Journal of Physical Chemistry A, Vol.106, No.23, 5563-5576, 2002
Nonstatistical translational energy distribution of H-2 elimination products from Co+(F-3(4)) plus propane
The velocity map imaging technique is applied to mass-selected CoC2H4+ + CH4 and CoC3H6+ + H-2 elimination products from the Co+(F-3(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, Theta), where E and Theta 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 Col. The fraction of the total available energy deposited into product translation is 0.13 for CH4 and 0.44 for H-2. For CH4 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 CoC2H4+ + CH4 products equilibrate in the Co+(C2H4)(CH4) 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 MCTSH2 is funneled efficiently, and highly nonstatistically, into product translation, although MCTSH2 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.