Rate constants and hydrogen isotope substitution effects in the CH ₃ + HCl and CH₃ + Cl₂ reactions

The kinetics of the CH₃ + Cl₂ (k_2a) and CD₃ + Cl₂ (k_2b) reactions were studied over the temperature range 188-500 K using laser photolysis-photoionization mass spectrometry. The rate constants of these reactions are independent of the bath gas pressure within the experimental range, 0.6-5.1 Torr (He). The rate constants were fitted by the modified Arrhenius expression, k_2a = 1.7 × 10^-13(T/300 K)^2.52exp(5520 J mol^-1/RT) and k_2b = 2.9 × 10^-13(T/300 K) ^1.84exp(4770 J mol^-1 VRT) cm³ molecule ^-1 s^-1. The results for reaction 2a are in good agreement with the previous determinations performed at and above ambient temperature. Rate constants of the CH₃ + Cl₂ and CD₃ + Cl₂ reactions obtained in this work exhibit minima at about 270-300 K. The rate constants have positive temperature dependences above the minima, and negative below. Deuterium substitution increases the rate constant, in particular at low temperatures, where the effect reaches ca. 45% at 188 K. These observations are quantitatively rationalized in terms of stationary points on a potential energy surface based on QCISD/6-311G(d,p) geometries and frequencies, combined with CCSD(T) energies extrapolated to the complete basis set limit. ID tunneling as well as the possibility of the negative energies of the transition state are incorporated into a transition state theory analysis, an approach which also accounts for prior experiments on the CH₃ + HCl system and its various deuterated isotopic substitutions [Eskola, A. J.; Seetula, J. A.; Timonen, R. S. Chem. Phys. 2006, 331, 26].