation of alkyl radical of MDCA along with H 2 O, HOCl, and CH 3 O • . Among the five pathways studied, the α-carbon atom (-CHCl 2 site) H atom abstraction reaction, which leads to the formation of the alkyl radical intermediate • CCl 2 C(O)OCH 3 (I1) is found to be more favorable with an energy barrier of 7.3 kcal/mol, and Cl-atom abstraction reaction is having high energy barrier of 21.3 kcal/mol at M06-2X/6-311++G(2df,2p) level. The calculated thermochemical parameters show that except Cl-atom abstraction channel the other initial reaction channels are highly exothermic. The rate constant is calculated for the initial H atom abstraction reactions using canonical variational transition state theory over the temperature range of 278 to 350 K. The Arrhenius plot shows positive temperature dependence for both the reactions. The results from the calculated thermochemical parameters and rate constants show that the formation of the alkyl radical intermediate (I1) is more favorable with the rate constant of 2.07 × 10 -13 cm 3 molecule -1 s -1 at 298 K. The calculated atmospheric lifetime of MDCA is 28 days at normal atmospheric OH concentration. The results obtained from secondary reactions show that the major product formed from the oxidation chemistry of MDCA is methyl-2-chloro-2-oxoacetate (or) methyl oxalyl chloride.

In the present work, the reaction mechanism of two segregated hydrofluoroethers (HFEs), CH 3 OCF 3 (HFE-143a) and CH 3 OCHF 2 (HFE-152a), and a nonsegregated HFE, CHF 2 OCH 2 CF 3 (HFE-245fa2), with OH radical is studied using electronic structure calculations. The initial reaction between HFE and OH radical is studied by considering two (three for CHF 2 OCH 2 CF 3 ) pathways, H-atom abstraction and C-O bond breaking, OH addition reaction and C-C bond breaking, and OH addition reaction, which leads to the formation of alkyl radical intermediate. The dominant atmospheric fate of initially formed alkyl radical intermediate is its reaction with O 2 . The peroxy radicals thus formed exit through the reaction with HO 2 radical and NO radical resulting in the formation of products, carbonyl fluoride (COF 2 ), trifluoromethylformate, trifluoro(hydroperoxymethoxy)methane, difluoro(hydroperoxy methoxy)methane, difluoromethylformate, 2-(difluoromethoxy)-1,1,1-trifluoro-2-hydroperoxyethane, and difluoromethyl ester. The rate constant is calculated for the initial H-atom abstraction reaction using canonical variational transition state theory with small curvature tunnelling corrections over the temperature range 272-350 K. The atmospheric lifetime and global warming potential of HFEs are obtained from the calculated reaction potential energy surface and rate constant. The results are discussed with respect to the atmospheric implications of CH 3 OCF 3 (HFE-143a), CH 3 OCHF 2 (HFE-152a), and CHF 2 OCH 2 CF 3 (HFE-245fa2).

The reaction of 3 CH 2 with 3 O 2 is of fundamental importance in combustion, and the reaction is complex as a result of multiple extremely exothermic product channels. In the present study, direct dynamics simulations were performed to study the reaction on both the singlet and triplet potential energy surfaces (PESs). The simulations were performed at the UM06/6-311++G(d,p) level of theory. Trajectories were calculated at a temperature of 300 K, and all reactive trajectories proceeded through the carbonyl oxide Criegee intermediate, CH 2 OO, on both the singlet and triplet PESs. The triplet surface leads to only one product channel, H 2 CO + O( 3 P), while the singlet surface leads to eight product channels with their relative importance as CO + H 2 O > CO + OH + H ∼ H 2 CO + O( 1 D) > HCO + OH ∼ CO 2 + H 2 ∼ CO + H 2 + O( 1 D) > CO 2 + H + H > HCO + O( 1 D) + H. The reaction on the singlet PES is barrierless, consistent with experiment, and the total rate constant on the singlet surface is (0.93 ± 0.22) × 10 -12 cm 3 molecule -1 s -1 in comparison to the recommended experimental rate constant of 3.3 × 10 -12 cm 3 molecule -1 s -1 . The simulation product yields for the singlet PES are compared with experiment, and the most significant differences are for H, CO 2 , and H 2 O. The reaction on the triplet surface is also barrierless, inconsistent with experiment. A discussion is given of the need for future calculations to address (1) the barrier on the triplet PES for 3 CH 2 + 3 O 2 → 3 CH 2 OO, (2) the temperature dependence of the 3 CH 2 + 3 O 2 reaction rate constant and product branching ratios, and (3) the possible non-RRKM dynamics of the 1 CH 2 OO Criegee intermediate.

Dissociation chemistry of the diproline anion [Pro 2 -H] - is studied using chemical dynamics simulations coupled with quantum-chemical calculations and RRKM analysis. Pro 2 - is chosen due to its reduced size and the small number of sites where deprotonation can take place. The mechanisms leading to the two dominant collision-induced dissociation (CID) product ions are elucidated. Trajectories from a variety of isomers of [Pro 2 -H] - were followed in order to sample a larger range of possible reactivity. While different mechanisms yielding y 1 - product ions are proposed, there is only one mechanism yielding the b 2 - ion. This mechanism leads to formation of a b 2 - fragment with a diketopiperazine structure. The sole formation of a diketopiperazine b 2 sequence ion is experimentally confirmed by infrared ion spectroscopy of the fragment anion. Furthermore, collisional and internal energy activation simulations are used in parallel to identify the different dynamical aspects of the observed reactivity.

In the present work, we investigated the reaction dynamics that will possibly lead to the formation of protonated glycine by an ion-molecule collision. In particular, two analogous reactions were studied: NH 3 OH + + CH 3 COOH and NH 2 OH 2 + + CH 3 COOH that were suggested by previous experiments to be able to form protonated glycine loosing a neutral water molecule. Chemical dynamics simulations show that both reactants can form a molecule with the mass of the protonated glycine but with different structures, if some translational energy is given to the system. The reaction mechanisms for the most relevant product isomers are discussed as well as the role of collision energy in determining reaction products. Finally, in comparing collision dynamics at room and at very low initial internal temperature of the reactants, the same behavior was obtained for forming the protonated glycine isomers products. This supports the use of standard gas phase ion-chemistry setups to study collision-induced reactivity as a model for astrophysical cold conditions, when some relative translation energy is given to the system.

Intermolecular energy transfer for the vibrationally excited propylbenzene cation (C 9 H 12 + ) in a helium bath was studied with chemical dynamics simulations. The bond energy bond order relationship and electronic structure calculations were used to develop an intramolecular potential for C 9 H 12 + . Spin component scaled MP2/6-311++G** calculations were used to develop an intermolecular potential for He + C 9 H 12 + . The He + He intermolecular potential was determined from a previous explicitly correlated Gaussian electronic structure calculation. For the simulations, C 9 H 12 + was prepared with a 100.1 kcal/mol excitation energy to compare with experiment. The average energy transfer from C 9 H 12 + , ⟨ΔE c ⟩, decreased as C 9 H 12 + was vibrationally relaxed and for the initial excitation energy ⟨ΔE c ⟩ = 0.64 kcal/mol. This result agrees well with the experimental ⟨ΔE c ⟩ value of 0.51 ± 0.26 kcal/mol for collisions of He with the ethylbenzene cation. The ⟨ΔE c ⟩ value found for He + C 9 H 12 + collisions is compared with reported values of ⟨ΔE c ⟩ for He colliding with other molecules.

The fragmentation dynamics of predissociative SO 2 (C̃ 1 B 2 ) is investigated on an accurate adiabatic potential energy surface (PES) determined from high level ab initio data. This singlet PES features non-C 2v equilibrium geometries for SO 2 , which are separated from the SO(X̃ 3 Σ - ) + O( 3 P) dissociation limit by a barrier resulting from a conical intersection with a repulsive singlet state. The ro-vibrational state distribution of the SO fragment is determined quantum mechanically for many predissociative states of several sulfur isotopomers of SO 2 . Significant rotational and vibrational excitations are found in the SO fragment. It is shown that these fragment internal state distributions are strongly dependent on the predissociative vibronic states, and the excitation typically increases with the photon energy.

Chemical dynamics simulations were performed to study collisional intermolecular energy transfer from a thermalized N 2 bath at 300 K to vibrationally "cold" C 6 F 6 . The vibrational temperature of C 6 F 6 is taken as 50 K, which corresponds to a classical vibrational energy of 2.98 kcal/mol. The temperature ratio between C 6 F 6 and the bath is 1/6, the reciprocal of the same ratio for previous "hot" C 6 F 6 simulations (J. Chem. Phys. 2014, 140, 194103). Simulations were also done for a C 6 F 6 vibrational temperature of 0 K. The average energy of C 6 F 6 versus time is well fit by a biexponential function which gives a slightly larger short time rate component, k 1 , but a four times smaller long time rate component, k 2 , compared to those obtained from the "hot" C 6 F 6 simulations. The average energy transferred per collision depends on the difference between the average energy of C 6 F 6 and the final C 6 F 6 energy after equilibration with the bath, but not on the temperature ratio of C 6 F 6 and the bath. The translational and rotational degrees of freedom of the N 2 bath transfer their energies to the vibrational degrees of freedom of C 6 F 6 . The energies of the N 2 vibrational mode and translational and rotational modes of C 6 F 6 remain unchanged during the energy transfer. It is also found that the energy distribution of C 6 F 6 broadens as energy is transferred from the bath, with an almost linear increase in the deviation of the C 6 F 6 energies from the average C 6 F 6 energy as the average energy of C 6 F 6 increases.

The high resolution spectroscopy of the SO 2 molecule is of great topical interest, in a wide variety of contexts ranging from origins of higher life, to astrophysics of the interstellar medium, to environmental chemistry. In particular, the C̃ 1 B 2 ← X̃ 1 A 1 UV photoabsorption spectrum has received considerable attention. This spectrum exhibits a highly regular progression of ∼20 or so strong peaks, spaced roughly 350 cm -1 apart, which is comparable to the C̃ 1 B 2 bending vibrational frequency. Accordingly, they have for decades been largely attributed to the (1, v 2 ' , 2) ← (0, 0, 0) bend progression. Using a highly accurate new ab initio potential energy surface (PES) for the C̃ 1 B 2 state, we compute vibrational energy levels and wave functions, and compare with a photoabsorption calculation obtained using the same PES and corresponding C̃ 1 B 2 ← X̃ 1 A 1 transition dipole surface (TDS). We find that the above putative assignment is incorrect, contradicting even general qualitative trends-thus necessitating a very different dynamical picture for this highly unusual molecule.

Anti-E2, syn-E2, inv-, and ret-S N 2 reaction channels for the gas-phase reaction of F - + CH 3 CH 2 I were characterized with a variety of electronic structure calculations. Geometrical analysis confirmed synchronous E2-type transition states for the elimination of the current reaction, instead of nonconcerted processes through E1cb-like and E1-like mechanisms. Importantly, the controversy concerning the reactant complex for anti-E2 and inv-S N 2 paths has been clarified in the present work. A positive barrier of +19.2 kcal/mol for ret-S N 2 shows the least feasibility to occur at room temperature. Negative activation energies (-16.9, -16.0, and -4.9 kcal/mol, respectively) for inv-S N 2, anti-E2, and syn-E2 indicate that inv-S N 2 and anti-E2 mechanisms significantly prevail over the eclipsed elimination. Varying the leaving group for a series of reactions F - + CH 3 CH 2 Y (Y = F, Cl, Br, and I) leads to monotonically decreasing barriers, which relates to the gradually looser TS structures following the order F > Cl > Br > I. The reactivity of each channel nearly holds unchanged except for the perturbation between anti-E2 and inv-S N 2. RRKM calculation reveals that the reaction of the fluorine ion with ethyl iodide occurs predominately via anti-E2 elimination, and the inv-S N 2 pathway is suppressed, although it is energetically favored. This phenomenon indicates that, in evaluating the competition between E2 and S N 2 processes, the kinetic or dynamical factors may play a significant role. By comparison with benchmark CCSD(T) energies, MP2, CAM-B3LYP, and M06 methods are recommended to perform dynamics simulations of the title reaction.

Density functional theory calculations were used to design an anion receptor that utilizes antimony(III) secondary bonding interactions. Calculations were performed on promising motifs found in the chemical literature where two antimony sites were found in close proximity to a halide anion. The study was extended to a structurally related class of 1,3,2-benzodioxastibole derivatives to elucidate their potential for binding halide ions. Multiple geometric conformations were evaluated and various ratios of halide anions were considered. According to the computation results, this class of anion receptors shows strong affinities toward charge-dense halides. These 1,3,2-benzodioxastibole derivatives were prepared to evaluate their synthetic accessibility. Structural characterization of one species revealed the ability to bind up to three electron donors through secondary bonding interactions. This gates the future experimental study of these antimony systems for anion binding and recognition.

Direct dynamics simulations, utilizing the RM1 semiempirical electronic structure theory, were performed to study the thermal dissociation of the doubly protonated tripeptide threonine-isoleucine-lysine ion, TIK(H + ) 2 , for temperatures of 1250-2500 K, corresponding to classical energies of 1778-3556 kJ/mol. The number of different fragmentation pathways increases with increase in temperature. At 1250 K there are only three fragmentation pathways, with one contributing 85% of the fragmentation. In contrast, at 2500 K, there are 61 pathways, and not one dominates. The same ion is often formed via different pathways, and at 2500 K there are only 14 m/z values for the product ions. The backbone and side-chain fragmentations occur by concerted reactions, with simultaneous proton transfer and bond rupture, and also by homolytic bond ruptures without proton transfer. For each temperature the TIK(H + ) 2 fragmentation probability versus time is exponential, in accord with the Rice-Ramsperger-Kassel-Marcus and transition state theories. Rate constants versus temperature were determined for two proton transfer and two bond rupture pathways. From Arrhenius plots activation energies E a and A-factors were determined for these pathways. They are 62-78 kJ/mol and (2-3) × 10 12 s -1 for the proton transfer pathways and 153-168 kJ/mol and (2-4) × 10 14 s -1 for the bond rupture pathways. For the bond rupture pathways, the product cation radicals undergo significant structural changes during the bond rupture as a result of hydrogen bonding, which lowers their entropies and also their E a and A parameters relative to those for C-C bond rupture pathways in hydrocarbon molecules. The E a values determined from the simulation Arrhenius plots are in very good agreement with the reaction barriers for the RM1 method used in the simulations. A preliminary simulation of TIK(H + ) 2 collision-induced dissociation (CID), at a collision energy of 13 eV (1255 kJ/mol), was also performed to compare with the thermal dissociation simulations. Though the energy transferred to TIK(H + ) 2 in the collisions is substantially less than the energy for the thermal excitations, there is substantial fragmentation as a result of the localized, nonrandom excitation by the collisions. CID results in different fragmentation pathways with a significant amount of short time nonstatistical fragmentation. Backbone fragmentation is less important, and side-chain fragmentation is more important for the CID simulations as compared to the thermal simulations. The thermal simulations provide information regarding the long-time statistical fragmentation.

Chemical dynamics simulations were performed to investigate collisional energy transfer from highly vibrationally excited azulene (Az*) in a N2 bath. The intermolecular potential between Az and N2, used for the simulations, was determined from MP2/6-31+G* ab initio calculations. Az* is prepared with an 87.5 kcal/mol excitation energy by using quantum microcanonical sampling, including its 95.7 kcal/mol zero-point energy. The average energy of Az* versus time, obtained from the simulations, shows different rates of Az* deactivation depending on the N2 bath density. Using the N2 bath density and Lennard-Jones collision number, the average energy transfer per collision ⟨ΔEc⟩ was obtained for Az* as it is collisionally relaxed. By comparing ⟨ΔEc⟩ versus the bath density, the single collision limiting density was found for energy transfer. The resulting ⟨ΔEc⟩, for an 87.5 kcal/mol excitation energy, is 0.30 ± 0.01 and 0.32 ± 0.01 kcal/mol for harmonic and anharmonic Az potentials, respectively. For comparison, the experimental value is 0.57 ± 0.11 kcal/mol. During Az* relaxation there is no appreciable energy transfer to Az translation and rotation, and the energy transfer is to the N2 bath.

A zero-point energy (ZPE) constraint model is proposed for classical trajectory simulations of unimolecular decomposition and applied to CH4* → H + CH3 decomposition. With this model trajectories are not allowed to dissociate unless they have ZPE in the CH3 product. If not, they are returned to the CH4* region of phase space and, if necessary, given additional opportunities to dissociate with ZPE. The lifetime for dissociation of an individual trajectory is the time it takes to dissociate with ZPE in CH3, including multiple possible returns to CH4*. With this ZPE constraint the dissociation of CH4* is exponential in time as expected for intrinsic RRKM dynamics and the resulting rate constant is in good agreement with the harmonic quantum value of RRKM theory. In contrast, a model that discards trajectories without ZPE in the reaction products gives a CH4* → H + CH3 rate constant that agrees with the classical and not quantum RRKM value. The rate constant for the purely classical simulation indicates that anharmonicity may be important and the rate constant from the ZPE constrained classical trajectory simulation may not represent the complete anharmonicity of the RRKM quantum dynamics. The ZPE constraint model proposed here is compared with previous models for restricting ZPE flow in intramolecular dynamics, and connecting product and reactant/product quantum energy levels in chemical dynamics simulations.

The kinetics of the reaction of N2H4 with oxygen depends sensitively on the initial conditions used. In oxygen-rich systems, the rate constant shows a conventional positive temperature dependence, while in hydrazine-rich setups the dependence is negative in certain temperature ranges. In this study, a theoretical model is presented that adequately reproduces the experimental results trend and values for hydrazine-rich environment, consisting of the hydrogen abstraction from the hydrazine (N2H4) dimer by an oxygen atom. The thermochemical properties of the reaction were computed using two quantum chemical approaches, the coupled cluster theory with single, double, and noniterative triple excitations (CCSD(T)) and the M06-2X DFT approach with the aug-cc-pVTZ and the maug-cc-pVTZ basis sets, respectively. The kinetic data were calculated with the improved canonical variational theory (ICVT) using a dual-level methodology to build the reaction path. The tunneling effects were considered by means of the small curvature tunneling (SCT) approximation. Potential wells on both sides of the reaction ((N2H4)2 + O → N2H4···N2H3 + OH) were determined. A reaction path with a negative activation energy was found leading, in the temperature range of 250-423 K, to a negative dependence of the rate constant on the temperature, which is in good agreement with the experimental measurements. Therefore, the consideration of the hydrazine dimer model provides significantly improved agreement with the experimental data and should be included in the mechanism of the global N2H4 combustion process, as it can be particularly important in hydrazine-rich systems.

In this study, we compute all of the dynamically relevant vibrational quantum states of benzene, using an "exact" quantum dynamics (EQD) methodology. Benzene (C6H6), in addition to being a very large molecule for EQD (12 atoms, 30 vibrational modes), also has a very large number of vibrational states-around 10(6) in all, lying within 6500 cm(-1) of the ground state. The EQD methodology developed here uses a phase space picture to optimize the truncation of a harmonic oscillator basis-not only with respect to the molecular system of interest but also with respect to the targeted spectral range. By employing several such EQD calculations, targeted to different spectral ranges, a "hybridized" data set is constructed that provides the most accurate results everywhere. In particular, more than 500,000 states are converged to 15 cm(-1) or better.

Direct chemical dynamics simulations, at collision energies Erel of 0.32 and 1.53 eV, were performed to obtain an atomistic understanding of the F(-) + CH3I reaction dynamics. There is only the F(-) + CH3I → CH3F + I(-) bimolecular nucleophilic substitution SN2 product channel at 0.32 eV. Increasing Erel to 1.53 eV opens the endothermic F(-) + CH3I → HF + CH2I(-) proton transfer reaction, which is less competitive than the SN2 reaction. The simulations reveal proton transfer occurs by two direct atomic-level mechanisms, rebound and stripping, and indirect mechanisms, involving formation of the F(-)···HCH2I complex and the roundabout. For the indirect trajectories all of the CH2I(-) is formed with zero-point energy (ZPE), while for the direct trajectories 50% form CH2I(-) without ZPE. Without a ZPE constraint for CH2I(-), the reaction cross sections for the rebound, stripping, and indirect mechanisms are 0.2 ± 0.1, 1.2 ± 0.4, and 0.7 ± 0.2 Å(2), respectively. Discarding trajectories that do not form CH2I(-) with ZPE reduces the rebound and stripping cross sections to 0.1 ± 0.1 and 0.7 ± 0.5 Å(2). The HF product is formed rotationally and vibrationally unexcited. The average value of J is 2.6 and with histogram binning n = 0. CH2I(-) is formed rotationally excited. The partitioning between CH2I(-) vibration and HF + CH2I(-) relative translation energy depends on the treatment of CH2I(-) ZPE. Without a CH2I(-) ZPE constraint the energy partitioning is primarily to relative translation with little CH2I(-) vibration. With a ZPE constraint, energy partitioning to CH2I(-) rotation, CH2I(-) vibration, and relative translation are statistically the same. The overall F(-) + CH3I rate constant at Erel of both 0.32 and 1.53 eV is in good agreement with experiment and negligibly affected by the treatment of CH2I(-) ZPE, since the SN2 reaction is the major contributor to the total reaction rate constant. The potential energy surface and reaction dynamics for F(-) + CH3I proton transfer are compared with those reported previously (J. Phys. Chem. A 2013, 117, 7162-7178) for the isoelectronic OH(-) + CH3I reaction.

Chemical dynamics simulations were used to study Bz + Na(+)(Bz) → Na(+)(Bz)2* association and the ensuing dissociation of the Na(+)(Bz)2* cluster (Bz = benzene). An interesting and unexpected reaction found from the simulations is direct displacement, for which the colliding Bz molecule displaces the Bz molecule attached to Na(+), forming Na(+)(Bz). The rate constant for Bz + Na(+)(Bz) association was calculated at 750 and 1000 K, and found to decrease with increase in temperature. By contrast, the direct displacement rate constant increases with temperature. The cross section and rate constant for direct displacement are approximately an order of magnitude lower than those for association. The Na(+)(Bz)2* cluster, formed by association, dissociates with a biexponential probability, with the rate constant for the short-time component approximately an order of magnitude larger than that for the longer time component. The latter rate constant agrees with that of Rice-Ramsperger-Kassel-Marcus (RRKM) theory, consistent with rapid intramolecular vibrational energy redistribution (IVR) and intrinsic RRKM dynamics for the Na(+)(Bz)2* cluster. A coupled phase space model was used to analyze the biexponential dissociation probability.

Classical chemical dynamics simulations were performed to study the intramolecular and unimolecular dissociation dynamics of the benzene dimer, Bz2 → 2 Bz. The dissociation of microcanonical ensembles of Bz2 vibrational states, at energies E corresponding to temperatures T of 700-1500 K, were simulated. For the large Bz2 energies and large number of Bz2 vibrational degrees of freedom, s, the classical microcanonical (RRKM) and canonical (TST) rate constant expressions become identical. The dissociation rate constant for each T is determined from the initial rate dN(t)/dt of Bz2 dissociation, and the k(T) are well-represented by the Arrhenius eq k(T) = A exp(-E(a)/RT). The E(a) of 2.02 kcal/mol agrees well with the Bz2 dissociation energy of 2.32 kcal/mol, and the A-factor of 2.43 × 10(12) s(-1) is of the expected order-of-magnitude. The form of N(t) is nonexponential, resulting from weak coupling between the Bz2 intramolecular and intermolecular modes. With this weak coupling, large Bz2 vibrational excitation, and low Bz2 dissociation energy, most of the trajectories dissociate directly. Simulations, with only the Bz2 intramolecular modes excited at 1000 K, were also performed to study intramolecular vibrational energy redistribution (IVR) between the intramolecular and intermolecular modes. Because of restricted IVR, the initial dissociation is quite slow, but N(t) ultimately becomes exponential, suggesting an IVR time of 20.7 ps.

The structural processes leading to dual fluorescence of 4-(dimethylamino)benzonitrile in the gas phase and in acetonitrile solvent were investigated using a combination of multireference configuration interaction (MRCI) and the second-order algebraic diagrammatic construction (ADC(2)) methods. Solvent effects were included on the basis of the conductor-like screening model. The MRCI method was used for computing the nonadiabatic interaction between the two lowest excited ππ* states (S2(La, CT) and S1(Lb, LE)) and the corresponding minimum on the crossing seam (MXS) whereas the ADC(2) calculations were dedicated to assessing the role of the πσ* state. The MXS structure was found to have a twisting angle of ∼50°. The branching space does not contain the twisting motion of the dimethylamino group and thus is not directly involved in the deactivation process from S2 to S1. Polar solvent effects are not found to have a significant influence on this situation. Applying Cs symmetry restrictions, the ADC(2) calculations show that CCN bending leads to a strong stabilization and to significant charge transfer (CT). Nevertheless, this structure is not a minimum but converts to the local excitation (LE) structure on releasing the symmetry constraint. These findings suggest that the main role in the dynamics is played by the nonadiabatic interaction of the LE and CT states and that the main source for the dual fluorescence is the twisted internal charge-transfer state in addition to the LE state.