State-to-state dynamics of atom+polyatom abstraction reactions. II. The H+C₂H₆/C₃H₈ → H₂ (v′,J′) + C₂H₅/C₃H₇ reactions

Abstract

The rotational and vibrational quantum state distributions for the H 2 products of the H + HR → H2 + R reactions (HR = C2H6 and C3H8) at 1.6 eV collision energy have been measured using coherent anti-Stokes Raman scattering. Total reaction cross sections have also been determined. For the total cross sections we find 1.5 ± 0.5 Å2 for the ethane reaction and 2.9 ± 0.8 Å2 for the propane reaction. Although several vibrational states are energetically accessible, we observe H2 products only in υ′ = 0 and υ′ = 1, with the majority in the ground vibrational state. The H2 products are on average rotationally cold as well, and 20% or less of the total energy is partitioned to H2 internal energy. However, the quantum state distributions show a positive correlation of H2 product rotational and vibrational energy. That is, the average rotational energy of the H2 in υ′ = 1 is substantially greater than the average rotational energy of the H 2 in υ′ = 0. Comparison with state-to-state dynamics results previously obtained for the kinematically and energetically similar H + CD4 → HD + CD3 and H + HC1 → H2 + C1 reactions seems to indicate that this anomalous energy disposal is an intrinsic characteristic of H + alkane hydrogen atom abstraction reactions at high collision energy. We speculate that this anomalous behavior is the result of inelastic encounters between the nascent H2 and alkyl radical products in the reaction exit channel. © 1992 American Institute of Physics.