STATISTICAL RATE THEORY CALCULATIONS OF THE CL-+CH3BR-]CLCH3+BR- RATE-CONSTANT VERSUS TEMPERATURE, TRANSLATIONAL ENERGY, AND H(D) ISOTOPIC-SUBSTITUTION

In the work presented here three different statistical theoretical models and three different potential energy surfaces are used to calculate the rate constant for the Cl- + CH3Br --> ClCH3 + Br- S(N)2 nucleophilic substitution reaction versus temperature, relative translational energy and CH3Br temperature, and H(D) isotopic substitution. The calculated rate constants are compared with experimental measurements. In applying one of the theoretical models along with one of the potential energy surfaces, the potential energy for the reaction's central barrier was chosen by fitting the 300 K Cl- + CH3Br experimental rate constant. Overall, there is poor agreement between the calculated and experimental results. The calculated temperature dependence of the Cl- + CH3Br S(N)2 rate constant is in qualitative agreement with experiment, but not quantitative. Using anharmonic vibrational frequencies, instead of harmonic, has only a small effect on the rate constant's calculated temperature dependence. The temperature dependence of the k(H)/k(D) isotope effect is fit by one of the theoretical models, microcanonical transition state theory, when using harmonic vibrational frequencies. However, this agreement is dramatically diminished if anharmonic vibrational frequencies are used. Differences between the calculated rate constant versus relative translational energy and CH3Br temperature, i.e., k(E(rel),T), and experiment are striking. The decrease in the calculated k(E(rel),T), as E(rel) is increased, is much less pronounced than observed in the experiments. Also, the calculated k(E(rel),T) increases as T is increased, while the experimental k(E(rel),T) is nearly independent of T. The inability of statistical theories to interpret the kinetics of the Cl- + CH3Br S(N)2 reaction is in accord with previous theoretical and experimental studies, which indicate the reaction's dynamics and kinetics is non-statistical.