Shock Tube Explorations of Roaming Radical Mechanisms: The Decompositions of Isobutane and Neopentane

The thermal decompositions of isobutane and neopentane have been studied using both shock tube experiments and ab initio transition state theory based master equation calculations. Dissociation rate constants for these molecules have been measured at high temperatures (1260–1566 K) behind reflected shock waves using high-sensitivity H-ARAS detection. The two major dissociation channels at high temperature are iso-C4H10 → CH3 + i-C3H7 (1a) and neo-C5H12 → CH3 + t-C4H9 (2a). Ultrahigh-sensitivity ARAS detection of H-atoms produced from the rapid decomposition of the product radicals, i-C3H7 in (1a) and t-C4H9 in (2a), through i-C3H7 + M → H + C3H6 + M (3a) and t-C4H9 + M → H + i-C4H8 + M (4a) allowed measurements of both the total decomposition rate constants, ktotal, and the branching to radical products, which were observed to be equivalent in both systems, k1a/ktotal and k2a/ktotal = 0.79 ± 0.05. Theoretical analyses indicate that in isobutane, the non-H-atom fraction has two contributions, the dominant ...