Quantum dynamics and kinetics of the abstraction reactions by H atoms of primary and secondary hydrogens in C3H8

Active vs. spectator modes in nonadiabatic photodissociation dynamics of the hydroxymethyl radical via the 22A(3s) Rydberg state

The choice of the active degrees of freedom (DOFs) is a pivotal issue in a reduced-dimensional model of quantum dynamics when a full-dimensional one is not feasible. Here, several five-dimensional (5D) models are used to investigate the nonadiabatic photodissociation dynamics of the hydroxymethyl (CH₂OH) radical, which possesses nine internal DOFs, in its lowest absorption band. A normal-mode based scheme is used to identify the active and spectator modes, and its predictions are confirmed by 5D quantum dynamical calculations. Our results underscore the important role of the CO stretching mode in the photodissociation dynamics of CH₂OH, originating from the photo-induced promotion of an electron from the half-occupied π*_CO antibonding orbital to a carbon Rydberg orbital.

Quantum effects in the abstraction reaction by H atoms of primary and secondary hydrogens in n-C4H10: a test of a new potential energy surface construction method

Recently, von Horsten et al. suggested an efficient method to construct two dimensional potential energy surfaces (PESs) for use in quantum scattering simulations, in which they utilised the minimum energy path (MEP) and assumed harmonic behaviour of the PES near the MEP. In the same paper, the authors applied this method to various H-abstraction reactions from C1-C3 alkane molecules. In this work we demonstrate an alternative PES construction method, and apply it to the more challenging H-abstraction from n-C(4)H(10) reactions. The geometry optimizations and frequency calculations are done at the MP2/cc-pVTZ level of theory, while the energies are calculated with the CCSD(T) method with the same basis set. The calculations give adiabatic energy barrier heights of 45.9 kJ mol(-1) and 34.4 kJ mol(-1) for the primary and secondary hydrogens in n-C(4)H(10). When compared to purely classical transition state theory, quantum scattering calculations show that quantum tunnelling and zero-point effects have large contributions at low temperatures, typically below 500 K. The branching ratio study suggests that the abstraction of secondary hydrogen in n-C(4)H(10) dominates the overall reaction rate at low temperatures. The rate constants for the two abstraction channels become more comparable as the temperature increases.

Theoretical studies on bimolecular reaction dynamics

This perspective discusses progress in the theory of bimolecular reaction dynamics in the gas phase. The examples selected show that definitive quantum dynamical computations are providing insights into the detailed mechanisms of chemical reactions.

Stereodynamics of Chlorine Atom Reactions with Organic Molecules

A series of recent experimental and computational studies has explored how the dynamics of hydrogen abstraction from organic molecules are affected by the presence of functional groups in the molecule and by basic structural motifs such as strained ring systems. Comparisons drawn between reactions of Cl atoms with alkanes such as ethane, Cl + CH3CH3--> HCl + CH3CH2, which serve as benchmark systems, and with functionalized molecules such as alcohols, amines, and alkyl halides, Cl + CH3X --> HCl + CH2X (X = OH, NH2, halogen, etc.) expose a wealth of mechanistic detail. Although the scattering dynamics, as revealed from measured angular distributions of the velocities of the HCl with quantum-state resolution, show many similarities, much-enhanced rotational excitation of the HCl products is observed from reactions of the functionalized molecules. The degree of rotational excitation of the HCl correlates with the dipole moment of the CH2X radical and is thus attributed, at least in part, to post-transition-state dipole-dipole interactions between the separating, polar reaction products. This interpretation is supported by direct dynamics trajectories computed on-the-fly, and the HCl rotation is thus argued to serve as an in situ probe of the angular anisotropy of the reaction potential energy surface in the post-transition-state region. Comparisons between the dynamics of reactions of dimethyl ether and the three- and four-membered-ring compounds oxirane (c-C2H4O) and oxetane (c-C3H6O) raise questions about the role of reorientation of the reaction products on a time scale commensurate with their separation. The shapes and structures of polyatomic molecules are thus demonstrated to have important consequences for the stereodynamics of these direct abstraction reactions.