Anharmonic and harmonic rate constants were calculated in the unimolecular reaction HCO 2 → H + CO 2 with Yao and Lin (YL) method. In microcanonical case, the total number of states and density of states increase reasonably with the increasing of the total energy and the temperature. Both in microcanonical and canonical cases, the harmonic rate constants are much higher than those for anharmonic cases, especially in the case of high total energy and temperature, which indicates that anharmonic effect of the reaction is so significant that it is not negligible. The results obtained in this research show that the YL method works well for such unimolecular reactions.

HOCO is the intermediate of the reaction H + CO 2 ® HO + CO. In this study, all the geometries in the collision reaction H + CO 2 were optimized at MP2/6-311++G** level with Gaussian 03 program and a potential energy surface which shows that three unimolecular reactions were in the process of HOCO ® HO + CO. For the three reactions, YL method proposed by L. Yao and S. H. Lin is applied to calculate the anharmonic and harmonic total number of states, the density of states and rate constants. The anharmonic values for rate constants calculated in this study are much lower than harmonic values, which indicate that anharmonic effects are significant and can not be neglected. After convert the experimental lifetime of HOCO into rate constants, the values are close to the calculations in our research, which shows that YL method used in our study is suitable for studying the rate constants of unimolecular reaction.

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