Theoretical Studies On The Polyatomic Reactions

The construction of the global potential energy surface (PES) and the calculations of the global dynamics for the polyatomic reactions have challenged the present computational algorithm and the computer technology. Based on the current status, using the ab initio calculations at the higher level and the experimental data, we recalibrated the global semiemperical PES(WB PES) for the H₂+C₂H system to obtain a new PES (MWB PES) with a lower barrier height. Then, with the variational transition state theory (VTST), we investigated systematically the dynamical properties of the H₂+C₂H→H+C₂H₂ reaction as well as its reverse and their isotopes to verify the reliability of the MWB PES.For the H₂+C₂H→H+C₂H₂ reaction, the transition state on the MWB PES is collinear and reactant-like and the barrier height is 2.29 kcal/mol, which are in good agreement with the previous theoretical calculations and the experimental results. In addition, the calculated rate constants based on the MWB PES for the H₂+C₂H→H+C₂H₂ reaction and its reverse are in line with the corresponding experimental data. Therefore, it may be concluded that the MWB PES is reliable to calculate the rate constants.It is found that the primary kinetic isotope effects (KIEs) dominate the KIEs of the H₂+C₂H→H+C₂H₂ reaction. Subsequently, comparing the effects on classical potential energy curves by replacing the different H atom in this system with D atom, we found that the outer H in the H₂ molecule cannot be considered as a spectator, while the H in the C₂H acts as a spectator in the reaction. It is thought this might be caused by the effective mass involved in this reaction.Additionally, the calculated rate constants for the isotopic reactions are in good agreement with the corresponding experimental results, which lends a good support to our MWB PES. Finally, it is found that most of the KIEs belong to"normal"for the considered range of temperature, which is mainly caused by the two factors of the vibrational partition function and the tunneling correction, which vary with the temperature.The TST overestimating significantly the rate constants of the reaction in liquid and biological enzymes has been identified. VTST is an improvement version of TST, but the reliability of VTST has been doubted. In this paper, we compared and analyzed a series of the general gas-phase bimolecular reactions, which have been investigated by VTST, quantum dynamics (QD) and quasiclassical trajectory (QCT) approaches for the rate constants in the literatures. It is found that the level of the agreement between VTST and QCT (QD) strongly depends on the density of states at the transition state, the number of the DOFs (Degree of Freedoms) involved and the shape of the barrier for the given reaction.