Nonseparable transition state theory for nonzero total angular momentum: Implications for J shifting and application to the OH+H2 reaction

Dynamics and resonances of the H(2S) + CH+(X1Σ+) reaction in the electronic ground state: a detailed quantum wavepacket study

Initial state-selected and energy resolved channel-specific reaction probabilities, integral cross sections and thermal rate constants of the H(²S) + CH⁺(X¹Σ⁺) reaction are calculated within the coupled states approximation by a time-dependent wave packet propagation method. The new ab initio global potential energy surface (PES) of the electronic ground state (1 ²A') of the system, recently reported by Li et al. [J. Chem. Phys., 2015, 142, 124302], is employed for this purpose. All partial wave contributions up to the total angular momentum J = 60 are considered to obtain the converged integral reaction cross section up to a collision energy of 1.0 eV. Thermal rate constants are calculated by averaging the reaction cross sections over the Boltzmann distribution of energies and compared with the available theoretical and experimental results for the temperature range 10-1000 K. Investigation of the channel-specific reaction attributes shows that the H abstraction (CH⁺ destruction) channel is highly favored over the H exchange channel. The effect of rotational and vibrational excitations of the CH⁺ reagent on the dynamics is also studied. The resonances formed during the course of the reaction are also identified by calculating the transition state spectrum and characterized in terms of the eigenfunctions and lifetimes. More than 260 vibrational levels are obtained and their eigenfunctions are calculated, which are represented in terms of the nodal assignments and the eigenenergies. They reveal both the local and hyperspherical behavior for the bound and quasibound states of the CH₂⁺ complex in the ground 1 ²A' surface. The lifetime analysis of the quasibound states indicates that the CH₂⁺ resonances survive for as long as ∼400 fs at high energies (E ∼ 2.0 eV) and are expected to decay faster with further increasing energy. Finally, the type of mechanism for the formation of the product (C⁺ + H₂) is elucidated.

REACTION RATES: ACCURATE QUANTUM DYNAMICAL CALCULATIONS FOR POLYATOMIC SYSTEMS

Rate constants of chemical reactions can be computed directly: employing flux correlation functions, no scattering calculations are required. If the reaction is direct, the dynamical simulation can be restricted to the vicinity of the reaction barrier. Therefore accurate thermal rate constants and cumulative reaction probabilities can be calculated for rather large systems. Recent calculations have studied systems with up to six atoms, e.g., H + CH 4 → H 2 + CH 3. The calculations provide a full-dimensional quantum description of the reaction process.

In the article, an introduction to the theory of flux correlation functions is given. Methods for the efficient computation of accurate thermal rate constants and cumulative reaction probabilities are reviewed. Connections to transition state ideas are highlighted. The multi-configurational time-dependent Hartree (MCTDH) approach, which facilitates efficient multi-dimensional wave packet propagation, is described and its use in reaction rate calculation is discussed. As examples, recent results for the prototypical polyatomic reaction H + CH 4 → H 2 + CH 3 are presented and rotational effects on the reaction rates of H 2 + OH → H + H 2 O , O + HCl → OH + Cl , and H 2 + Cl → H + HCl are discussed.

Rotational excitation cross sections of para-H2+para-H2 collisions. A full-dimensional wave-packet propagation study using an exact form of the kinetic energy

A full-dimensional quantum dynamical study of the rotational excitation in para-para H2 + H2 collisions using the potential-energy surface of Boothroyd et al. [J. Chem. Phys. 116, 666 (2002)] is reported. The multiconfiguration time-dependent Hartree algorithm is utilized to propagate wave packets and the cross sections for collision energies up to 1.2 eV are determined by a flux analysis through the interaction of the wave packet with a complex absorbing potential. Calculations for a collection of total angular momenta up to J = 70 are performed; the missing channels are obtained with a J-interpolation algorithm.