Gas Phase Pulse Radiolysis of Hydrocarbon Mixtures; Determination of the Charge Recombination Rate Coefficient and Absolute Rate Constants of Ion-Molecule Reactions of the t-Butyl Ion through a Competitive Kinetic Method

The gas phase pulse radiolysis of several neopentane-d12-alkane mixtures has been studied with the purpose of examining the competition between the reaction of the fragment t-butyl ion with the alkane to form isobutane, t-C4D9+ +RH2→ i-C4D9H+RH+, and the neutralization of the t-butyl ion by an electron or SF6− ion, t-C4D9++X−→products. Using a computer calculation which takes into account the pulse characteristics as a function of time, the yield of isobutane observed in the reaction with 2,3-dimethylbutane (for which krn was accurately determined to be 5.7 ± 0.5 × 10−11 cm3/molecule·sec) and the dose per pulse, rate coefficients of the competing neutralization reactions were determined. These are α=1.92 ± 0.2 × 10−6 cm3/molecule· sec for neutralization by an electron, and 0.40± 0.04 × 10−6 cm3/molecule· sec for neutralization by the SF6− ion. The latter value is independent of SF6 concentration and both are independent of pressure between 40 and 200 torr. Finally, these values of the neutralization rate coefficients and yields of isobutane measured in the pulse radiolysis of neopentane in the presence of other added alkanes (2-methylpentane, 3-methylpentane, methylcyclopentane) are used to calculate absolute rate coefficients of the hydride transfer reactions between the t-C4D9+ ion and these compounds; the values determined are in good agreement with those measured by other techniques. The unique feature of the competitive method described here consists of the fact that the positive ion which is neutralized can be identified by the conventional analysis of a known reaction product.