Mode- and Bond-Selective Chemistry on Metal Surfaces: The Dissociative Chemisorption of CHD₃ on Ni(111)

A quantum approach based on an expansion in vibrationally adiabatic eigenstates is used to explore CHD3 dissociation on Ni(111). Symmetric stretch vibrations are shown to have larger efficacies for promoting reaction and bond selectivity than antisymmetric stretches. Mode softening, associated with a localization of vibrational energy on the reacting bond, correlates adiabatically with the symmetric stretch. This lowers the barrier on that vibrationally adiabatic reaction path. At energies below these barriers, reaction occurs via nonadiabatic transitions to lower-energy vibrational states at the avoided crossings. This sudden behavior preserves energy localization on the reactive bond and converts vibrational energy into bond breaking. One of the antisymmetric stretches can typically not couple to the other states due to symmetry, eliminating these pathways. The ν1 stretch has a large efficacy and can give 100% C–H cleavage selectivity. Both CD3 stretches have reasonable efficacies, and both promote C–D relative to C–H cleavage. These effects are stronger for the symmetric stretch. We observe some minor surface-induced intramolecular vibrational energy redistribution (IVR), where excitation of C–H or C–D stretches can promote C–D or C–H cleavage, respectively. Our results are consistent with experiment with regard to the magnitude of the dissociative sticking and the increase in selectivity with excitation of the ν1 stretch.