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We report quantum calculations involving the dynamics of rotational energy-transfer processes, by collision with He atoms in interstellar environments, of the title molecular species which share the presence of the CN backbone and are considered of importance in those environments. The latter structural feature is taken to be especially relevant for prebiotic chemistry and for its possible role in the processing of the heterocyclic rings of RNA and DNA nucleobases in the interstellar space. We carry out ab initio calculations of their interaction potentials with He atoms and further obtain the state-to-state rotationally inelastic cross sections and rate coefficients over the relevant range of temperatures. The similarities and differences between such species and other similar partners which have been already detected are analyzed and discussed for their significance on internal state populations in interstellar space for the two title molecular radicals. [ABSTRACT FROM AUTHOR]

A new set of six accurate ab initio potential energy surfaces (PESs) is presented for the first three singlet and triplet states of LiH2+ (1,21A′, 11A′′, 1,23A′, and 13A′′ states, where four of them are investigated for the first time), which have allowed new detailed studies gaining a global view on this interesting system. These states are relevant for the study of the most important reactions of lithium chemistry in the early universe. More than 45 000 energy points were calculated using the multi-reference configuration interaction level of theory using explicitly correlated methods (ic-MRCI-F12), and the results obtained for each individual electronic state were fitted to an analytical function. Using quasiclassical trajectories and considering the initial diatomic fragment in the ground rovibrational state, we have determined the integral cross sections for the H + LiH+(X2Σ+, C2Π) and H+ + LiH(X1Σ+, B1Π) reactions. In these calculations all available reaction channels were considered: the chemically most important H or H+ transfer/abstraction as well as atom exchange and collision induced dissociation for up to 1.0 eV of collision energy. [ABSTRACT FROM AUTHOR]

The non-adiabatic quantum dynamics of the H + H 2 + → H 2 + H+ charge transfer reactions, and some isotopic variants, is studied with an accurate wave packet method. A recently developed 3 × 3 diabatic potential model is used, which is based on very accurate ab initio calculations and includes the long-range interactions for ground and excited states. It is found that for initial H 2 + (v = 0), the quasi-degenerate H2(v′ = 4) non-reactive charge transfer product is enhanced, producing an increase in the reaction probability and cross section. It becomes the dominant channel from collision energies above 0.2 eV, producing a ratio between v′ = 4 and the rest of v's, which that increase up to 1 eV. The H + H 2 + → H 2 + + H exchange reaction channel is nearly negligible, while the reactive and non-reactive charge transfer reaction channels are of the same order, except that corresponding to H2(v′ = 4), and the two charge transfer processes compete below 0.2 eV. This enhancement is expected to play an important vibrational and isotopic effect that needs to be evaluated. For the three proton case, the problem of the permutation symmetry is discussed when using reactant Jacobi coordinates. [ABSTRACT FROM AUTHOR]

Analytical derivatives and non-adiabatic coupling matrix elements are derived for Hn+ systems (n = 3-5). The method uses a generalized Hellmann-Feynman theorem applied to a multi-state description based on diatomics-in-molecules (for H3+) or triatomics-in-molecules (for H4+ and H5+) formalisms, corrected with a permutationally invariant many-body term to get high accuracy. The analytical non-adiabatic coupling matrix elements are compared with ab initio calculations performed at multi-reference configuration interaction level. These magnitudes are used to calculate H2(v' = 0, j' = 0) + 2+ (v,j = 0) collisions, to determine the effect of electronic transitions using a molecular dynamics method with electronic transitions. Cross sections for several initial vibrational states of 2+ are calculated and compared with the available experimental data, yielding an excellent agreement. The effect of vibrational excitation of 2+ reactant and its relation with non-adiabatic processes are discussed. Also, the behavior at low collisional energies, in the 1 meV-0.1 eV interval, of interest in astrophysical environments, is discussed in terms of the long range behaviour of the interaction potential which is properly described within the triatomics-in-molecules formalism. [ABSTRACT FROM AUTHOR]

Full dimensional analytical fits of the coupled potential energy surfaces for the three lower singlet and triplet adiabatic states of H +3 are developed, providing analytic derivatives and non-adiabatic coupling matrix elements. The fits are highly accurate and include an improved description of the long range interactions, including new terms for the description of the long range in the diatomic fits and the atom–diatom dissociation channels. The fits are based on the DIM formalism including three body terms in Hamiltonian matrix elements, each of them obeying S2 permutational symmetry, where the positive charge is placed in either of the three hydrogen atoms, but the full system obeys S3 permutational symmetry, invariant under all permutations of the nuclei. The ab initio points used in the fitting are obtained from a complete basis set extrapolation, made for all electronic states. Total root mean square errors of the fits are 27 and 12 cm−1, for the singlet and triplet states, respectively. The errors in the channels are lower than 2 cm−1 and 6 cm−1 for the H + H +2 and H+ + H2 channels respectively. The new fits have been used to calculate the rovibrational bound states of the lowest singlet and lowest triplet states showing very good agreement with previous calculations in the literature. [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]

In this work, we present a global potential energy surface for the ground electronic state of the H4+ based on ab initio calculations. The final fit is based on triatomics-in-molecules (TRIM) approximation and it includes extra four-body terms for the better description of some discrepancies found on the TRIM model. The TRIM method itself allows a very accurate description of the asymptotic regions. The global fit uses more than 19 000 multireference configuration interaction ab initio points. The global potential energy surface has an overall root mean square error of 0.013 eV for energies up to 2 eV above the global minimum. This work presents an analysis of the stationary points, reactant and product channels, and crossing between the two lowest TRIM adiabatic states. It is as well included a brief description of the two first excited states of the TRIM matrix, concluding that TRIM method is a very good approximation not only for the ground state but also for at least two of the excited states of H4+ system. [ABSTRACT FROM AUTHOR]

Recently, the predissociation lifetimes of the NeBr2(B) complex for different initial vibrational excitation (10≤v′≤20) have been measured using time-resolved optical pump-probe spectroscopy [Taylor et al., J. Chem. Phys., 132, 104309 (2010)]. In the vibrational interval studied, the vibrational predissociation (VP) proceeds by the transfer of a single vibrational quantum and the lifetimes are expected to decrease smoothly with increasing v′, as predicted by the energy gap law. However, the experimental lifetimes show strong oscillations with v′, which were attributed to the occurrence of electronic predissociation into two possible dissociative electronic states of Br2(1g,2g), based on a Franck–Condon spectator model. In this work we reproduce the experimental findings by performing full three-dimensional wave packet calculations for the competition of vibrational and electronic predissociation, including the B(0u+), 2g, and C(1u) electronic states. Model potential energy surfaces were used based on previous theoretical simulations of the VP dynamics on the B state and on ab initio calculations on the NeCl2 related system. Thus, only two parameters, the strength of the electronic couplings, are fit to achieve the excellent theoretical/experimental agreement. [ABSTRACT FROM AUTHOR]

Abstract: We present the first comprehensive ab initio study of the Ne–Br2 potential energy surfaces and the non-adiabatic couplings between the valence excited electronic states. These ab initio results are used to obtain 3-D approximate potentials for each electronic state, and these potentials are used in a wave packet calculation of the competing electronic predissociation and vibrational predissociation dynamics. The results of this calculation are in excellent agreement with both experimental results and a previous empirical fit to the experiments. The calculations allow us to observe not only the competition between vibrational and electronic dynamics for the dimer, but also the competition between two different electronic channels. Coupling to the 2 g state dominates for the levels studied here, but coupling to the C state is progressively more important for low vibrational levels, and may dominate at levels below which the current results pertain. The ability of ab initio surfaces and couplings to so accurately reproduce experimental data raises the hope of a complete understanding of the VP and EP dynamics for other Rg-halogen dimers. Success in the case presented here is largely due to the fact that the VP dynamics for the vibrational levels in this study are in the simple, direct regime. Understanding the simple case so thoroughly provides new hope that the more complicated examples, such as ArI2 and NeCl2, for which experiment and theory are not currently in accord, may yet yield to analysis. [Copyright &y& Elsevier]

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]