Nonstatistical Translational Energy Distribution of H₂ Elimination Products from Co⁺(³F₄) + Propane

The velocity map imaging technique is applied to mass-selected CoC 2 H 4 + + CH 4 and CoC 3 H 6 + + H 2 elimination products from the Co + ( 3 F 4 ) + propane reaction studied under crossed-beam conditions at 0.19 eV collision energy. For both products, we obtain the joint scattering probability distribution P( E,ϑ), where E and ϑ are the product translational energy and scattering angle. Both angular distributions are forward-backward symmetric and only modestly anisotropic. Both product translational energy distributions P( E ) are in fairly good agreement with earlier results of Bowers and co-workers, indicating that the earlier distributions were dominated by reaction of ground-state Co + . The fraction of the total available energy deposited into product translation is 0.13 for CH 4 and 0.44 for H 2 . For CH 4 products, P( E ) is cold and would be readily fit by orbiting-transition-state phase-space theory (OTS−PST) without an exit-channel barrier. Evidently, incipient CoC 2 H 4 + + CH 4 products equilibrate in the Co + (C 2 H 4 )(CH 4 ) exit-channel well, from which they decay statistically. In sharp contrast, for H 2 products P( E ) exhibits a substantial hot, nonstatistical tail toward high energy, extending to the limits of the available energy. Although it is imaginable that the H 2 channel has a late potential energy barrier some 0.5 eV above products, we view this as highly unlikely. Instead, we suggest that the potential energy from an earlier multi-center transition state MCTS H 2 is funneled efficiently, and highly nonstatistically, into product translation, although MCTS H 2 is far from product-like. We believe that H 2 escapes on the same time scale as intramolecular vibrational redistribution (IVR) due to its light mass, the early potential energy decoupling of the migrating H atom from the incipient alkene moiety, and the weak kinetic energy coupling across the massive Co + ion. This surprising conclusion seemingly applies to H 2 products for the entire family of reactions of the late-3d series transition metal cations Fe +, Co +, and Ni + with alkanes.