ELECTRONIC-STATE-SPECIFIC TRANSITION-METAL CATION CHEMISTRY - FE++C3H8 AND N-C4H10

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+, 3d(6)4s(6D), 3d7(4F), and 3d(6)4s(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, 3d(6)4s and 3d7 configurations, and a range of 1.1 eV electronic energy. Product branching between H-2 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.