Deconstructing experimental rate constant measurements: Obtaining intrinsic reaction parameters, kinetic isotope effects, and tunneling coefficients from kinetic data for OH + methane, ethane and cyclohexane

Kinetic data for multiple OH–alkane reactions, including primary kinetic isotope data, can be fit simultaneously to extract constrained contributions from transition state vibrational activation, zero-point energy, and tunneling. We use data for single-pathway reactions: OH + methane, ethane, and cyclohexane, to obtain optimal ‘intrinsic’ parameters common to all reactions. This is possible because parameters specific to individual reactions, such as the pre-exponential term, can be accurately calculated with ease. Several intrinsic parameters – the barrier height, the tunneling temperature (imaginary frequency) and a transition state bending frequency (the radical attack angle) – scale together from reaction to reaction. This scaling is so precise that we can accurately fit data for OH + methane using intrinsic parameters derived for OH + ethane and OH + cyclohexane using only a single free parameter (the scaling factor). This strongly confirms our underlying hypothesis about the physics controlling the key transition state parameters and suggests that extrapolations in temperature and predictions for unmeasured reactions are potentially robust.