Collisional Quenching of Competitive Unimolecular Reactions. Vibrational Energy Transfer

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Abstract
Two competitive–consecutive unimolecular reaction systems were investigated. An H+olefin chemical activation technique was used to produce highly vibrationally excited polyatomic alkyl radicals (Rp*) in a narrow, nonequilibrium distribution of energy states centered near 45 kcal mole−1. The radicals studied, 3,3-dimethylhexyl-2·* and 3-methylhexyl-2·*, each decompose unimolecularly by several competitive C–C rupture reaction channels to stable olefins and smaller radical fragments; also, isomerization by H-atom transfer to secondary radicals, 4,4-dimethylhexyl-2·* and 4-methylhexyl-2·*, respectively, competes with C–C rupture. The isomerized species retain the vibrational excitation and undergo further consecutive unimolecular reactions including C–C rupture and H-atom isomerization. The spectrum of competitive reactions of Rp* via various paths, each characterized by a different energy threshold, provides a type of competitive chemical reaction “spectroscopy” wherein the amounts of the various decomposition events associated with each threshold act as transducers of the steady-state energy level populations of the reactive species. The competition between collisional stabilization and reaction via the several reaction paths of Rp* was measured for H2 and CF4 heat-bath molecules. The experimental results are compared with calculations made on a stochastic basis. Various collisional transition probability models characterized by different distributions of vibrational energy jump sizes were examined. Based on a step ladder model of transition probabilities, H2 transfers ∼ 400 cm−1 per collision and CF4 >̃ 1600 cm−1 per collision; for an exponential distribution of transition probabilities, a larger amount of vibration energy, e.g., ∼ 500 cm−1 for H2, is appropriate. The conclusions obtained by this technique are independent of information on the magnitudes of the collision cross sections. Some of the more important aspects of the radical chemistry are considered and several disproportionation–recombination ratios were obtained.