Multicoefficient Gaussian-3 calculation of the rate constant for the OH + CH ₄ reaction and its ¹²C/ ¹³C kinetic isotope effect with emphasis on the effects of coordinate system and torsional treatment

Rate constants and ¹²CV ¹³C kinetic isotope effects are calculated by direct dynamics for the OH + CH ₄ → H ₂O + CH ₃ reaction. The electronic structure calculations required to generate the implicit potential energy surface were carried out by the high-level multicoefficient Gaussian-3/version-3 (MCG3) method and compared to two other multilevel methods, MC3BB and MC3MPW, and three density functional methods, M06-2X, BB1K, and MPW1K. The rate constants and ¹²C/ ¹³C kinetic isotope effects are shown to depend strongly on the coordinate system used to calculate the frequencies as well as on the method used to account for the torsional anharmonicity of the lowest-frequency vibrational mode of the generalized transition states.