Rate Constants and Branching Ratios for H-Atom Abstractions in Small Alkanes

摘要

The reactions of D/H with n-C_4H_(10) and i-C_4H_(10) have been studied with both shock-tube experiments and ab initio transition state theoretical calculations. H/D-atoms were measured behind reflected shock waves using atomic resonance absorption spectrometry (ARAS) in mixtures with C_2D_5I (D-atom precursor) and the alkane of interest in excess, at high temperatures (>1000 K). D-atom depletion is sensitive to only the reaction of interest and fits to the D-atom profiles allowed direct measurements for the total rate constants for D + n-C_4H_(10) and i-C_4H_(10) at high temperatures. On the other hand, fits to the H-atom profiles were sensitive to only the abstraction rate constant at a specific C-H bond. In combination with the total rate constant measurements using D-atoms, H-atom measurements were then used to directly determine site-specific branching ratios. These reactions have also been characterized using electronic structure theory. Over the T-range of the present experiments, the theoretical kinetics predictions for the isotope effects, k_D/k_H, are near unity. Results from the present studies on n-C_4H_(10) and i-C_4H_(10) combined with prior studies on C_2H_6 and C_3H_8 suggest the applicability of rate rules for H + Alkanes that are based on generic primary, secondary, and tertiary abstraction sites.