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This paper presents two new adiabatic, global potential energy surfaces (PESs) for the two lowest 3A′ and 3A″ electronic states of the O(3P) + H2 system. For each of these states, ab initio electronic energies were calculated for more than 5000 geometries using internally contracted multireference configuration interaction methods. The calculated points were then fitted using the ansatz by Aguado et al. [Comput. Phys. Commun. 108, 259 (1998)] leading to very accurate analytical potentials well adapted to perform reaction dynamics studies. Overall, the topographies of both PESs are in good agreement with the benchmark potentials of Rogers et al. [J. Phys. Chem. A 104, 2308 (2000)], but those presented in this work reproduce better the height and degeneracy of the two states at the saddle point. Moreover, the long range potential in the entrance channel does not require any cutoff. These features make the new PESs particularly suitable for a comparison of the dynamics on each of them. The new set of PESs was then used to perform quantum mechanics and quasiclassical trajectory calculations to determine differential and integral cross sections, which are compared to the experimental measurements by Garton et al. [J. Chem. Phys. 118, 1585 (2003)]. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics leading to the C–N bond cleavage in methylamine (CH3NH2) are investigated upon photoexcitation in the blue edge of the first absorption A-band, in the 198–204 nm range. Velocity map images of the generated methyl (CH3) fragment detected in specific vibrational modes, i.e., ν = 0, ν1 = 1, and ν2 = 1, through resonance enhanced multiphoton ionization, are presented along with the corresponding translational energy distributions and the angular analysis. The experimental results are complemented by high-level ab initio calculations of potential energy curves as a function of the C–N bond distance. While a similar single Boltzmann-type contribution is observed in all the translational energy distributions measured, the speed-dependent anisotropy parameter obtained through the angular analysis reveals the presence of two different mechanisms. Prompt dissociation through the conical intersection between the A ̃ 1 A ′ first excited state and the ground state located in the exit channel is, indeed, revealed as a minor channel. In contrast, slow dissociation on the ground state, presumably from frustrated N–H bond cleavage trajectories, constitutes the major reaction pathway leading to the methyl formation. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of methylamine (CH3NH2) upon excitation in the blue edge of the first absorption A-band, in the 198–203 nm range, are investigated by means of nanosecond pump–probe laser pulses and velocity map imaging combined with H(2S)-atom detection through resonance enhanced multiphoton ionization. The images and corresponding translational energy distributions for the H-atoms produced show three different contributions associated with three reaction pathways. The experimental results are complemented by high-level ab initio calculations. The potential energy curves computed as a function of the N–H and C–H bond distances allow us to draw a picture of the different mechanisms. Major dissociation occurs through N–H bond cleavage and it is triggered by an initial geometrical change, i.e., from a pyramidal configuration of the C–NH2 with respect to the N atom to a planar geometry. The molecule is then driven into a conical intersection (CI) seam where three outcomes can take place: first, threshold dissociation into the second dissociation limit, associated with the formation of CH3NH ( A ̃) , is observed; second, direct dissociation after passage through the CI leading to the formation of ground state products; and third, internal conversion into the ground state well in advance to dissociation. While the two last pathways were previously reported at a variety of wavelengths in the 203–240 nm range, the former had not been observed before to the best of our knowledge. The role of the CI and the presence of an exit barrier in the excited state, which modify the dynamics leading the two last mechanisms, are discussed considering the different excitation energies used. [ABSTRACT FROM AUTHOR]

In order to study the Au- + H2 collision, a new global potential energy surface (PES) describing the ground electronic state of AuH-2 system is developed and compared with the PESs of the neutral [Zanchet et al., J. Chem. Phys. 132, 034301 (2010)] and cationic systems [Anaís et al., J. Chem. Phys. 135, 091102 (2011)].We found that Au- - H2 presents a H-Au-H insertion minimum attributed to the stabilization of the LUMO 3b2 orbital, which can be considered as the preamble of the chemisorption well appearing in larger gold clusters. While the LUMO orbital is stabilized, the HOMO 6a1 is destabilized, creating a barrier at the geometry where the energy orbitals' curves are crossing. In the anion, this HOMO is doubly occupied, while in the neutral system is half-filled and completely empty in the cation, explaining the gradual disappearance of the well and the barrier as the number of electrons decreases. The cation presents a well in the entrance channel partially explained by electrostatic interactions. The three systems' reactions are highly endothermic, by 1.66, 2.79, and 3.23 eV for AuH, AuH+, and AuH- products, respectively. The reaction dynamics is studied using quasi-classical trajectory method for the three systems. The one corresponding to the anionic system is new in this work. Collision energies between 1.00 and 8.00 eV, measured for the cation, are in good agreement with the simulated cross section for the AuH+. It was also found that the total fragmentation, in three atoms, competes becoming dominant at sufficiently high energy. Here, we study the competition between the two different reaction pathways for the anionic, cationic, and neutral species, explaining the differences using a simple model based on the topology of the potential energy surfaces. [ABSTRACT FROM AUTHOR]

Despite being a very strong oxidizing agent, most organic molecules are not oxidized in the presence of O2 at room temperature because O2 is a diradical whereas most organic molecules are closed‐shell. Oxidation then requires a change in the spin state of the system, which is forbidden according to non‐relativistic quantum theory. To overcome this limitation, oxygenases usually rely on metal or redox cofactors to catalyze the incorporation of, at least, one oxygen atom into an organic substrate. However, some oxygenases do not require any cofactor, and the detailed mechanism followed by these enzymes remains elusive. To fill this gap, here the mechanism for the enzymatic cofactor‐independent oxidation of 3,5‐dihydroxyphenylacetyl‐CoA (DPA‐CoA) is studied by combining multireference calculations on a model system with QM/MM calculations. Our results reveal that intersystem crossing takes place without requiring the previous protonation of molecular oxygen. The characterization of the electronic states reveals that electron transfer is concomitant with the triplet–singlet transition. The enzyme plays a passive role in promoting the intersystem crossing, although spontaneous reorganization of the water wire connecting the active site with the bulk presets the substrate for subsequent chemical transformations. The results show that the stabilization of the singlet radical‐pair between dioxygen and enolate is enough to promote spin‐forbidden reaction without the need for neither metal cofactors nor basic residues in the active site. [ABSTRACT FROM AUTHOR]

The correlation between chemical structure and predissociation dynamics has been evaluated for a series of linear and branched alkyl iodides with increasing structural complexity by means of femtosecond time-resolved velocity map imaging experiments following excitation on the second absorption band (B-band) at around 201 nm. The time-resolved images for the iodine fragment are reported and analyzed in order to extract electronic predissociation lifetimes and the temporal evolution of the anisotropy while the experimental results are supported by ab initio calculations of the potential energy curves as a function of the C-I distance. Remarkable similarities are observed for all molecules consistent with a major predissociation of the initially populated bound Rydberg states 6A″ and 7A′ through a crossing with the purely repulsive states 7A″, 8A′ and 8A″ leading to a major R + I*(2P1/2) (R = CH3, C2H5, n-C3H7, n-C4H9, i-C3H7 and t-C4H9) dissociation channel. The reported electronic predissociation lifetimes are found to decrease for an increasing size of the linear radical, reflecting the shifts observed in the position of the crossings in the potential energy curves, and very likely a greater non-adiabatic coupling between the initially populated Rydberg states and the repulsive states leading to dissociation induced by other coordinates associated to key vibrational normal modes. The loss of anisotropy is fully accounted for by the parent molecular rotation during predissociation and the rotational temperature of the parent molecule in the molecular beam is reasonably derived. [ABSTRACT FROM AUTHOR]

A quasi-classical study of the endoergic Au+(1S) + H2(X1Σg+) → AuH+ (2Σ+) + H(2S) reaction, and isotopic variants, is performed to compare with recent experimental results [F. Li, C. S. Hinton, M. Citir, F. Liu, and P. B. Armentrout, J. Chem. Phys. 134, 024310 (2011)]. For this purpose, a new global potential energy surface has been developed based on multi-reference configuration interaction ab initio calculations. The quasi-classical trajectory results show a very good agreement with the experiments, showing the same trends for the different isotopic variants of the hydrogen molecule. It is also found that the total dissociation into three fragments, Au++H+H, is the dominant reaction channel for energies above the H2 dissociation energy. This results from a well in the entrance channel of the potential energy surface, which enhances the probability of H-Au-H insertion. [ABSTRACT FROM AUTHOR]

The first calculations of state-to-state reaction probabilities and product state-resolved integral cross sections at selected collision energies (0.05, 0.1, 0.5, and 1.0 eV) for the title reaction on the ab initio potential energy surface of [Zanchet et al. J. Phys. Chem. A 110, 12017 (2006)] with the OH reagent in selected rovibrational states (v=0–2, j=0–5) have been carried out by means of the real wave packet (RWP) and quasiclassical trajectory (QCT) methods. State-selected total reaction probabilities have been calculated for total angular momentum J=0 in a broad range of collision energies. Integral cross sections and state-specific rate coefficients have been obtained from the corresponding J=0 RWP reaction probabilities for initially selected rovibrational states by means of a capture model. The calculated RWP and QCT state-selected rate coefficients are practically temperature independent. Both RWP and QCT reaction probabilities, integral cross sections, and rate coefficients are almost independent of the initial rotational excitation. The RWP results are found to be in an overall good agreement with the corresponding QCT results. The present results have been compared with earlier wave packet calculations carried out on the same potential energy surface. [ABSTRACT FROM AUTHOR]

Quasiclassical trajectory calculations have been carried out for the C(3P)+OH(X 2Π)→CO(X 1Σ+)+H(2S) reaction using a recent ab initio potential energy surface for the ground electronic state X 2A′ of COH. Differential cross sections (DCSs), and product vibrational, rotational and translational distributions have been determined for a wide range of collision energies (0.001–1 eV). The role of excitations (rotation or vibration) of the OH reactant on these quantities has been investigated. Product vibrational, rotational, and translational distributions are found to be almost independent on the rovibrational state of OH, whereas DCSs show a weak dependence on the initial rotational state of OH. We also analyze the results using a study based on the lifetime of the intermediate complex and on the kinematic constraint associated with the mass combination. [ABSTRACT FROM AUTHOR]

First quasiclassical trajectory calculations have been carried out for the C(3P)+OH(X 2Π)→CO(X 1Σ+)+H(2S) reaction using a recent ab initio potential energy surface for the ground electronic state, X 2A′, of HCO/COH. Total and state-specific integral cross sections have been determined for a wide range of collision energies (0.001–1 eV). Then, thermal and state-specific rate constants have been calculated in the 1–500 K temperature range. The thermal rate constant varies from 1.78×10-10 cm3 s-1 at 1 K down to 5.96×10-11 cm3 s-1 at 500 K with a maximum value of 3.39×10-10 cm3 s-1 obtained at 7 K. Cross sections and rate constants are found to be almost independent of the rovibrational state of OH. [ABSTRACT FROM AUTHOR]

The rate constants for the formation, destruction, and collisional excitation of SH+ are calculated from quantum mechanical approaches using two new SH+2 SH 2 + $ {\rm SH}_2^+ $ potential energy surfaces (PESs) of 4A″ and 2A″ electronic symmetry. The PESs were developed to describe all adiabatic states correlating to the SH+(3Σ−) + H(2S) channel. The formation of SH+ through the S+ + H2 reaction is endothermic by ≈9860 K, and requires at least two vibrational quanta on the H2 molecule to yield significant reactivity. Quasi-classical calculations of the total formation rate constant for H2(v = 2) are in very good agreement with the quantum results above 100 K. Further quasi-classical calculations are then performed for v = 3, 4, and 5 to cover all vibrationally excited H2 levels significantly populated in dense photodissociation regions (PDR). The new calculated formation and destruction rate constants are two to six times larger than the previous ones and have been introduced in the Meudon PDR code to simulate the physical and illuminating conditions in the Orion bar prototypical PDR. New astrochemical models based on the new molecular data produce four times larger SH+ column densities, in agreement with those inferred from recent ALMA observations of the Orion bar. [ABSTRACT FROM AUTHOR]

Nonequilibrium Green's functions calculations based on density functional theory show a direct link between the initial stages of H2 dissociation on a gold atomic wire and the electronic current supported by the gold wire. The simulations reveal that for biases below the stability threshold of the wire, the minimum-energy path for H2 dissociation is not affected. However, the electronic current presents a dramatic drop when the molecule initiates its dissociation. This current drop is traced back to quantum interference between electron paths when the molecule starts interacting with the gold wire. [ABSTRACT FROM AUTHOR]

Ammonia ice has been detected on different astrophysical media ranging from interstellar medium (ISM) particles to the surface of various icy bodies of our solar system, where nitrogen is also present. We have carried out a detailed study of amorphous NH3 ice and NH3:N2 ice mixtures, based on infrared (IR) spectra in the mid-IR (MIR) and near-IR (NIR) regions, supported by theoretical quantum chemical calculations. Spectra of varying ice thicknesses were obtained and optical constants were calculated for amorphous NH3 at 15 K and 30 K and for a NH3:N2 mixture at 15 K over a 500-7000 cm–1 spectral range. These spectra have improved accuracy over previous data, where available. Moreover, we also obtained absolute values for the band strengths of the more prominent IR features in both spectral regions. Our results indicate that the estimated NH3 concentration in ISM ices should be scaled upward by ∼30%. [ABSTRACT FROM AUTHOR]

State-to-state rate constants for the title reaction are calculated using the electronic ground state potential energy surface and an accurate quantum wave-packet method. The calculations are performed for H2 in different rovibrational states, v = 0, 1 and J = 0 and 1. The simulated reaction cross section for v = 0 shows a rather good agreement with the experimental results of Gerlich et al., both with a threshold of 0.36 eV and within the experimental error of 20%. The total reaction rate coefficients simulated for v = 1 are two times smaller than those estimated by Hierl et al. from cross sections measured at different temperatures and neglecting the contribution from v > 1 with an uncertainty factor of two. Thus, part of the disagreement is attributed to the contributions of v > 1. The computed state-to-state rate coefficients are used in our radiative transfer model code applied to the conditions of the Orion Bar photodissociation region, and leads to an increase of the line fluxes of high-J lines of CH+. This result partially explains the discrepancies previously found with measurements and demonstrates that CH+ excitation is mostly driven by chemical pumping. [ABSTRACT FROM AUTHOR]

An analytical potential energy surface for a rigid Rb2 in the 3σu+ state interacting with one helium atom based on accurate ab initio computations is proposed. This 2-dimensional potential is used, together with the pair approximation approach, to investigate Rb2 attached to small helium clusters HeN with N = 1-6, 12, and 20 by means of quantum Monte Carlo studies. The limit of large clusters is approximated by a flat helium surface. The relative orientation of the dialkali axis and the helium surface is found to be parallel. Dynamical investigations of the pendular and of the in-plane rotation of the rigid Rb2 molecule on the surface are presented. [ABSTRACT FROM AUTHOR]

[Display omitted] • Unprecedented exhaustive QCT calculations have been carried out in the range 5–800 K. • Rate coefficients fall rapidly from 5 K to 150 K and then level-off. • The abrupt decrease is mainly due to the electronic degeneration factor. • Cross sections and rate coeffcients are very sensitive to the rotational excitation of NH. • Two capture models based on the long-range interaction have been developed. Rate coefficients of the complex-forming C + NH → H + CN reaction have been calculated at temperatures ranging from 5 K to 800 K using quasi-classical trajectories on a potential energy surface which accurately describes the attractive long-range interaction, along with results using two capture models. In contrast with the constant value recommended in astrochemical databases, a steep decrease of the rates has been found up to 150 K, and then they tends to be nearly constant. Such behavior is analyzed in terms of the rovibrational state-selected rate coefficients and cross sections singling out the role played by the rotational excitation of the initial diatom. The effect of the electronic degeneracy is also discussed. [ABSTRACT FROM AUTHOR]

We present multipolar potentials at large intermolecular distances for the 18 doubly degenerate spin-orbit states arising from the interaction between the two open-shell systems, C(3P) and OH(X 2Π). With OH fixed at its ground vibrational state-averaged distance r0, the long-range potentials are two-dimensional potential energy surfaces (PESs) that depend on the intermolecular distance R and the angle γ=CG⁁H, where G represents the mass center of OH. The 18×18 diabatic potential matrix elements are built up from the perturbation theory up to second order and from a two-center expansion of the Coulombic interaction potential, resulting in a multipolar expansion of the potential expressed as a series of terms varying in R-n. The expressions for the long-range coefficients of the expansion are explicitly given in terms of monomer properties such as permanent multipole moments, and static and dynamic polarizabilities. Accurate values for the monomer properties are used to properly determine the long-range interaction coefficients. The diagonalization of the full 18×18 potential matrix generates adiabatic long-range PESs in good agreement with their ab initio counterparts. [ABSTRACT FROM AUTHOR]

Sub‐nanometer metal clusters have special physical and chemical properties, significantly different from those of nanoparticles. However, there is a major concern about their thermal stability and susceptibility to oxidation. In situ X‐ray Absorption spectroscopy and Near Ambient Pressure X‐ray Photoelectron spectroscopy results reveal that supported Cu5 clusters are resistant to irreversible oxidation at least up to 773 K, even in the presence of 0.15 mbar of oxygen. These experimental findings can be formally described by a theoretical model which combines dispersion‐corrected DFT and first principles thermochemistry revealing that most of the adsorbed O2 molecules are transformed into superoxo and peroxo species by an interplay of collective charge transfer within the network of Cu atoms and large amplitude "breathing" motions. A chemical phase diagram for Cu oxidation states of the Cu5‐oxygen system is presented, clearly different from the already known bulk and nano‐structured chemistry of Cu. [ABSTRACT FROM AUTHOR]

The C–C bond dissociation channel of the ethyl (CH 3 –CH 2 ) radical is investigated at 197.4, 199.98 and 201 nm by velocity map imaging and resonance enhanced multiphoton ionisation (REMPI) detection of the produced CH 3 fragments. On the light of high-level abinitio calculations performed in this work, up to four dissociation pathways are identified leading, respectively, to ground state CH 3 radicals in correlation with CH 2 fragments in the first four electronic states. The major pathway is associated with threshold dissociation over an exit barrier located at the point of a conical intersection between the initially populated 3 p A ′ state and a dissociative valence state. As discussed in the text, the concomitance of both features opens the narrow window in which C–C bond dissociation occurs for ethyl and, presumably, for other similar alkyl radicals. [ABSTRACT FROM AUTHOR]

The impact of the number and positions of Rb ions in the RbCo[Fe(CN)6] Prussian blue analogue is analyzed by means of complete active space self-consistent field (CASSCF) and subsequent second-order perturbation theory (CASPT2) calculations performed upon embedded cluster models. It is shown that the geometries and corresponding electronic structures of the monomeric [CoIII(NC)5(OH2)]2- and [CoII(NC)5(OH2)]3- units are differently affected when the apical cyanide ligand is substituted by a water molecule. The CoIII ion moves away from the equatorial plane by ca. 0.15 Å, whereas the CoII is almost not sensitive to the environmental change. Furthermore, we find that this phenomenon is rather independent of the positions and numbers of the nearest-neighbour alkali ions. The Co ion displacement directly controls the overlap between the bridging cyanide and metal ion orbitals, a scenario which might be favorable to trigger electron transfer in the photomagnetic dimeric CoFe units of the CoFe Prussian blue analogues. [ABSTRACT FROM AUTHOR]

In-situ XPS and XANES spectroscopy experiments, along with theoretical characterizations of Cu5-(O SB 2 sb ) SB I n i sb complexes, provide direct evidence of the Cu SB 5 sb clusters' O SB 2 sb stability. Keywords: density functional calculations; nanotechnology; oxidation; photoelectron spectroscopy; X-ray absorption spectroscopy EN density functional calculations nanotechnology oxidation photoelectron spectroscopy X-ray absorption spectroscopy 1 1 1 09/05/23 20230901 NES 230901 B Bare clusters of five Cu atoms b , synthesized electrochemically, display remarkable oxidation stability on highly oriented pyrolytic graphite. Density functional calculations, nanotechnology, oxidation, photoelectron spectroscopy, X-ray absorption spectroscopy. [Extracted from the article]

Graphical abstract Highlights • Aimed at filling the gap in the theoretical studies of the CH 3 's photo-dissociation. • Our results look promising reproducing the same trend as state of the art experiments. • The method used allows the interpretation of the internal mechanisms of the process. Abstract The photodissociation dynamics of methyl radical from selected vibrational levels of the 3 s and 3 p z Rydberg states has been studied using the trajectory surface hopping method on one-dimensional potential energy curves along the carbon-hydrogen dissociative coordinate and the non-adiabatic couplings between them, calculated by ab initio methods. Lifetimes and product distributions have been obtained for all the studied vibrational levels and compared with previous theoretical and experimental works. The theoretical lifetimes tend to decrease as vibrational excitation increases in accordance with experimental results. The agreement, however, is only qualitative considering the reduced dimensionality of the model. [ABSTRACT FROM AUTHOR]

In this work we present a dynamically biased statistical model to describe the evolution of the title reaction from statistical to a more direct mechanism, using quasi-classical trajectories (QCT). The method is based on the one previously proposed by Park and Light [J. Chem. Phys. 126, 044305 (2007)]. A recent global potential energy surface is used here to calculate the capture probabilities, instead of the long-range ion-induced dipole interactions. The dynamical constraints are introduced by considering a scrambling matrix which depends on energy and determine the probability of the identity/hop/exchange mechanisms. These probabilities are calculated using QCT. It is found that the high zero-point energy of the fragments is transferred to the rest of the degrees of freedom, what shortens the lifetime of H5+ complexes and, as a consequence, the exchange mechanism is produced with lower proportion. The zero-point energy (ZPE) is not properly described in quasi-classical trajectory calculations and an approximation is done in which the initial ZPE of the reactants is reduced in QCT calculations to obtain a new ZPE-biased scrambling matrix. This reduction of the ZPE is explained by the need of correcting the pure classical level number of the H5+ complex, as done in classical simulations of unimolecular processes and to get equivalent quantum and classical rate constants using Rice-Ramsperger-Kassel-Marcus theory. This matrix allows to obtain a ratio of hop/exchange mechanisms, α(T), in rather good agreement with recent experimental results by Crabtree et al. [J. Chem. Phys. 134, 194311 (2011)] at room temperature. At lower temperatures, however, the present simulations predict too high ratios because the biased scrambling matrix is not statistical enough. This demonstrates the importance of applying quantum methods to simulate this reaction at the low temperatures of astrophysical interest. [ABSTRACT FROM AUTHOR]