Full-dimensional quantum dynamics study of exchange processes for the D + H2O and D + HOD reactions

Two-photon chemistry of tetrahydrofuran in clathrate hydrates

High-lying electronic states hold the potential for new and unusual photochemical reactions. However, for conventional single-photon excitation in the condensed phase, reaching these states is often not possible because the vacuum-ultraviolet (VUV) light required is competitively absorbed by the surrounding matrix rather than the molecule of interest. Here, this hurdle is overcome by leveraging nonresonant two-photon absorption (2PA) at 265 nm to achieve preferential photolysis of tetrahydrofuran (THF) trapped within a clathrate hydrate network at 77 K. Electron spin resonance (ESR) spectroscopy enables direct observation and identification of otherwise short-lived organic radicals stabilized by the clathrate cages, providing clues into the rapid dynamics that immediately follow photoexcitation. 2PA induces extensive fragmentation of enclathrated THF yielding 1-alkyl, acyl, allyl and methyl radicals-a stark departure from the reactive motifs commonly reported in γ-irradiated hydrates. We speculate on the undetected transient dynamics and explore the potential role of trapped electrons generated from water and THF. This demonstration of nonresonant two-photon chemistry presents an alternative approach to targeted condensed phase photochemistry in the VUV energy range.

Toward a Comprehensive Understanding of Mode-Specific Dynamics of Polyatomic Reactions: A Full-Dimensional Quantum Dynamics Study of the H + NH3 Reaction

Mode specificity not only sheds light on reaction dynamics but also opens the door for chemical reaction control. This work reports a state-of-the-art full-dimensional quantum dynamics study on the prototypical hydrogen abstraction reaction of hydrogen with ammonia, which serves as a benchmark for advancing our fundamental understanding of polyatomic reaction dynamics. By taking advantage of the (3 + 1) Radau-Jacobi coordinates, the bond-specific probabilities are resolved with the reactant NH₃ initiated from either a non-degenerate or degenerate stretching vibrational state. The observed different atom-specific abstraction probabilities from individual states of the degenerate pair are rationalized in the local mode representation according to the different vibrational energy deposited in each N-H bond. It is verified that the three H atoms in NH₃ have the same abstraction probability as that from the degenerate pair and the linear combination of the degenerate pair gives the same reaction probability. In addition, the symmetric and asymmetric stretching modes of the reactant NH₃ have comparable efficacies on driving the reaction.

Femtochemistry of bimolecular reactions from weakly bound complexes: computational study of the H + H'OD → H'OH + D or HOD + H' exchange reactions

A full-dimensional wavepacket propagation describing the bimolecular exchange reactions H + H'OD → H'OH + D or HOD + H' initiated by photolysis of HCl in the hydrogen-bound complex (HCl)⋯(HOD) is reported. The dynamics of this reaction is carried out with the MCTDH method on an ab initio potential energy surface (PES) of H₃O and the initial state is derived from the ground state wavefunction of the complex obtained by relaxation on its own electronic ground state ab initio PES. The description of the system makes use of polyspherical coordinates parametrizing a set of Radau and Jacobi vectors. The calculated energy- and time-resolved reaction probabilities show, owing to the large collision energies at play stemming from the (almost full) photolysis of HCl, that the repulsion between oxygen in the H'OD molecule and the incoming hydrogen atom is the main feature of the collision and leads to non-reactive scattering. No abstraction reaction products are observed. However, both exchange processes are still observable, with a preference in O-H' bond dissociation over that of O-D. The selectivity is reversed upon vibrational pre-excitation of the O-D stretching mode in the H'OD molecule. It is shown that, after the collision, the hydrogen atom of HCl does most likely not encounter the almost stationary chlorine atom again but we also consider the limit case where the H atom is forced to collide multiple times against H'OD as a result of being pushed back by the Cl atom.

Mode- and Bond-Selected Reaction of H with Local Mode Molecule HDS

Understanding mode- and bond-selected dynamics of elementary chemical reactions is of central importance in molecular reaction dynamics. The initial state-selected time-dependent wave packet method is employed to study the mode and bond selectivity, isotopic branching ratio, and temperature dependence of rate constants of the two-channel reaction of H with local mode molecule HDS. For the abstraction channel, fundamental excitation of the HS (DS) bond of the reactant HDS significantly enhances the H-abstraction (D-abstraction) reaction, whose efficacy is higher than the same amount of translational energy except at low energies just above the energy threshold. This is in sharp contrast to the prediction of Polanyi rules: translational energy is more efficient than vibrational energy in enhancing a reaction with an early barrier. The recent sudden vector projection model is then applied to rationalize the observed mode specificity, which, however, shows that the translational mode vector has a larger coupling with the reaction coordinate than the stretching vector of the active bond, implying a reversed relative efficacy on promoting the reaction as well. In contrast, the mode and bond specificity for the exchange channel is not as strong as for the abstraction channel due to the regulation of the shallow well along the reaction path.

Recent advances in quantum scattering calculations on polyatomic bimolecular reactions

This review surveys quantum scattering calculations on chemical reactions of polyatomic molecules in the gas phase published in the last ten years.

Recent Advances in Quantum Dynamics of Bimolecular Reactions

In this review, we survey the latest advances in theoretical understanding of bimolecular reaction dynamics in the past decade. The remarkable recent progress in this field has been driven by more accurate and efficient ab initio electronic structure theory, effective potential-energy surface fitting techniques, and novel quantum scattering algorithms. Quantum mechanical characterization of bimolecular reactions continues to uncover interesting dynamical phenomena in atom-diatom reactions and beyond, reaching an unprecedented level of sophistication. In tandem with experimental explorations, these theoretical developments have greatly advanced our understanding of key issues in reaction dynamics, such as microscopic reaction mechanisms, mode specificity, product energy disposal, influence of reactive resonances, and nonadiabatic effects.

SEVEN-DIMENSIONAL QUANTUM DYNAMICS STUDY OF THE H2 + NH2 → H + NH3 REACTION ON AN INTERPOLATED POTENTIAL ENERGY SURFACE

Initial-state-selected time-dependent wave packet dynamics studies have been performed for the H 2 + NH 2 → H + NH 3 reaction with a seven-dimensional model on a new interpolated ab initio potential energy surface (PES). The PES is constructed using modified Shepard interpolation Scheme and contains 1967 data points with ab initio calculations carried out on UCCSD(T)/aug-cc-pVTZ level. In the seven-dimensional model, NH 2 group keeps C2v symmetry and two NH bonds are fixed at their equilibrium values. The total reaction probabilities are calculated when (1) the two reactants are initially at their ground states; (2) NH 2 bending mode is excited, and (3) H 2 is on its first vibrational excited state. The integral cross sections are also reported for these initial states with centrifugal-sudden approximation. Thermal rate constants are calculated for the temperature range of 200–2000 K and compared with the previous calculated values and available experimental data. Good agreements between theory and experiments for the rate constants at intermediate temperature are achieved on this PES.