State-to-state dynamics of atom+polyatom abstraction reactions. II. The H+C sub 2 H sub 6 /C sub 3 H sub 8 r arrow H sub 2 ( v prime , J prime ) +C sub 2 H sub 5 /C sub 3 H sub 7 reactions

The rotational and vibrational quantum state distributions for the H{sub 2} products of the H+HR{r arrow}H{sub 2}+R reactions (HR=C{sub 2}H{sub 6} and C{sub 3}H{sub 8} ) 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{plus minus}0.5 A{sup 2} for the ethane reaction and 2.9{plus minus}0.8 A{sup 2} for the propane reaction. Although several vibrational states are energetically accessible, we observe H{sub 2} products only in {ital v}{prime}=0 and {ital v}{prime}=1, with the majority in the ground vibrational state. The H{sub 2} products are on average rotationally cold as well, and 20% or less of the total energy is partitioned to H{sub 2} internal energy. However, the quantum state distributions show a positive correlation of H{sub 2} product rotational and vibrational energy. That is, the average rotational energy of the H{sub 2} in {ital v}{prime}=1 is substantially greater than the average rotational energy of the H{sub 2} in {ital v}{prime}=0. Comparison with state-to-state dynamics results previously obtained for the kinematically and energetically similar H+CD{sub 4}{r arrow}HD+CD{sub 3} and H+HCl{r arrow}H{sub 2}+Cl 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 H{sub 2} and alkyl radical products in the reaction exit channel.