Theoretical chemical kinetics of atmospheric reactions with near-zero and negative barriers

Abstract

The atmosphere consists of many gases and radicals and many reactions could occur, thereby forming a very dynamic system. Due to the unstable nature of radicals, they are short-lived which make experimental studies difficult. Moreover, reactions involving radicals (such as radical-radical reactions) have low or negative barriers. The life time of these radicals short, which make them and their reactions difficult to observe. Furthermore, some conditions can be challenging to achieve in laboratory, such as very high or very low temperature and/or pressure, or the complete desirable ranges. In view of these experimental challenges of gas phase studies, theoretical methods (ab initio (wavefunction-based) methods, density functional theory (DFT), and the transition state theory (TST)) provide alternative ways to study gas phase reactions without subject to the aforementioned difficulties, especially for reactions involving radicals. This work aims to establish reliable and practical methods to calculate rate coefficients for chemical reactions with low or negative barriers, which involves the investigation of the interrelationships between the rate coefficients, the reaction surface and the computed barrier heights. In particular, two atmospherically important reactions were studied computationally, namely the CH3C(O)OCH3 + Cl reaction and the BrO + HO2 reaction. The former is a reaction with a low barrier while the major channel of the latter is a reaction with a negative barrier. Rate coefficient calculations showed that one vibrational mode changed dramatically along the reaction coordinate, leading to a dramatic change of the zero-point energy (ΔZPE) along the reaction coordinate. Consequently, large variational effects resulted for reactions with flat and very flat reaction surfaces, which were characterized by small classical adiabatic ground-state transmission factors (CAG factors). This approach is effective in compensating for the deficiency of the TST method. The results also showed that the locations of the maxima of the ΔG curves at certain temperatures were a bit away from the saddle point at a negative reaction coordinate (i.e. in the reactant side), especially for reactions with low and negative barriers. This indicated that the choice of the intrinsic reaction coordinate in rate coefficient calculations had to cover the maximum of a ΔG curve in order to obtain accurate rate coefficients. It was also found that the pre-exponential entropic terms, which were computed using the vibrational frequencies and geometrical parameters, had significant effects on the computed rate coefficients, especially for chemical reactions with low and negative barriers. Thus, in addition to the accuracies of the computed barrier heights, the accuracies of the computed harmonic frequencies and geometries along the reaction coordinate also play crucial roles as they in part determined the accuracies of rate coefficients. This work has also provided the computed barrier heights, reaction mechanisms, reaction enthalpies and rate coefficients of the two reactions, so that they can be used for further kinetic modeling.