Unimolecular Decomposition of the Neopentyl Radical

Article abstract of DOI:10.1021/j100173a034

The kinetics of the unimolecular decomposition of the neopentyl radical has been investigated.Experimentally, the decomposition was monitored in time-resolved experiments by using a heatable tubular reactor coupled to a photoionization mass spectrometer.The radicals were produced indirectly by pulsed excimer laser photolysis of CCl4 (to produce CCl3 + Cl) followed by the rapid reaction between the Cl atoms and neopentane to produce neo-C5H11 + HCl.Unimolecular rate constants were determined as a function of bath gas (He, N2, and Ar), temperature (10 temperatures between 560 and 650 K), and bath gas density ((3-30) * 10¹⁶ molecules cm^-3 (He) and (6-12) * 10¹⁶ (N2, Ar)).The data were fitted within the framework of RRKM theory by using a vibrational model.The high-pressure rate constant in the temperature range studied was determined to be k(neopentyl -> isobutene + CH3) = 10^{13.9 +/- 0.5} exp(-30.9 +/- 1.0 kcal mol^-1/RT)s^-1.The average step sizes down for the bath gases used (adjusted parameters in the RRKM calculations) are comparable: 200 (He), 130 (N2), and 140 (Ar) cm^-1 (all +/- 60 cm^-1).The high-pressure-limit rate constant expression for the reverse reaction, nonterminal addition of CH3 to isobutene, was obtained from thermochemical calculations by using the results of this study: k(CH3 + isobutene -> neopentyl) = 3.7 * 10^-13 exp(-10.6 kcal mol^-1/RT) cm³ molecule^-1 s^-1.The Arrhenius parameters of this addition reaction indicate that nonterminal addition is inhibited relative to terminal addition mostly by a larger energy barrier to addition, as opposed to possible entropic effects.