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

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).