| We have investigated the bimolecular reaction dynamics of gas-phase transition metal ion Co+ with small alkanes, propane, n-butane, and isobutane. We use a state-specific, crossed-beam, time-of-flight mass spectrometry experiment to measure time-resolved branching fractions for Co+ + n-butane and Co+ + isobutane; studies of Co+ + isobutane and Co+ + propane are performed under the same crossed-beam conditions employing the velocity map imaging technique to obtain product translational energy distributions and product angular distributions.;Co+(3F4) reacts with n-butane under single-collision conditions at 0.01 and 0.22 eV collision energy to eliminate C2H6, CH4, and H2. In the Co+(3F4) + isobutane reaction, CH4 and H2 elimination products are observed. Long-lived Co(C4H10)+ complexes formed in both reactions decay to elimination products and back to reactants on a wide range of timescales, ranging from 300 ns to 25 mus. We propose plausible mechanisms for these reactions based on insights from our recent statistical modeling based on density functional theory calculations for the related systems Ni+ + propane, Co+ + propane, and Ni+ + n-butane.;We use the velocity map imaging technique to obtain three-dimensional product velocity distributions, P(E,theta), for the Co + + isobutane and Co+ + propane reactions. We report product translation energy distributions, P(E) and product angular distributions, T(theta) for the CH4 and H2 elimination products. In both reactions, the P(E) for the CH4 elimination product is in accord with a statistical process; only 10% of the available energy for reaction is released into translation. In contrast, both reactions give rise to considerably hotter, non-statistical H2 elimination product distributions, with a remarkably large 35--40% of the available energy becoming product translation. The new data are in good agreement with earlier measurements from the Bowers group for both reactions. We report the first measurements of product angular distributions for metal ion/alkane reactions. The T(theta)s for CH4 elimination are anisotropic and are well-described by the statistical complex model, while the H2 elimination product distributions are nearly isotropic in disagreement with the model. We postulate that H2 and CH4 elimination occur via single pathways at low collision energy, as predicted by density functional theory calculations. Direct, non-statistical dynamics are proposed to explain the unusually hot H 2 elimination product energy distributions. |
