This report was prepared as an account of work sponsored-by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not neoessarily state or reflect those of the United States Government or any agency thereof.-3-3/21/93 molecules: a) the study of radical kinetics, b) the use of negative ion thermochemical cycles, and c) photoionization mass spectroscopic techniques. It is essential to stress the complcmc#tarity ofthese three experimental methods; theyarcall interrelated. Our goal in this essayistodissect eachofourmethodstodescribe how themeasurements arecarried out, whatthelimitations are, andtodemonstrate by direct comparison that all givethesame bondenergies. An introduction tothese three experimental programs isnow inorder. a)Radical Kinetics Supposeone measuresthekinetics of equilibrium of a halogenatom,X, witha substrate, RH. RH + X = R +XH (I) By monitoring thetime dependenceof [X] and [R] after flash photolysis, by atomic fluorescence, and/orresonance lamp photoionization detection, one can determine the absolute rateconstants kI and k.1. These rateconstants fixtheequilibrium constant, Kequi(1), which permits one todetermine AGrxn(1), fromwhichthecnthalpy, AHrxn(1), can bc extracted. Iftheheatsof formation (AHf°(RH),AHf°(X),and AHf°(XH)) are known,AHrxn(1) permits one tofind AHf°(R)whichfixes thebond energy, BDE(R-H). b)Negative IonCycles Ionchemistry canbc usedtodeducethegasphaseacidity ofa target molecule, RH. The acidity, AHacid, isthecnthalpy for theproton abstraction reaction.

The reactions of alkyl radicals (R = CHI, 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-' s-I), E,/(kJ mol-')): R = CH3, 299-536 K ((-1.57 i 0.26) X lo-", 1.6 i 0.6); R = C2H5, 297-530 K ((1.70 f 0.55) X -4.2 i 1.2); R = i-C3H7, 298-530 K ((1.58 i 0.38) X -6.4 i 0.9); R = t-C4Hg, 298-530 K ((1.37 i 0.47) X lo-'*, -7.8 i 1.4). R + HBr rate constants are approximately a factor of 2 higher than previously reported. The source of this disparity is explained. The kinetics of reverse reactions, Br + RH (R = C&, C3H8, n-C4Hl,,, i-C&Il0), have also been investigated using laser flash photolysis/resonance fluorescence methods. Rate constants were obtained in the following temperature ranges and provided Arrhenius parameters for each reaction (same units): RH = C2&, 473-621 K ((2.35 i 1.12) X 1O-Io, 53.3 i 2.1); RH = C3H8, 476-667 K ((8.78 f 3.00) X lo-", 36.0 i 2.0); RH = n-C4Hlo, 447-625 K ((2.86 i 0.90) X 10-lo, 37.7 i 2.0); RH = i-C4H10, 423-621 K ((1.61 f 0.60) X lO-'O, 28.8 i 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 C& alkyl radicals involved. Second-and third-law determinations agreed extremely closely (differences were under 1.3 kJ mol-'). The heats 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-'. 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-'): C2Hs, 121.0 i 1.5; i-C3H7, 90.0 i 1.7; sec-C4Hg, 67.5 i 2.2; t-C4Hg, 51.3 i 1.8. Accurate determinations of carbon-hydrogen bond enthalpies (298 K) are provided that are based on these heats of formation (kJ mol-'): primary C-H in C2H6 (422.8 i 1.5); secondary C-H in C3H8 (412.7 i 1.7) and in n-C4Hlo (41 1.1 i 2.2); tertiary C-H in i-CIHIO (403.5 i 1.8).

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