Electronic-state-specific transition metal cation chemistry: Fe++C3H8 and n-C4H1

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We present total reaction cross sections and product branching fractions for collisions of specific electronic states of Fe+ with the linear alkanes C3H8 and n-C4H10 at two collision energies each, 0.2 and 1.0 eV. Resonant two-photon ionization prepares specific electronic state distributions of Fe+, as described in the preceding paper. A crossed beam experiment using pulsed, time-of-flight mass spectrometry measures total reaction cross sections averaged over known state distributions, from which we extract state-specific cross sections. The three lowest energy electronic terms of Fe+, 3d64s(6D), 3d7(4F), and 3d64s(4D), show remarkably similar reactivity with both alkanes. The relative cross section varies only a factor of 4 in Fe++C3H8 and less than a factor of 2 in Fe++C4H10, in spite of sampling both sextet and quartet spins, 3d64s and 3d7 configurations, and a range of 1.1 eV electronic energy. Product branching between H2 and alkane elimination is insensitive to initial electronic state as well. All reactions are inefficient compared with the Langevin cross section. We propose a model of Fe++alkane chemistry that assumes that all those Fe+ that insert in a C–H bond of the alkane proceed to elimination products. Spin-changing surface hops between diabatic potential surfaces correlating to different reactant asymptotes (i.e., electronic quenching) competes with bond insertion, explaining the low reactivity of the 3d7(4F) term.