Unimolecular decomposition of the neopentyl radical

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 CCl{sub 4} (to produce CCl{sub 3} + Cl) followed by the rapid reaction between the Cl atoms and neopentane to produce neo-C{sub 5}H{sub 11} + HCl. Unimolecular rate constants were determined as a function of bath gas (He, N{sub 2}, and Ar), temperature (10 temperatures between 560 and 650 K), and bath gas density ((3{minus}30) {times} 10{sup 16} molecules cm{sup {minus}3} (He) and (6{minus}12) {times} 10{sup 16} (N{sub 2}, 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 {yields} isobutene + CH{sub 3}) = 10{sup 13.9 {plus minus} 0.5} exp({minus}30.9 {plus minus} 1.0 kcal mol{sup {minus}1}/RT) s{sup {minus}1}. The average step sizes down for the bath gases used (adjusted parameters in the RRKM calculations) are comparable: 200 (He), 130 (N{sub 2}), and 140 (Ar) cm{sup {minus}1} (all {plus minus} 60 cm{sup {minus}1}). The high-pressure-limit rate constant expression for the reverse reaction, nonterminal addition of CH{sub 3} to isobutene, was obtained from thermochemical calculations by using the results of this study: k(CH{sub 3} + isobutene {yields} neopentyl) = 3.7 {times} 10{sup {minus}13} exp({minus}10.6 kcal mol{sup {minus}1}/RT) cm{sup 3} molecule{sup {minus}1} s{sup {minus}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.