In this paper we present a model Hamiltonian to treat the dynamics of reactions of the general type X+YCZ3→XY+CZ3, where the CZ3 group has C3v symmetry. The main assumption of the model is that CZ3 maintains its symmetry during the reaction. The Hamiltonian we derive can be used as the starting point for more simplified models. We present here one of these simpler models and use it to study the role of the umbrella vibrational motion in the reaction O(3P)+CH4→OH+CH3. Finally, we compare our results with previous theoretical and experimental results.

Extending the fully quantum-state-resolved description of elementary chemical reactions beyond three or four atom systems is a crucial issue in fundamental chemical research. Reactions of methane with F, Cl, H or O are key examples that have been studied prominently. In particular, reactive resonances and nonintuitive mode-selective chemistry have been reported in experimental studies for the F+CH4 →HF+CH3 reaction. By investigating this reaction using transition-state spectroscopy, this joint theoretical and experimental study provides a clear picture of resonances in the F+CH4 system. This picture is deduced from high-resolution slow electron velocity-map imaging (SEVI) spectra and accurate full-dimensional (12D) quantum dynamics simulations in the picosecond regime.

Six-dimensional (6D) quantum scattering calculations of reaction probabilities are reported for the reaction. An arrangement channel hyperspherical coordinate method is used.H potential energy surface due to Ochoa and Clary is employed. The results agree well with those calculated using the rotating bond approximation (RBA) and the quasi-classical trajectory (QCT) method. 6D quantum, RBA and QCT calculations of rate constants for the reaction agree well with experiment. In OH ] H 2 addition, RBA calculations of di †erential cross sections for the reaction and the OH ] D 2 ] HOD ] D photodetachment spectrum for also agree well with experiment. These results suggest that the new H 3 Op otential surface is reliable for this reason.

Motivated by recent photodetachment experiments studying resonance structures in the transition-state region of the F + CH(4) → HF + CH(3) reaction, the vibrational dynamics of the precursor complex CH(4)·F(-) is investigated. Delocalized vibrational eigenstates of CH(4)·F(-) are computed in full dimensionality employing the multiconfigurational time-dependent Hartree (MCTDH) approach and a recently developed iterative diagonalization approach for general multiwell systems. Different types of stereographic coordinates are used, and a corresponding general N-body kinetic energy operator is given. The calculated tunneling splittings of the ground and the lower vibrational excited states of the CH(4)·F(-) complex do not significantly exceed 1 cm(-1). Comparing the converged MCTDH results for localized vibrational excitations with existing results obtained by normal-mode-based (truncated) vibrational configuration interaction calculations, significantly lower frequencies are found for excitations in the intermolecular modes.

Quantum scattering calculations on the SN2 reaction Cl−+CH3Cl→ClCH3+Cl− are reported. The rotating bond approximation (RBA) has been adapted so that three degrees of freedom including the C–Cl stretching vibration and the CH3 umbrella mode are treated explicitly. The calculations have been done with minor modifications of a potential due to Vande Linde and Hase. It is found that initial excitation of the C–Cl vibration has a large effect on the reaction probabilities, while excitation of the CH3 umbrella vibration is less significant. The reaction is dominated by scattering resonances with lifetimes ranging from 0.1 to 10 ps. It is found that the length of the C–Cl bond at the transition state of the reaction has a particularly pronounced effect on the reaction probabilities. The magnitude of the quantum reaction probabilities compares quite well with those calculated using the quasiclassical trajectory method.