Rules for vibrational mode selectivity in polyatomic reactions
In plain words
For three-atom reactions, empirical rules (the Polanyi rules) say whether energy put into vibration or into collision speed better promotes reaction. For larger molecules such as methane these rules fail in some measured cases, and no general replacement is established.
Precise statement
For abstraction reactions $X$ + CH4 and $X$ + CHD3 ($X = \mathrm{H}, \mathrm{F}, \mathrm{Cl}, \mathrm{O}$) and H + H2O, predict the vibrational efficacy $\eta_v$ (reactivity gain per unit energy in mode $v$ divided by that per unit translational energy) for each normal mode, and the bond selectivity, from properties of the potential near the transition state. An answer is a criterion tested against full-dimensional quantum scattering and crossed-beam data, including measured cases where C-H stretch excitation is no more effective than translation (Cl + CHD3) or suppresses reaction (F + CHD3 at low collision energy).
What would settle it
Full-dimensional quantum scattering on accurate potentials for the reaction family compared with mode-selected crossed-beam measurements.
Status in the literature
The sudden vector projection model (Jiang and Guo, introduced 2013; reviewed in Acc. Chem. Res. 2014, https://doi.org/10.1021/ar500350f) accounts for many measured efficacies; low-collision-energy exceptions remain unexplained by it.