Formaldehyde And Ch <sub> 3 </ Sub>, H, O [~ 3p], The Oh Radical Reaction Pathways And The Variational Rate Constant Calculation

The mechanisms and rate constants of four hydrogen abstraction reactions of with CHa, H, O[3P] and OH are investigated by using ab initio molecular orbital theory and the variational transition state theory. At QCISD/6-311g[d, p] level, the geometries of the reactants, transition states and products of reactions mentioned about have been optimized. Energies of Stationary points on the minimum energy paths (MEP) for each reaction are improved by UQCISD(t, full) single-point energy correction. Further considering zero-point energy correction, the activation barriers of the reaction CH2O with CH3, H, O[3P] and OH, are 40.55, 25.22, 24.59, 2.19 kJ/mol, respectively. Such calculated barriers are close to the corresponding experimental data 33.06, 29.69, 14.8, 1.8 kJ/mol.For each reaction, by investigating the changes of the interatomic distances, it is found that the rupture of old bonds and formation of new bond for hydrogen abstraction reaction are concerted. Some one defined the "effective reaction region " as a region of the new bond forming and the old bond breaking. This region is delimited on the MEP where the geometrical structure of reaction molecules would change mightily. So it can be realized that the effective reaction regions for the reactions mentioned about are -1.0~0.74,-0.4~0.55, -0.4-0.75 and -1.0-1.0(amu) l/2bohr, respectively.By discussing the changes of vibrational mode for all the four reactions, it can be found that there exists s special vibrational mode, which is called "reactive vibrational mode" And its frequencies change is relevant to the formation and rupture of the covalent bond.Based on canonical variational transition state theory (CVT) and improved canonical variational transition state theory (ICVT), each reaction rate constants are calculated. The calculated results show that the variational effect and the tunnel effect are small and not significant for these hydrogen abstraction reactions in the temperature range 300-3200 K. The calculated reaction rate constants by CVT/SCT orICVT/SCT theory are in good agreement with the experimental results reported in wide temperature rage.