KINETICS AND THERMOCHEMISTRY OF R+HBR-REVERSIBLE-ARROW-RH+BR REACTIONS - DETERMINATIONS OF THE HEAT OF FORMATION OF C2H5, I-C3H7, SEC-C4H9, AND T-C4H9

被引:206
作者
SEAKINS, PW
PILLING, MJ
NIIRANEN, JT
GUTMAN, D
KRASNOPEROV, LN
机构
[1] UNIV OXFORD,PHYS CHEM LAB,OXFORD OX1 3QZ,ENGLAND
[2] CATHOLIC UNIV AMER,DEPT CHEM,WASHINGTON,DC 20064
[3] NOVOSIBIRSK CHEM KINET & COMBUST INST,NOVOSIBIRSK 630090,USSR
关键词
D O I
10.1021/j100203a050
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 reactions of alkyl radicals (R = CH3, C2H5, i-C3H7, and t-C4H9) with HBr have been studied by excimer laser flash photolysis coupled with photoionization mass spectrometry. Rate constants were obtained in the following temperature ranges and provided Arrhenius parameters for each reaction (A/(CM3 molecule-1 s-1), E(a)/(kJ mol-1)): R = CH3, 299-536 K ((-1.57 +/- 0.26) X 10(-12), 1.6 +/- 0.6); R = C2H5, 297-530 K ((1.70 +/- 0.55) X 10(-12), -4.2 +/- 1.2); R = i-C3H7, 298-530 K ((1.58 +/- 0.38) x 10(-12), -6.4 +/- 0.9); R = t-C4H9, 298-530 K ((1.37 +/- 0.47) X 10(-12), -7.8 +/- 1.4). R + HBr rate constants are approximately a factor of 2 higher than previously reported. The source of this disparity is explained. The kinetic of reverse reactions, Br + RH (R - C2H6, C3H8, n-C4H10, i-CH10), have also been investigated using laser flash photolysis/resonance fluorescence methods. Rate constants were obtained m the following temperature ranges and provided Arrhenius parameters for each reaction (same units): RH - C2H6, 473-621 K ((2.35 +/- 1.12) x 10(-10) 53.3 +/- 2.1); RH = C3H8, 476-667 K ((8.78 +/- 3.00) X 10(-11), 36.0 +/- 2.0); RH - n-C4H10, 447-625 K ((2.86 +/- 0.90) x 10(-10), 37.7 +/- 2.0); RH = i-C4H10, 423-621 K ((1.61 +/- 0.60) x 10(-10), 28.8 +/- 1.5). These results, combined with previously obtained kinetic information, were used in second- and third-law thermochemical calculations to obtain accurate determinations of the heats of formation of the C2-C4 alkyl radicals involved. Second- and third-law determinations agreed extremely closely (differences were under 1.3 kJ mol-1). The beats of formation of the radicals thus obtained are in excellent agreement with values obtained from studies of dissociation/association equilibria, within 2.6 kJ mol-1. Recommended alkyl-radical heats of formation (with uncertainties) at 298 K are provided that are based on an assessment of all the results of the current study and a review of other recent determinations (kJ mol-1): C2H5, 121.0 +/- 1.5; i-C3H7, 90.0 +/- 1.7; sec-C4H9, 67.5 +/- 2.2; t-C4H9, 51.3 +/- 1.8. Accurate determinations of carbon-hydrogen bond enthalpies (298 K) are provided that are based on these heats of formation (kJ mol-l): primary C-H in C2H6 (422.8 +/- 1.5); secondary C-H in C3H8 (412.7 +/-1.7) and in n-C4H1 (411.1 +/- 2.2); tertiary C-H in i-C4H10 (403.5 +/- 1.8).
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页码:9847 / 9855
页数:9
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