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The use of a tip-shaped sample for the atom probe tomography technique offers the unique opportunity to analyze the dynamics of molecular ions in strong DC fields. We investigate here the stability of AlN2+ and GaN2+ dications emitted from an Al0.25Ga0.75N sample in a joint theoretical and experimental study. Despite the strong chemical resemblance of these two molecules, we observe only stable AlN2+, while GaN2+ can only be observed as a transient species. We simulate the emission dynamics of these ions on field-perturbed potential energy surfaces obtained from quantum chemical calculations. We show that the dissociation is governed by two independent processes. For all bound states, a mechanical dissociation is induced by the distortion of the potential energy surface in the close vicinity of the emitting tip. In the specific case of GaN2+, the relatively small electric dipole of the dication in its ground 13Σ− and excited 11Δ states induces a weak coupling with the electric field so that the mechanical dissociation into Ga+ + N+ lasts for sufficient time to be observed. By contrast, the AlN2+ mechanical dissociation leads to Al2+ + N which cannot be observed as a correlated event. For some deeply bound singlet excited states, the spin-orbit coupling with lower energy triplet states gives another chance of dissociation by system inter-system crossing with specific patterns observed experimentally in a correlated time of flight map. [ABSTRACT FROM AUTHOR]

We investigate from a theoretical point of view the stability of AlN2+ and GaN2+ dications produced under high static electric fields like those reached in Atom Probe Tomography (APT) experiments. By means of quantum chemical calculations of the electronic structure of these molecules, we show that their stability is governed by two independent processes. On the one hand, the spin-orbit coupling allows some molecular excited states to dissociate by inter-system crossing. On the other hand, the action of the electric field lowers the potential energy barrier, which ensures the dication stability in standard conditions. We present a detailed example of field emission dynamics in the specific case of the 11Δ states for a parabolic tip, which captures the essentials of the process by means of a simplified model. We show that the dissociation dynamics of AlN2+ and GaN2+ is completely different despite the strong resemblance of their electronic structure. [ABSTRACT FROM AUTHOR]

Non-equilibrium molecular-dynamics simulations have been performed for dissolution of planar propane-hydrate/water interfaces in externally-applied electromagnetic (e/m) fields in the microwave to far infrared range (∼2.45-200 GHz) at electric-field intensities up to 2.0 V/nm and at roughly 20 K over/under temperatures vis-à-vis the zero-field propane-hydrate melting point. Upon e/m-field application, there is a field-frequency threshold above which the dissociation rate drops significantly, with a plateau therein for larger-frequencies. It was found that higher intensity and lower frequency facilitates dissociation. Except in the presence of a thermal driving-force, the 10 GHz frequency shows more substantial rate-enhancement effect vis-à-vis static electric fields or, indeed, lower-frequency e/m fields. [ABSTRACT FROM AUTHOR]

The dissociative chemisorption of singly deuterated methane (CH3D) has been studied on the steps and terraces of a Pt(211) surface by quantum state resolved molecular beam methods. At incident translational energy (Et) below 50 kJ/mol, CH3D dissociates only on the more reactive steps of Pt(211), where both C–H and C–D cleavage products CH2D(ads) and CH3(ads) can be detected by reflection absorption infrared spectroscopy. Vibrational excitation of a slow beam of CH3D (Et = 10 kJ/mol), prepared with one quantum of antisymmetric C–H stretch excitation by infrared laser pumping, allows for fully bond- and site-selective dissociation forming exclusively CH2D(ads) on the step sites. At higher kinetic energies (Et > 30 kJ/mol), bond selective dissociation by C–H bond cleavage is observed on the terrace sites for stretch excited CH3D (ν4) while on the steps, the C–H/C–D cleavage branching ratio approaches the statistical 3/1 limit. Finally, at Et > 60 kJ/mol, both C–H and C–D cleavages are observed on both step and terrace sites of Pt(211). Our experiments show how careful control of incident translational and vibrational energy can be used for site and bond selective dissociation of methane on a catalytically active Pt surface. [ABSTRACT FROM AUTHOR]

The electron affinity of the deuterium atom has been determined to be 6086.81(27) cm−1 from a measurement of the difference between the D+ + H− and H+ + D− ion-pair dissociation energies and a thermochemical cycle involving the electron affinity of H and the ionization energies of H and D. Heavy-Rydberg states and the ion-pair dissociation thresholds of HD were accessed with good efficiency using a three-photon excitation sequence through the B Σ u + 1 ( v = 22, N = 1) and H ¯ Σ g + 1 ( v = 9, N = 0) intermediate levels and the threshold positions were determined using the method of threshold-ion-pair-production spectroscopy. [ABSTRACT FROM AUTHOR]

Using the stimulated-emission-pumping/resonant 2-photon ionization (SEP-R2PI) method, we have determined accurate intermolecular dissociation energies D0 of supersonic jet-cooled intermolecular complexes of 1-naphthol (1NpOH) with alkanes, 1NpOH·S, with S = methane, ethane, propane, and n-butane. Experimentally, the smaller alkanes form a single minimum-energy structure, while 1-naphthol·n-butane forms three different isomers. The ground-state dissociation energies D0(S0) for the complexes with propane and n-butane (isomers A and B) were bracketed within ±0.5%, being 16.71 ± 0.08 kJ/mol for S = propane and 20.5 ± 0.1 kJ/mol for isomer A and 20.2 ± 0.1 kJ/mol for isomer B of n-butane. All 1NpOH·S complexes measured previously exhibit a clear dissociation threshold in their hot-band detected SEP-R2PI spectra, but weak SEP-R2PI bands are observed above the putative dissociation onset for the methane and ethane complexes. We attribute these bands to long-lived complexes that retain energy in rotation-type intermolecular vibrations, which couple only weakly to the dissociation coordinates. Accounting for this, we find dissociation energies of D0(S0) = 7.98 ± 0.55 kJ/mol (±7%) for S = methane and 14.5 ± 0.28 kJ/mol (±2%) for S = ethane. The D0 values increase by only 1% upon S0 → S1 excitation of 1-naphthol. The dispersion-corrected density functional theory methods B97-D3, B3LYP-D3, and ωB97X-D predict that the n-alkanes bind dispersively to the naphthalene “Face.” The assignment of the complexes to Face structures is supported by the small spectral shifts of the S0 → S1 electronic origins, which range from +0.5 to −15 cm−1. Agreement with the calculated dissociation energies D0(S0) is quite uneven, the B97-D3 values agree within 5% for propane and n-butane, but differ by up to 20% for methane and ethane. The ωB97X-D method shows good agreement for methane and ethane but overestimates the D0(S0) values for the larger n-alkanes by up to 20%. The agreement of the B3LYP-D3 D0 values is intermediate between the other two methods. [ABSTRACT FROM AUTHOR]

Phenol can serve as a model for examining the deactivation of the aromatic amino acid tyrosine following UV excitation, which mainly occurs through a repulsive πσ* state along the O–H bond. The reaction barrier formed by the conical intersection between the optically bright S1 (ππ*) state and the dissociative S2 (πσ*) state does not inhibit O–H bond rupture even though the excitation energy is below the barrier height. To examine the O–H bond-rupture dynamics in association with the initial excited vibrational modes, we used a picosecond laser to investigate the vibrational-mode-dependent excited-state lifetime of phenol under the S1/S2 conical intersection. Unexpectedly short lifetimes were observed in the S1 state for a″ symmetric vibrational modes (including v4, v16a, τOH, and v5). These results clarify recent theoretical calculations showing that the relaxation from S1 to S2 either occurs via symmetry-allowed non-adiabatic transitions or is topographically linked to a lower energy minimum on the multidimensional potential energy surface. [ABSTRACT FROM AUTHOR]

In recent years, ozone and its isotopologues have been a topic of interest in many fields of research, due to its importance in atmospheric chemistry and its anomalous isotopic enrichment—or the so-called “mass-independent fractionation.” In the field of potential energy surface (PES) creation, debate over the existence of a potential barrier just under the dissociation threshold (referred to as a “potential reef”) has plagued research for some years. Recently, Dawes and co-workers [Dawes, Lolur, Li, Jiang, and Guo (DLLJG) J. Chem. Phys. 139, 201103 (2013)] created a highly accurate global PES, for which the reef is found to be replaced with a (monotonic) “plateau.” Subsequent dynamical calculations on this “DLLJG” PES have shown improved agreement with experiment, particularly the vibrational spectrum. However, it is well known that reaction dynamics is also highly influenced by the rovibrational states, especially in cases like ozone that assume a Lindemann-type mechanism. Accordingly, we present the first significant step toward a complete characterization of the rovibrational spectrum for various isotopologues of ozone, computed using the DLLJG PES together with the ScalIT suite of parallel codes. Additionally, artificial neural networks are used in an innovative fashion—not to construct the PES function per se but rather to greatly speed up its evaluation. [ABSTRACT FROM AUTHOR]

A hydrogen atom can either physisorb or chemisorb onto a graphene surface. To describe the interaction of H with graphene, we trained the C—C, H—H, and C—H interactions of the ReaxFF CHO bond order potential to reproduce Density Functional Theory (DFT) generated values of graphene cohesive energy and lattice constant, H2 dissociation energy, H on graphene adsorption potentials, and H2 formation on graphene using the Eley-Rideal (ER) and Langmuir-Hinshelwood (LH) processes. The results, generated from the trained H-graphene potentials, are in close agreement with the corresponding results from DFT. The advantage of using optimized CH potentials is, for example, the inclusion of physisorption interactions and quantum mechanical features of chemical bonding in the functional forms of the potentials. The trained CH potentials are utilized to study the energetics of formation of an H2 molecule on graphene using the Eley-Rideal and Langmuir-Hinshelwood processes. Potential energy surfaces for the formation of H2 through ER are generated for the collinear and oblique approach of the second hydrogen atom. Energetics of the formation of H2 through LH is studied for a variety of cases such as when hydrogen atoms are chemisorbed or physisorbed and when hydrogen occupies ortho, meta, or para chemisorption sites. The likelihood of H2 formation through LH for various configurations is discussed. Furthermore, the tunneling probability of an atom through a continuous symmetric/asymmetric barrier is calculated and applied to an adsorbed hydrogen atom on graphene. [ABSTRACT FROM AUTHOR]

A coupled-channel Schrödinger equation model of predissociation in the B Σ u − 3 state of S2 is developed and optimized by comparison with recent photoabsorption spectra of the B Σ u − 3 − X Σ g − 3 ( v , 0 ) bands for 11 ≤ v ≤ 27, covering the energy range 35 800–41 500 cm−1. All bands in this range exhibit varying degrees of diffuseness, with corresponding predissociation linewidths Γ ≈ 4–60 cm−1 full-width at half-maximum. Model comparisons with both low-temperature (T = 370 K) and high-temperature (T = 823 K) spectra indicate, for many bands, significant dependence of the linewidth on both the rotational quantum number J and the fine-structure component Fi. Just as in the analogous case of O2, the B( v )-state predissociation in S2 is caused principally by spin-orbit interaction with 3Πu, 1Πu, 5Πu, and Σ u + 3 states. The inner-limb crossing with B″3Πu is responsible for the predissociation of B( v = 11) and provides a significant slowly varying contribution for B( v ≥ 12). The outer crossings with the 1Πu, 5Πu, and 2 Σ u + 3 states are responsible for oscillatory contributions to the predissociation widths, with first peaks at v = 13, 20, and 24, respectively, and the 5Πu contribution dominant. Prior to the photodissociation imaging study of Frederix et al. [J. Phys. Chem. A 113, 14995 (2009)], which redefined the dissociation energy of S2, the prevailing paradigm was that only the 1Πu interaction was responsible for the B( v = 11–16) predissociation: this view is not supported by our model. [ABSTRACT FROM AUTHOR]

This paper reports a time-dependent quantum mechanical wave packet study for bond-selective excitation and dissociation of HOD into the H + OD and D + OH channels in the first absorption band. Prior to excitation, the HOD molecule is randomly oriented with respect to a linearly polarized laser field and accurate static dipole moment and polarizability surfaces are included in the interaction potential. Vibrational excitation is obtained with intense, non-resonant 800 nm few-cycle excitation using dynamic Stark effect/impulsive Raman scattering. Dissociation is accomplished by another ultrashort vacuum ultraviolet-laser excitation. A laser control scheme is designed with a train of simple, non-resonant laser pulses in order to enhance the selectivity between the fragmentation channels. The effect of the carrier-envelope-phase of the ultrashort laser pulses is also investigated. [ABSTRACT FROM AUTHOR]

In the present contribution, we study dissociative electron attachment to 1-methyl-2-thiouracil that has been synthesized and purified prior to the measurements. We compare the results with those previously obtained from 2-thiouracil. The comparison of the yield of the dehydrogenated parent anion from both the compounds allows us to assign the site from which the H atom is expulsed and to predict the mechanism that is involved in the formation of the peaks within the ion yield curve. It appears that the dehydrogenation observed for 2-thiouracil arising from the vibrational Feshbach resonances (at 0.7 and 1.0 eV) and a π*/σ* transition (at 0.1 eV) involves the bond cleavage at the N1 site, while that at the N3 site operates via the π*/σ* transition and occurs in the energy range of 1.1–3.3 eV. Besides the loss of the H atom from 1-methyl-2-thiouracil, we observe a relatively strong signal due to the loss of an entire methyl group (not observed from methyl-substituted thymine and uracil) that is formed from the N1–CH3 bond cleavage and can mimic the N-glycosidic bond cleavage within the DNA macromolecule. [ABSTRACT FROM AUTHOR]

Slow electron velocity-map imaging spectroscopy of cryogenically cooled TiO3H2− anions is used to probe the simplest titania/water reaction, TiO20/− + H2O. The resultant spectra show vibrationally resolved structure assigned to detachment from the cis-dihydroxide TiO(OH)2− geometry based on density functional theory calculations, demonstrating that for the reaction of the anionic TiO2− monomer with a single water molecule, the dissociative adduct (where the water is split) is energetically preferred over a molecularly adsorbed geometry. This work represents a significant improvement in resolution over previous measurements, yielding an electron affinity of 1.2529(4) eV as well as several vibrational frequencies for neutral TiO(OH)2. The energy resolution of the current results combined with photoelectron angular distributions reveals Herzberg-Teller coupling-induced transitions to Franck-Condon forbidden vibrational levels of the neutral ground state. A comparison to the previously measured spectrum of bare TiO2− indicates that reaction with water stabilizes neutral TiO2 more than the anion, providing insight into the fundamental chemical interactions between titania and water. [ABSTRACT FROM AUTHOR]

We report the absorption profile of isolated Flavin Adenine Dinucleotide (FAD) mono-anions recorded using photo-induced dissociation action spectroscopy. In this charge state, one of the phosphoric acid groups is deprotonated and the chromophore itself is in its neutral oxidized state. These measurements cover the first four optical transitions of FAD with excitation energies from 2.3 to 6.0 eV (210–550 nm). The S0 → S2 transition is strongly blue shifted relative to aqueous solution, supporting the view that this transition has a significant charge-transfer character. The remaining bands are close to their solution-phase positions. This confirms that the large discrepancy between quantum chemical calculations of vertical transition energies and solution-phase band maxima cannot be explained by solvent effects. We also report the luminescence spectrum of FAD mono-anions in vacuo. The gas-phase Stokes shift for S1 is 3000 cm−1, which is considerably larger than any previously reported for other molecular ions and consistent with a significant displacement of the ground and excited state potential energy surfaces. Consideration of the vibronic structure is thus essential for simulating the absorption and luminescence spectra of flavins. [ABSTRACT FROM AUTHOR]

Experimental investigation of electron ionization (EI) of and electron attachment (EA) onto dicyclohexyl phthalate (DCHP) was carried out using a crossed electron and molecular beam technique. Formation of positive and negative ions by EI and EA with the corresponding dissociation processes was studied and discussed. Due to a low ion yield of the parent positive ion, we were not able to estimate the ionization energy of DCHP. However, we estimated the appearance energies for the protonated phthalate anhydride (m/z 149) to be 10.5 eV and other significant ionic fragments of m/z 249 [DCHP—(R—2H)]+, m/z 167 [DCHP—(2R—3H)]+, and m/z 83 [C6H11]+. The reaction mechanisms of the dissociative ionization process were discussed. In the case of negative ions, we estimated the relative cross sections for a transient negative ion (TNI) and for several detected ions. At low electron energies (close to 0 eV), the TNI of DCHP molecules was the dominant ion, with products of dissociative EA dominating in broad resonances at 7.5 and 8.5 eV. [ABSTRACT FROM AUTHOR]

The observed anomalous self-diffusivity of ice VII in the region of 10 GPa at ∼400 K has been suggested to arise from a change in proton-hopping mechanism involving a transition from ionic-defect-driven diffusivity to that dominated by diffusion of rotational defects. Here, we report ab initio molecular dynamics to study the structural, hydrogen bonding, electronic, vibrational, and Raman properties of ice VII at this temperature and between 5 and 20 GPa to elucidate any possible hints of intramolecular strain that may serve as precursor events for proton hopping to unfold. We determine such equilibrium properties to be in reasonable agreement with experimental Raman spectra, although we do not detect any water-dissociation and proton-hopping events per se, owing to still-large water-dissociation free-energy barriers. [ABSTRACT FROM AUTHOR]

We report quantum-state resolved measurements of angular and velocity distributions of the associative desorption of H2, HD, and D2 from Cu(111) and Cu(211) surfaces. The desorbing molecules have bimodal velocity distributions comprising a “fast” channel and a “slow” channel on both facets. The “fast channel” is promoted by both hydrogen incidence translational and vibrational energy, while the “slow channel” is promoted by vibrational energy but inhibited by translational energy. Using detailed balance, we determine state-specific reaction probabilities for dissociative adsorption and compare these to theoretical calculations. The results for the activation barrier for the “fast channel” on Cu(111) are in agreement with theory within “chemical accuracy” (1 kcal/mole). Results on the Cu(211) facet provide direct information on the effect of increasing step density, which is commonly believed to increase reactivity. Differences in reactivity on the (111) and (211) facets are subtle – quantum state specific reactivity on the (211) surface is characterized by a broader distribution of barrier heights whose average values are higher than for reaction on (111). We fully characterize the “slow channel,” which has not been found in theoretical calculations although it makes up a large fraction of the reactivity in these experiments. [ABSTRACT FROM AUTHOR]

A new approach for preventing zero-point energy (ZPE) violation in quasi-classical trajectory (QCT) simulations is presented and applied to H2CO “roaming” reactions. Zero-point energy may be problematic in roaming reactions because they occur at or near bond dissociation thresholds and these channels may be incorrectly open or closed depending on if, or how, ZPE has been treated. Here we run QCT simulations on a “ZPE-corrected” potential energy surface defined as the sum of the molecular potential energy surface (PES) and the global harmonic ZPE surface. Five different harmonic ZPE estimates are examined with four, on average, giving values within 4 kJ/mol—chemical accuracy—for H2CO. The local harmonic ZPE, at arbitrary molecular configurations, is subsequently defined in terms of “projected” Cartesian coordinates and a global ZPE “surface” is constructed using Shepard interpolation. This, combined with a second-order modified Shepard interpolated PES, V, allows us to construct a proof-of-concept ZPE-corrected PES for H2CO, Veff, at no additional computational cost to the PES itself. Both V and Veff are used to model product state distributions from the H + HCO → H2 + CO abstraction reaction, which are shown to reproduce the literature roaming product state distributions. Our ZPE-corrected PES allows all trajectories to be analysed, whereas, in previous simulations, a significant proportion was discarded because of ZPE violation. We find ZPE has little effect on product rotational distributions, validating previous QCT simulations. Running trajectories on V, however, shifts the product kinetic energy release to higher energy than on Veff and classical simulations of kinetic energy release should therefore be viewed with caution. [ABSTRACT FROM AUTHOR]

Numerous processes in atmospheric and combustion chemistry produce the vinoxy radical (•CH2CHO). To understand the fate of this radical and to provide reliable energies needed for kinetic modeling of such processes, we have examined its reaction with O2 using highly reliable theoretical methods. Utilizing the focal point approach, the energetics of this reaction and subsequent reactions were obtained using coupled-cluster theory with single, double, and perturbative triple excitations [CCSD(T)] extrapolated to the complete basis set limit. These extrapolated energies were appended with several corrections including a treatment of full triples and connected quadruple excitations, i.e., CCSDT(Q). In addition, this study models the initial vinoxy radical + O2 reaction for the first time with multireference methods. We predict a barrier for this reaction of approximately 0.4 kcal mol−1. This result agrees with experimental findings but is in disagreement with previous theoretical studies. The vinoxy radical + O2 reaction produces a 2-oxoethylperoxy radical which can undergo a number of unimolecular reactions. Abstraction of a β-hydrogen (a 1,4-hydrogen shift) and dissociation back to reactants are predicted to be competitive to each other due to their similar barriers of 21.2 and 22.3 kcal mol−1, respectively. The minimum-energy β-hydrogen abstraction pathway produces a hydroperoxy radical (QOOH) that eventually decomposes to formaldehyde, CO, and •OH. Two other unimolecular reactions of the peroxy radical are α-hydrogen abstraction (38.7 kcal mol−1 barrier) and HO2• elimination (43.5 kcal mol−1 barrier). These pathways lead to glyoxal + •OH and ketene + HO2• formation, respectively, but they are expected to be uncompetitive due to their high barriers. [ABSTRACT FROM AUTHOR]

Photon-induced fragmentation of a full set of chlorinated methanes (CH3Cl, CH2Cl2, CHCl3, CCl4) has been investigated both experimentally and computationally. Using synchrotron radiation and electron-ion coincidence measurements, the dissociation processes were studied after chlorine 2p electron excitation. Experimental evidence for CH3Cl and CH2Cl2 contains unique features suggesting that fast dissociation processes take place. By contrast, CHCl3 and CCl4 molecules do not contain the same features, hinting that they experience alternative mechanisms for dissociation and charge migration. Computational work indicates differing rates of charge movement after the core-excitation, which can be used to explain the differences observed experimentally. [ABSTRACT FROM AUTHOR]

In gas phase synthesis systems, clusters form and grow via condensation, in which a monomer binds to an existing cluster. While a hard-sphere equation is frequently used to predict the condensation rate coefficient, this equation neglects the influences of potential interactions and cluster internal energy on the condensation process. Here, we present a collision rate theory-molecular dynamics simulation approach to calculate condensation probabilities and condensation rate coefficients. We use this approach to examine atomic condensation onto 6-56-atom Au andMgclusters. The probability of condensation depends upon the initial relative velocity (v) between atom and cluster and the initial impact parameter (b). In all cases, there is a well-defined region of b-v space where condensation is highly probable, and outside of which the condensation probability drops to zero. For Au clusters with more than 10 atoms, we find that at gas temperatures in the 300--1200 K range, the condensation rate coefficient exceeds the hard-sphere rate coefficient by a factor of 1.5-2.0. Conversely, for Au clusters with 10 or fewer atoms and for 14- and 28-atom Mg clusters, as cluster equilibration temperature increases, the condensation rate coefficient drops to values below the hard-sphere rate coefficient. Calculations also yield the self-dissociation rate coefficient, which is found to vary considerably with gas temperature. Finally, calculations results reveal that grazing (high b) atom-cluster collisions at elevated velocity (>1000 m s-1) can result in the colliding atom rebounding (bounce) from the cluster surface or binding while another atom dissociates (replacement). The presented method can be applied in developing rate equations to predict material formation and growth rates in vapor phase systems. [ABSTRACT FROM AUTHOR]

This study presents the method of fixed-photon-energy double-imaging photoelectron photoion coincidence (i2PEPICO) utilized to investigate the dissociation of state-selected ions. Vacuum ultraviolet (VUV) synchrotron radiation at one fixed photon energy of hν = 21.2 eV, the He(I) atomic resonance energy, is employed as a light source to ionize molecules. Various dynamic information including time-of-flight (TOF) mass spectra, mass-selected photoelectron spectra (PES), and electron and ion kinetic energy correlation diagrams corresponding to each mass are obtained efficiently with the multiplexed capabilities of i2PEPICO, thereby revealing the detailed dissociation mechanisms of ions. As representative examples, dissociation of state-selected O2+ ions prepared in the b4∑g− and B2∑g− electronic states and CH3F+ ions in the X2E, A2A1, and B2E states were selected and investigated. [ABSTRACT FROM AUTHOR]

The reactivity of various Pd ensembles on the Au-Pd(100) alloy catalyst toward CO oxidation was investigated by using density functional theory (DFT). This study was prompted by the search for efficient catalysts operating at low temperature for the CO oxidation reaction that is of primary environmental importance. To this aim, we considered Pd modified Au(100) surfaces including Pd monomers, Pd dimers, second neighboring Pd atoms, and Pd chains in a comparative study of the minimum energy reaction pathways. The effect of dispersion interactions was included in the calculations of the O2 dissociation reaction pathway by using the DFT-D3 scheme. The addition of the dispersion interaction strongly improves the adsorption ability of O2 on the Au-Pd surface but does not affect the activation energy barriers of the Transitions States (TSs). As for O2 to dissociate, it is imperative that the TS has lower activation energy than the O2 desorption energy. DFT-D3 is found to favor, in some cases, O2 dissociation on configurations being identified from uncorrected DFT calculations as inactive. This is the case of the second neighboring Pd configuration for which uncorrected DFT predicts positive Gibbs free energy (ΔG) of the O2 adsorption, therefore an endergonic reaction. With the addition of D3 correction, ΔG becomes negative that reveals a spontaneous O2 adsorption. Among the investigated Au-Pd (100) ensembles, the Pd chain dissociates most easily O2 and highly stabilizes the dissociated O atoms; however, it has an inferior reactivity toward CO oxidation and CO2 formation. Indeed, CO strongly adsorbs on the palladium bridge sites and therefore poisoning the surface Pd chain. By contrast, the second neighboring Pd configuration that shows somewhat lower ability to dissociate O2 turns out to be more reactive in the CO2 formation step. These results evidence the complex effect of Pd ensembles on the CO oxidation reaction. Associative CO oxidation proceeds with high energy barriers on all the considered Pd ensembles and should be excluded, in agreement with experimental observations. [ABSTRACT FROM AUTHOR]

Methane dissociation on the step and terrace sites of a Pt(211) single crystal was studied by reflection absorption infrared spectroscopy (RAIRS) at a surface temperature of 120 K. The C—H stretch RAIRS signal of the chemisorbed methyl product species was used to distinguish between adsorption on step and terrace sites allowing methyl uptake to be monitored as a function of incident kinetic energy for both sites. Our results indicate a direct dissociation mechanism on both sites with higher reactivity on steps than on terraces consistent with a difference in an activation barrier height of at least 30 kJ/mol. State-specific preparation of incident CH4 with one quantum of antisymmetric (ν3) stretch vibration further increases the CH4 reactivity enabling comparison between translational and vibrational activation on both steps and terraces. The reaction is modeled with first principles quantum theory that accurately describes dissociative chemisorption at different sites on the surface. [ABSTRACT FROM AUTHOR]

The effect of aqueous environment on fast heavy-ion radiation damage of biomoleculeswas studied by comparative experiments using liquid- and gas-phase amino acid targets. Three types of amino acids with different chemical structures were used: glycine, proline, and hydroxyproline. Ion-induced reaction products were analyzed by time-of-flight secondary-ion mass spectrometry. The results showed that fragments from the amino acids resulting from the C--Cα bond cleavage were the major products for both types of targets. For liquid-phase targets, specific products originating from chemical reactions in solutions were observed. Interestingly, multiple dissociated atomic fragments were negligible for the liquid-phase targets. We found that the ratio of multifragment to total fragment ion yields was approximately half of that for gas-phase targets. This finding agreed with the results of other studies on biomolecular cluster targets. It is concluded that the suppression of molecular multifragmentation is caused by the energy dispersion to numerous water molecules surrounding the biomolecular solutes. [ABSTRACT FROM AUTHOR]

We report on the low energy anion spectra of 5-cyanateuracil (5-OCNU) and 5-thiocyanateouracil (5-SCNU), which have been the suggested potential radiosensitizers for use in cancer therapy [L. Chomicz et al., J. Phys. Chem. Lett. 4, 2853-2857 (2013)]. Employing bound state and scattering calculations, we obtained, for both molecules, a dipole bound state, a π* valence bound state, and four π* resonances, besides a σ* SCN resonance for 5-SCNU. The cyanate and thiocyanate substituents give rise to additional long-lived π* resonances, compared to 5-halouracil radiosensitizers. From the reaction thresholds and the expected vibronic couplings among the anion states, efficient production of SCN and CN anions and radical fragments should be observed in dissociative electron attachment measurements for 5-SCNU. The corresponding dissociation processes in 5-OCNU are expected to be less effective in view of the lack of a long-lived σ* OCN shape resonance and the little σ* admixture into the π* resonances located on the cyanate group. The present results thus indicate 5-SCNU as a more promising radiosensitizer at sub-excitation energies. [ABSTRACT FROM AUTHOR]

Previouswork has shown that atomic samarium cations react with carbonyl sulfide to form SmS+ +CO in an exothermic and barrierless process. To characterize this reaction further, the bond energy of SmS+ is determined in the present study using guided ion beam tandem mass spectrometry. Reactions of SmS+ with Xe, CO, andO2 are examined. Results for collision-induced dissociation processes with all three molecules along with the endothermicity of the SmS+ +CO→Sm+ + COS exchange reaction are combined to yield D0(Sm+-S) = 3.37 ± 0.20 eV. The CO and O2 reactions also yield a SmSO+ product, with measured endothermicities that indicate D0(SSm+-O) = 3.73 ± 0.16 eV and D0(OSm+-S) = 1.38 ± 0.27 eV. The SmS+ bond energy is compared with theoretical values characterized at several levels of theory, including CCSD(T) complete basis set extrapolations using all-electron basis sets. Multireference configuration interaction calculations with explicit spin-orbit calculations along with composite thermochemistry using the Feller-Peterson-Dixon method and all-electron basis sets were also explored for SmS+, and for comparison, SmO, SmO+, and EuO. [ABSTRACT FROM AUTHOR]

Previouswork has shown that atomic samarium cations react with carbonyl sulfide to form SmS+ +CO in an exothermic and barrierless process. To characterize this reaction further, the bond energy of SmS+ is determined in the present study using guided ion beam tandem mass spectrometry. Reactions of SmS+ with Xe, CO, andO2 are examined. Results for collision-induced dissociation processes with all three molecules along with the endothermicity of the SmS+ +CO→Sm+ + COS exchange reaction are combined to yield D0(Sm+-S) = 3.37 ± 0.20 eV. The CO and O2 reactions also yield a SmSO+ product, with measured endothermicities that indicate D0(SSm+-O) = 3.73 ± 0.16 eV and D0(OSm+-S) = 1.38 ± 0.27 eV. The SmS+ bond energy is compared with theoretical values characterized at several levels of theory, including CCSD(T) complete basis set extrapolations using all-electron basis sets. Multireference configuration interaction calculations with explicit spin-orbit calculations along with composite thermochemistry using the Feller-Peterson-Dixon method and all-electron basis sets were also explored for SmS+, and for comparison, SmO, SmO+, and EuO. [ABSTRACT FROM AUTHOR]

Protonated azobenzene (AB), H+(C6H5N2C6H5), has been studied using threshold collision-induced dissociation in a guided ion beam tandem mass spectrometer. Product channels observed are C6H5N2 + + C6H6 and C6H5 + + N2 + C6H6. The experimental kinetic energy-dependent cross sections were analyzed using a statistical model that accounts for internal and kinetic energy distributions of the reactants, multiple collisions, and kinetic shifts. From this analysis, the activation energy barrier height of 2.02 ± 0.11 eV for benzene loss is measured. To identify the transition states (TSs) and intermediates (IMs) for these dissociations, relaxed potential energy surface (PES) scans were performed at the B3LYP/aug-cc-pVTZ level of theory. The PES indicates that there is a substantial activation energy along the dissociation reaction coordinate that is the rate-limiting step for benzene loss and at some levels of theory, for subsequent N2 loss as well. Relative energies of the reactant, TSs, IMs, and products were calculated at B3LYP, wB97XD, M06, PBEPBE, and MP2(full) levels of theory using both 6-311++G(2d,2p) and aug-cc-pVTZ basis sets. Comparison of the experimental results with theoretical values from various computational methods indicates how well these theoretical methods can predict thermochemical properties. In addition to these density functional theory and MP2 methods, several high accuracy multi-level calculations such as CBS-QB3, G3, G3MP2, G3B3MP2, G4, and G4MP2 were performed to determine the thermochemical properties of AB including the proton affinity and gas-phase basicity, and to compare the performance of different theoretical methods. [ABSTRACT FROM AUTHOR]

The design and production of novel 2-dimensional materials have seen great progress in the last decade, prompting further exploration of the chemistry of such materials. Doping and hydrogenating graphene are an experimentally realised method of changing its surface chemistry, but there is still a great deal to be understood on how doping impacts on the adsorption of molecules. Developing this understanding is key to unlocking the potential applications of these materials. High throughput screening methods can provide particularly effective ways to explore vast chemical compositions of materials. Here, alchemical derivatives are used as a method to screen the dissociative adsorption energy of water molecules on variousBNdoped topologies of hydrogenated graphene. The predictions from alchemical derivatives are assessed by comparison to density functional theory. This screening method is found to predict dissociative adsorption energies that span a range of more than 2 eV, with a mean absolute error <0.1 eV. In addition, we show that the quality of such predictions can be readily assessed by examination of the Kohn-Sham highest occupied molecular orbital in the initial states. In this way, the root mean square error in the dissociative adsorption energies of water is reduced by almost an order of magnitude (down to ~0.02 eV) after filtering out poor predictions. The findings point the way towards a reliable use of first order alchemical derivatives for efficient screening procedures. [ABSTRACT FROM AUTHOR]

New molecular models, parameterized to ab initio calculations, were developed to describe HBr and HI at the air-water interface. These were used to compare how the air-water interface influenced dissociation of NaX and HX, with X being Cl, Br, or I, and also their propensity for the interface. The polarizable multistate empirical valence bond method, which explicitly describes proton sharing, was used to model HX. Results showed that the air-water interface suppressed HX dissociation from a contact ion pair to a solvent separated to a greater degree than NaX dissociation. Furthermore, HX had a greater propensity for the interface than NaX, whichwas a consequence of the hydronium ion having a greatest interfacial activity of all species studied. As a consequence of this, the average configuration of dissociated HX, while in both contact ion and solvent separated ion pairs near the air-water interface, is with the dissociated hydrogen oriented more towards the air than the X atom. [ABSTRACT FROM AUTHOR]

The association and dissociation of particles via specific anisotropic interactions is a fundamental process, both in biology (proteins) and in soft matter (colloidal patchy particles). The presence of alternative binding sites can lead to multiple productive states and also to non-productive "decoy" or intermediate states. Besides anisotropic interactions, particles can experience non-specific isotropic interactions. We employ single replica transition interface sampling to investigate how adding a non-productive binding site or a nonspecific isotropic interaction alters the dimerization kinetics of a generic patchy particle model. The addition of a decoy binding site reduces the association rate constant, independent of the site's position, while adding an isotropic interaction increases it due to an increased rebinding probability. Surprisingly, the association kinetics becomes non-monotonic for a tetramer complex formed by multivalent patchy particles. While seemingly identical to twoparticle binding with a decoy state, the cooperativity of binding multiple particles leads to a kinetic optimum. Our results are relevant for the understanding and modeling of biochemical networks and self-assembly processes. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of the tert-butyl peroxy (t-BuOO) radical are studied by fast-radical-beam coincidence translational spectroscopy. The neutral t-BuOO radical is formed by photodetachment of the corresponding t-BuOO- anion at 700 nm (1.77 eV), followed by dissociation at 248 nm (5.00 eV). Photofragment mass and translational energy distributions are obtained. The major channel is found to be three-body fragmentation to form O, CH3, and acetone (83%), with minor two-body fragmentation channels leading to the formation of O2 + tert-butyl radical (10%) and HO2 + isobutene (7%). Experimental results show that the translational energy distribution for two-body dissociation peaks is close to zero translational energy, with an isotropic angular distribution of fragments. These results indicate that two-body fragmentation proceeds via internal conversion to the ground electronic state followed by statistical dissociation. For three-body dissociation, the translational energy distribution peaks closer to the maximal allowed translational energy and shows an anisotropic distribution of the plane of the dissociating fragments, implying rapid dissociation on an excited-state surface.Asmall shoulder in the three-body translational energy distribution suggests that some three-fragment dissociation events proceed by a different mechanism, involving internal conversion to the ground electronic state followed by sequential dissociation. [ABSTRACT FROM AUTHOR]

Among various two-dimensional (2D) materials, monolayer group-III monochalcogenides (GaS, GaSe, InS, and InSe) stand out owing to their potential applications in microelectronics and optoelectronics. Devices made of these novel 2D materials are sensitive to environmental gases, especially O2 molecules. To address this critical issue, here we systematically investigate the oxidization behaviors of perfect and defective group-III monochalcogenide monolayers by first-principles calculations. The perfect monolayers show superior oxidation resistance with large barriers of 3.02-3.20 eV for the dissociation and chemisorption of O2 molecules. In contrast, the defective monolayers with single chalcogen vacancy are vulnerable to O2, showing small barriers of only 0.26-0.36 eV for the chemisorption of an O2 molecule. Interestingly, filling an O2 molecule to the chalcogen vacancy of group-III monochalcogenide monolayers could preserve the electronic band structure of the perfect system--the bandgaps are almost intact and the carrier effective masses are only moderately disturbed. On the other hand, the defective monolayers with single vacancies of group-III atoms carry local magnetic moments of 1-2 μB. These results help experimental design and synthesis of group-III monochalcogenides based 2D devices with high performance and stability. [ABSTRACT FROM AUTHOR]

This paper clarifies the disagreement in the depth of the potential energy curve of the cesium dimer singlet ground state which has lasted for nearly a decade. We point out that the origin of this disagreement must be a technical misprint in the values of the three binding energies reported by Danzl et al. [Science 321, 1062 (2008)], while the X¹Σg+ state potential reported by Coxon and Hajigeorgiou [J. Chem. Phys. 132, 094105 (2010)], based on experimental data by Amiot and Dulieu [J. Chem. Phys. 117, 5155 (2002)], is quite correct. We have recalculated the potential energy function of the triplet ground state a³Σu+ by using the available experimental data spanning both the attractive and the repulsive branches so that the potential energy function complies asymptotically with the singlet ground state X¹Σg+ potential energy function by Coxon and Hajigeorgiou. This is important for the simulation of the near dissociation properties such as Feshbach resonances, which are typically observed in modern experiments with ultracold atoms and molecules. [ABSTRACT FROM AUTHOR]

Dissociative electron attachment to nicotine, pyridine, and N-methyl-pyrrolidine was studied in the gas phase in order to assess their stability with respect to low-energy electron interactions. Anion yield curves for different products at electron energies ranging from zero to 15 eV were measured, and the molecular fragmentation pathways were proposed. Nicotine does not form a stable parent anion or a dehydrogenated anion, contrary to other biological systems. However, we have observed complex dissociation pathways involving fragmentation at the pyrrolidine side accompanied by isomerization mechanisms. Combining structure optimization and enthalpy calculations, performed with the Gaussian09 package, with the comparison with a deuterium-labeled N-methyl-d3-pyrrolidine allowed for the determination of the fragmentation pathways. In contrast to nicotine and N-methylpyrrolidine, the dominant pathway in dissociative electron attachment to pyridine is the loss of hydrogen, leading to the formation of an [M--H] anion. The presented results provide important new information about the stability of nicotine and its constituent parts and contribute to a better understanding of the fragmentation mechanisms and their effects on the biological environment. [ABSTRACT FROM AUTHOR]

The nonadiabatic alignment dynamics of weakly bound molecule-atom complexes, induced by a moderately intense 300 fs nonresonant laser pulse, is calculated by direct numerical solution of the time-dependent Schrödinger equation. Our method propagates the wave function according to the coupled channel equations for the complex, which can be done in a very efficient and stable manner out to large times. We present results for two van der Waal complexes, CS2-He and HCCH-He, as respective examples of linear molecules with large and small moments of inertia. Our main result is that at intensities typical of nonadiabatic alignment experiments, these complexes rapidly dissociate. In the case of the CS2-He complex, the ensuing rotational dynamics resembles that of isolated molecules, whereas for the HCCH-He complex, the detachment of the He atom severely perturbs and essentially quenches the subsequent rotational motion. At intensities of the laser pulse ≤ 2.0 x 1012 W/cm², it is shown that the molecule-He complex can rotate and align without breaking apart. We discuss the implications of our findings for recent experiments on iodine molecules solvated in helium nanodroplets. [ABSTRACT FROM AUTHOR]

Conical intersections are now firmly established to be the key features in the excited electronic state processes of polyatomic energetic molecules. In the present work, we have explored conical intersection-mediated nonadiabatic chemical dynamics of a simple analogue nitramine molecule, dimethylnitramine (DMNA, containing one N-NO2 energetic group), and its complex with an iron atom (DMNA-Fe). For this task, we have used the ab initio multiple spawning (AIMS) dynamics simulation at the state averaged-complete active space self-consistent field(8,5)/6-31G(d) level of theory. We have found thatDMNArelaxes back to the ground (S0) state following electronic excitation to the S1 excited state [which is an (n,π*) excited state] with a time constant of approximately 40 fs. This AIMS result is in very good agreement with the previous surface hopping-result and femtosecond laser spectroscopy result. DMNA does not dissociate during this fast internal conversion from the S1 to the S0 state. DMNA-Fe also undergoes extremely fast relaxation from the upper S1 state to the S0 state; however, this relaxation pathway is dissociative in nature. DMNA-Fe undergoes initial Fe-O, N-O, and N-N bond dissociations during relaxation from the upper S1 state to the ground S0 state through the respective conical intersection. The AIMS simulation reveals the branching ratio of these three channels as N-N:Fe-O:N-O = 6:3:1 (based on 100 independent simulations). Furthermore, the AIMS simulation reveals that the Fe-O bond dissociation channel exhibits the fastest (time constant 24 fs) relaxation, while the N-N bond dissociation pathway features the slowest (time constant 128 fs) relaxation. An intermediate time constant (30 fs) is found for the N-O bond dissociation channel. This is the first nonadiabatic chemical dynamics study of metal-contained energetic molecules through conical intersections. [ABSTRACT FROM AUTHOR]

Methane dissociation is a prototypical system for the study of surface reaction dynamics. The dissociation and recombination rates of CH4 through the Ni(111) surface are calculated by using the quantum instanton method with an analytical potential energy surface. The Ni(111) lattice is treated rigidly, classically, and quantum mechanically so as to reveal the effect of lattice motion. The results demonstrate that it is the lateral displacements rather than the upward and downward movements of the surface nickel atoms that affect the rates a lot. Compared with the rigid lattice, the classical relaxation of the lattice can increase the rates by lowering the free energy barriers. For instance, at 300 K, the dissociation and recombination rates with the classical lattice exceed the ones with the rigid lattice by 6 and 10 orders of magnitude, respectively. Compared with the classical lattice, the quantum delocalization rather than the zero-point energy of the Ni atoms further enhances the rates by widening the reaction path. For instance, the dissociation rate with the quantum lattice is about 10 times larger than that with the classical lattice at 300 K. On the rigid lattice, due to the zero-point energy difference between CH4 and CD4, the kinetic isotope effects are larger than 1 for the dissociation process, while they are smaller than 1 for the recombination process. The increasing kinetic isotope effect with decreasing temperature demonstrates that the quantum tunneling effect is remarkable for the dissociation process. [ABSTRACT FROM AUTHOR]

Association and dissociation of particles are elementary steps in many natural and technological relevant processes. For many such processes, the presence of multiple binding sites is essential. For instance, protein complexes and regular structures such as virus shells are formed from elementary building blocks with multiple binding sites. Here we address a fundamental question concerning the role of multivalency of binding sites in the association kinetics of such complexes. Using single replica transition interface sampling simulations, we investigate the influence of the multivalency on the binding kinetics and the association mechanism of patchy particles that form polyhedral clusters. When the individual bond strength is fixed, the kinetics naturally is very dependent on the multivalency, with dissociation rate constants exponentially decreasing with the number of bonds. In contrast, we find that when the total bond energy per particle is kept constant, association and dissociation rate constants turn out rather independent of multivalency, although of course still very dependent on the total energy. The association and dissociation mechanisms, however, depend on the presence and nature of the intermediate states. For instance, pathways that visit intermediate states are less prevalent for particles with five binding sites compared to the case of particles with only three bonds. The presence of intermediate states can lead to kinetic trapping and malformed aggregates. We discuss implications for natural forming complexes such as virus shells and for the design of artificial colloidal patchy particles. [ABSTRACT FROM AUTHOR]

The original ion imaging technique has low velocity resolution, and currently, photodissociation is mostly investigated using velocity map ion imaging. However, separating signals from the background (resulting from undissociated excited parent molecules) is difficult when velocity map ion imaging is used for the photodissociation of large molecules (number of atoms ≥ 10). In this study, we used the photodissociation of phenol at the S1 band origin as an example to demonstrate how our multimass ion imaging technique, based on modified spatial map ion imaging, can overcome this difficulty. The photofragment translational energy distribution obtained when multimass ion imaging was used differed considerably from that obtained when velocity map ion imaging and Rydberg atom tagging were used.We used conventional translational spectroscopy as a second method to further confirm the experimental results, and we conclude that data should be interpreted carefully when velocity map ion imaging or Rydberg atom tagging is used in the photodissociation of large molecules. Finally, we propose a modified velocity map ion imaging technique without the disadvantages of the current velocity map ion imaging technique. [ABSTRACT FROM AUTHOR]

We demonstrate a visualization of quantum mechanical phenomena with the velocity map imaging (VMI) technique, combining vibrationally mediated photodissociation (VMP) of a simple diatomic HCl with the VMI of its H-photofragments. Free HCl molecules were excited by a pump infrared (IR) laser pulse to particular rotational J levels of the v = 2 vibrational state, and subsequently a probe ultraviolet laser photodissociated the molecule at a fixed wavelength of 243.07 nm where also the H-fragments were ionized. The molecule was aligned by the IR excitation with respect to the IR laser polarization, and this alignment was reflected in the angular distribution of the H-photofragments. In particular, the highest degree of molecular alignment was achieved for the J = 1 ← 0 transition, which exclusively led to the population of a single rotational state with M = 0. The obtained images were analyzed for further details of the VMP dynamics, and different J states were studied as well. Additionally, we investigated the dynamic evolution of the excited states by changing the pumpprobe laser pulse delay; the corresponding images reflected dephasing due to a coupling between the molecular angular momentum and nuclear spin. Our measurements confirmed previous observation using the time-of-flight technique by Sofikitis et al. [J. Chem. Phys. 127, 144307 (2007)]. We observed a partial recovery of the originally excited state after 60 ns in agreement with the previous observation. [ABSTRACT FROM AUTHOR]

Collisions between K(12p) Rydberg atoms and CH3NO2 target molecules are studied. Whereas CH3NO2 can form long-lived valence-bound CH3NO-2 ions, the data provide no evidence for production of long-lived K+± ± ± CH3NO-2 ion pair states. Rather, the data show that collisions result in unusually strong Rydberg atom scattering. This behavior is attributed to ion-ion scattering resulting from formation of transient ion pair states through transitions between the covalent K(12p) + CH3NO2 and ionic K+ + (dipole bound) CH3NO-2 terms in the quasimolecule formed during collisions. The ion-pair states are destroyed through rapid dissociation of the CH3NO-2 ions induced by the field of the K+ core ion, the detached electron remaining bound to the K+ ion in a Rydberg state. Analysis of the experimental data shows that ion pair lifetimes &10 ps are sufficient to account for the present observations. The present results are consistent with recent theoretical predictions that Rydberg collisions with CH3NO2 will result in strong collisional quenching. The work highlights a new mechanism for Rydberg atom scattering that could be important for collisions with other polar targets. For purposes of comparison, results obtained following K(12p)-SF6 collisions are also included. [ABSTRACT FROM AUTHOR]

Calculations of the electronic structures and properties of M(CO)5 and M(CO)4, where M = Ru, Os, and Hs, have been performed using a variety of relativistic methods such as density functional theory and Dirac-Coulomb correlated ones implemented in program packages such as ADF, DIRAC, and ReSpect. The obtained results show that trends in spectroscopic properties of the M(CO)5 species in group 8 follow the same pattern as that of other compounds of group 4 through group 8 elements. The calculated first M-CO bond dissociation energy (FBDE) of Hs(CO)5 turned out to be significantly weaker than that of Os(CO)5. This was obtained both at the scalar relativistic and spin-orbit levels of theory. The reason for that is the relativistic destabilization and the expansion of the 6d AOs, responsible for weaker σ-forth and π-back donations in the Hs compound. Thus, the FBDEs of M(CO)5 have a Δ-shape behavior in the row Ru-Os-Hs. The non-relativistic FBDEs steadily increase in this row. Using the results of the molecular calculations and a molecule-slab dispersion interaction model, the volatility of the group-8 carbonyls was estimated as adsorption enthalpies, ΔHads, on surfaces of quartz and Teflon used in gas-phase chromatography experiments. It was found that Hs(CO)5 should be almost as volatile as the homologs; however, its interaction strength with these surfaces should be somewhat larger than that of both Ru(CO)5 and Os(CO)5, while the M(CO)4 (M = Ru, Os, and Hs) molecules should be non-volatile. It will, therefore, be difficult to distinguish between group- 8 M(CO)5species by measurements of their volatility as ΔHads on inert surfaces with error bars of ~4 kJ/mol. [ABSTRACT FROM AUTHOR]

In this work, we present a linear scaling formulation of the coupled-cluster singles and doubles with perturbative inclusion of triples (CCSD(T)) and explicitly correlated geminals. The linear scaling implementation of all post-mean-field steps utilizes the SparseMaps formalism [P. Pinski et al., J. Chem. Phys. 143, 034108 (2015)]. Even for conservative truncation levels, the method rapidly reaches near-linear complexity in realistic basis sets, e.g., an effective scaling exponent of 1.49 was obtained for n-alkanes with up to 200 carbon atoms in a def2-TZVP basis set. The robustness of the method is benchmarked against the massively parallel implementation of the conventional explicitly correlated coupled-cluster for a 20-water cluster; the total dissociation energy of the cluster (~186 kcal/mol) is affected by the reduced scaling approximations by only ~0.4 kcal/mol. The reduced scaling explicitly correlated CCSD(T) method is used to examine the binding energies of several systems in the L7 benchmark data set of noncovalent interactions. [ABSTRACT FROM AUTHOR]

Nickel-based alloys are widely applied materials in high-temperature applications because they exhibit superior corrosion resistance and mechanical properties. The effects of sulfur, which is invariably present in industrial atmospheres, on the early stages of oxidation of Ni-based surfaces are not well understood. Here we use density functional theory to investigate the interactions of sulfur, SO, andSO2 with the Ni(111) and Cr-doped Ni(111) surface and elucidate their electronic interactions and potential energy surfaces. The results show that Cr doping of the Ni(111) surface increases the adsorption energies of sulfur, oxygen on the sulfur pre-adsorbed condition, SO and SO2. Further, this increase positively correlates with Cr concentration on top of the Ni(111) surface, although sulfur does not have any preferential interaction with Cr. This explains why Cr doping has little effect on the activation energy of sulfur for the most preferable diffusion path. Nevertheless, the increase in adsorption energies indicates a strong interaction with Cr-doped surfaces, which is due to the Cr-enhanced charge transfer to sulfur adsorbates. The existence of pre-adsorbed sulfur is shown to have a destabilizing effect on the oxygen interactions with the surfaces. Our results show that Cr doping helps to stabilize the protective oxide scale on Ni(111) surfaces and enhances its corrosion resistance. [ABSTRACT FROM AUTHOR]

An improved atomic spin-orbit model is presented, which is designed to be used within the framework of the effective relativistic coupling by asymptotic representation method. This method is used for the generation of highly accurate coupled potential energy surfaces (PESs) to represent the fine structure energies of appropriate systems. The approach is demonstrated using CH3I and its photodissociation as a typical example. The method is based on a specific diabatization of electronic spin-space ("spinfree") states with respect to the asymptote at which a single relativistic atom is separated from a molecular non-relativistic fragment. Thus, the relativistic coupling effects can be treated entirely within the atomic framework. So far, an effective spin-orbit coupling operator which only accounts for intra-state coupling within each atomic spin-space state was used. In the present work, this approach is extended to account for inter-state couplings among different atomic spin-space states as well. It is shown that this extended approach improves the accuracy of the PESs significantly for higher excited states and also enhances the accuracy of low energy states. In particular, it improves the representation of the spin-orbit induced conical intersection among the 3Q0 and 1Q1 states of CH3I, which is of high relevance for the nonadiabatic quantum dynamics of the photodissociation. [ABSTRACT FROM AUTHOR]

The H-atom photodissociation reaction from the pyridinyl radical (C5H5NH) via the low-lying πσ* excited electronic state is investigated by nonadiabatic time-dependent quantum wave-packet dynamics calculations. A model comprising three electronic states and three nuclear coordinates has been constructed using ab initio multi-configurational self-consistent-field and multi-reference perturbation theory methods. Two conical intersections among the three lowest electronic states have been characterized in the framework of the linear vibronic-coupling model. Time-dependent wave-packet simulations have been performed using the multi-configuration time-dependent Hartree method. The population dynamics of the diabatic and adiabatic electronic states and the time-dependent dissociation behavior are analyzed for various vibrational initial conditions. The results provide detailed mechanistic insight into the photoinduced H-atom dissociation process from a hypervalent aromatic radical and show that an efficient dissociation reaction through two conical intersections is possible. [ABSTRACT FROM AUTHOR]

We investigate dissociative electron attachment to 5-fluorouracil (5-FU) employing a crossed electron-molecular beam experiment and quantum chemical calculations. Upon the formation of the 5-FU− anion, 12 different fragmentation products are observed, the most probable dissociation channel being H loss. The parent anion, 5-FU−, is not stable on the experimental timescale (~140 µs), most probably due to the low electron affinity of FU; simple HF loss and F− formation are seen only with a rather weak abundance. The initial dynamics upon electron attachment seems to be governed by hydrogen atom pre-dissociation followed by either its full dissociation or roaming in the vicinity of the molecule, recombining eventually into the HF molecule. When the HF molecule is formed, the released energy might be used for various ring cleavage reactions. Our results show that higher yields of the fluorine anion are most probably prevented through both faster dissociation of an H atom and recombination of F− with a proton to form HF. Resonance calculations indicate that F− is formed upon shape as well as core-excited resonances. [ABSTRACT FROM AUTHOR]

Due to high storage capacity, high dissociation enthalpy, and the appropriate melting point of gas hydrates, these compounds have the potential for many industrial applications. Tetra-n-butylammonium halides are molecules that can form semiclathrate hydrates. This manuscript employed the Clausius Clapeyron equation to evaluate the dissociation enthalpy of methane/nitrogen/carbon dioxide + tetra-n-butylammonium chloride (TBAC) semiclathrate hydrates (SCHs). Phase equilibrium data are measured in a batch reactor with an effective volume of 460 cc. The data of dissociation enthalpy were evaluated in the temperatures of (275.15 to 304.75) K and the pressures of (0.36 to 10.57) MPa at (0 - 0.36) mass fraction of TBAC. The results showed that the utilization of TBAC increases the amount of dissociation enthalpy of semiclathrate hydrates per mole of the hydrated gas. By increasing the amount of TBAC in the system, the quantity of dissociation enthalpy per mole of hydrated gas increased. [ABSTRACT FROM AUTHOR]