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

Abstract

The velocity map imaging technique is applied to mass-selected CoC2H4 + + CH4 and CoC3H6 + + H2 elimination products from the Co+(3F4) + 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 CH4 and 0.44 for H2. 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 H2 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 H2 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 MCTSH 2 is funneled efficiently, and highly nonstatistically, into product translation, although MCTSH 2 is far from product-like. We believe that H2 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 H2 products for the entire family of reactions of the late-3d series transition metal cations Fe+, Co+, and Ni+ with alkanes.

Imaging ion-molecule reactions: Charge transfer and C-N bond formation in the C+ + NH3 system

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Imaging ion-molecule reactions: Charge transfer and halide transfer reactions of O+ with CH3Cl, CH3Br, and CH3I

“…Detecting products at variable delay times after extracting products from the collision region allows products of different masses to be imaged. The experimental method is similar to schemes employed in other laboratories [21][22][23][24][25][26].…”

Ion imaging study of dissociative charge transfer in the N2+ + CH4 system

“…The experimental method is based upon important developments from other laboratories. [15][16][17][18][19][20] The primary ion beam is formed by impact of 90 eV electrons on a mixture of 10% N 2 in He. 21 The primary product of electron impact on this mixture is He + , which then undergoes charge transfer with N 2 to form both parent (N 2 + ) and fragment (N + ) ions.…”