ON THE QUESTION OF NEGATIVE ACTIVATION-ENERGIES - ABSOLUTE RATE CONSTANTS BY RRKM AND G1 THEORY FOR CH3 + HX-]CH4 + X (X = CL, BR) REACTIONS

The kinetics and mechanisms of the reactions CH3 + HX --> CH4 + X (X = Cl, Br) were investigated. Both reactions proceed via a loosely bonded complex which is formed without activation energy. Rate constants of each reaction have been calculated based on RRKM theory with corrections for tunneling evaluated using the Wigner method. For the chloro system, a quantitative agreement is found between theoretical results and available experimental data. In the case of the bromo system, the theoretical barrier must be lowered to -0.11 kcal/mol to match experimental results. Isotope effects are predicted; k(H)/k(D) values follow (296 K): CH3 + DX, 5.08 (X = Cl), 1.98 (X = Br); CD3 + HX, 0.62 (X = Cl), 0.87 (X = Br); CD3 + DX, 3.08 (X = Cl), 1.62 (X = Br). Quantitative agreement is found for the single experimental value at 296 K for CD3 + HBr vs CH3 + HBr: k(H)/k(D) = 0.83 +/- 0.08.