Deuterium and carbon-13 kinetic isotope effects for the reaction of OH with CH₄

Interpolated variational transition state theory calculations with centrifugal-dominant, small-curvature tunneling coefficients have been carried out for the case of the deuterium kinetic isotope effect (KIE) in the reaction OH + ¹²CD₄ → HDO+¹²CD₃ and for the ¹³C KIE for the reaction OH + ¹³CH₄ → H₂O + ¹³CH₃. The interpolated variationally optimized generalized transition states predict notably different nontunneling KIEs than the conventional ones, and factorization analyses of the KIEs are presented to illustrate the origin of the differences. The zero-point energies at the variational transition states differ from those at the saddle point by up to 0.19 kcal/mol for the OH + ¹²CD₄ reaction and by up to 0.34 kcal/mol for the OH + ¹²CH₄ reaction. The incorporation of multidimensional tunneling effects partly cancels the effect of variational optimization of the transition state.