UNIMOLECULAR DECOMPOSITION OF THE NEOPENTYL RADICAL

被引:43
作者
SLAGLE, IR
BATT, L
GMURCZYK, GW
GUTMAN, D
TSANG, W
机构
[1] CATHOLIC UNIV AMER,DEPT CHEM,WASHINGTON,DC 20064
[2] NATL INST STAND & TECHNOL,DIV CHEM KINET,GAITHERSBURG,MD 20899
[3] UNIV ABERDEEN,DEPT CHEM,ABERDEEN AB9 1FX,SCOTLAND
关键词
D O I
10.1021/j100173a034
中图分类号
O64 [物理化学(理论化学)、化学物理学];
学科分类号
070304 ; 081704 ;
摘要
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) x 10(16) molecules cm-3 (He) and (6-12) x 10(16) (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 x 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.
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页码:7732 / 7739
页数:8
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