Paper SS2-TuA10
Non Statistical Patterns of Chemical Reactivity on Metals

Tuesday, October 21, 2008, 4:40 pm, Room 209

Facile electron- and phonon-mediated energy exchange channels on metal surfaces have the potential to support rapid and extensive energy exchange during a reactive collision, but recent state-resolved reactivity measurements of methane's dissociative chemisorption on Ni have produced clear evidence for incomplete (non-statistical) energy redistribution prior to reaction. Examples include vibrational-mode selective chemistry (in which reactivity is influenced not only by the amount of vibrational energy available, but also by the identity of the excited vibrational state), the observation that vibrational and translational energy differ in their ability to activate reaction, and even bond-selective chemistry (selective excitation of the C-H stretch in CHD₃ prior to surface impact increased the C-H : C-D bond cleavage ratio among products nearly 100-fold relative to an isoenergetic ensemble of CHD₃ whose vibrations were statistically populated). We will describe a mechanistic framework for understanding the origin of observed non-statistical reactivity and predicting the propensity for such behavior in other systems. The model builds on numerous experimental and theoretical studies of intramolecular vibrational energy flow and quenching in the gas phase, in liquids, and at surfaces, and it focuses on the timescales for key energy exchange processes relative to reaction. It provides a qualitative explanation for observed reactivity patterns, intuitive guidance for understanding and predicting reactivity patterns, and it and identifies attractive targets for future experimental and theoretical work to better understand and exploit these effects.