Application of one-dimensional semiclassical transition state theory to the CH ₃ OH + H ⇌ CH ₂ OH/CH ₃ O + H ₂ reactions

Abstract

The rate constants of the two branches of H-abstractions from CHOH by the H-atom and the corresponding reactions in the reverse direction are calculated using the one-dimensional semiclassical transition state theory (1D SCTST). In this method, only the reaction mode vibration of the transition state (TS) is treated anharmonically, while the remaining internal degrees of freedom are treated as they would have been in a standard TS theory calculation. A total of eight single-point energy calculations are performed in addition to the computational cost of a standard TS theory calculation. This allows a second-order Richardson extrapolation method to be employed to improve the numerical estimation of the third- and fourth-order derivatives, which in turn are used in the calculation of the anharmonic constant. Hindered-rotor (HR) vibrations are identified in the equilibrium states of CH and CHOH, and the TSs of the reactions. The partition function of the HRs are calculated using both a simple harmonic oscillator model and a more sophisticated one-dimensional torsional eigenvalue summation (1D TES) method. The 1D TES method can be easily adapted in 1D SCTST computation. The resulting 1D SCTST with 1D TES rate constants show good agreement to previous theoretical and experimental works. The effects of the HR on rate constants for different reactions are also investigated.This article is part of the theme issue 'Modern theoretical chemistry'.

Calculations on the unimolecular decomposition of the nerve agent VX

“…62 In the DT correction proposed by Wagner et al 62 eqn (3) is replaced by a piece-wise continuous asymmetric Eckart barrier model. It improves the results at temperatures typically lower than 500 K. 21,36,[39][40][41]54 The additional computational cost of constructing the DT barrier is only the reverse barrier height.…”

“…In practice, the numerical computations of these anharmonic constants can introduce unnecessary error in a FD calculation. In the current study, we employ the one-dimensional (1D) SCTST, 21,36,[39][40][41] which is a much less expensive and yet sufficiently accurate approximation to the FD SCTST. We have used the 1D SCTST in a combined experimental and theoretical study of a pericyclic H-transfer reaction of sarin, 21 similar to the title reactions, and found excellent agreement between the two.…”

Spiers Memorial Lecture : Introductory lecture: quantum dynamics of chemical reactions

“…Further research on the SCTST has examined the excellent accuracy of a onedimensional 1D-SCTST compared to a full dimensional SCTST for the reactions of H with CH 4 and C 2 H 6 117,118 , and for the CH 4 and CH 3 hydrogen transfer reaction, in which also a hindered rotor modification is applied 130 . The simplest form of the 1D-SCTST only requires 4 single point MP2 calculations in addition to those required for conventional TST 131 . All the evidence suggests that the 1D-SCTST is a highly computationally inexpensive yet reliable method for predicting rate constants for bimolecular reactions.…”

Low-Temperature Kinetic Isotope Effects in CH3OH + H → CH2OH + H2 Shed Light on the Deuteration of Methanol in Space

“…Gas-phase kinetic measurements for temperatures of 200 K and above are available, 24,25 as well as simulations. 20,21,[26][27][28] In previous work, 17 the title reaction was investigated to low temperature using instanton theory based on density functional theory. To improve on the limits of that method, we now describe the reaction based on coupled cluster theory.…”