KINETICS AND THERMOCHEMISTRY OF THE REACTIONS OF ALKYL RADICALS (CH3, C2H5, I-C3H7, S-C4H9, AND T-C4H9) WITH HI - A RECONCILIATION OF THE ALKYL RADICAL HEATS OF FORMATION

The kinetics of the reactions of CH3, C2H5, i-C3H7, s-C4H9, and t-C4H9 with HI were studied in a tubular reactor coupled to a photoionization mass spectrometer. Rate constants were measuredas a function of temperature (typically between 295 and 648 K) to determine Arrhenius parameters. These results were combined with determinations of the rate constants of the reverse reactions (I + hydrocarbon) determined previously by others to obtain equilibrium constants for the following reaction: R + HI ⟺ R-H + I. Second and Third Law based analyses using these equilibrium constants yielded heats of formation for the five alkyl radicals whose R + HI reactions were studied. The Third Law heats of formation (obtained using calculated entropies) are extremely accurate, within ±2kJ mol-1 of the current best values. (The Second Law heats of formation are less accurate, within ±8 kJ mol-1 of the current best values.) Each of the R + HI reaction rate constants has a negative activation energy, decreasing from -1 kJ mol-13 for the CH3+ HIreaction to -6 kJ mol-1 for the t-C4H9 + HI reaction. The cause of the long-standing disparity that has existed between the heats of formation of the alkyl radicals derived from studies of R + HI ⟺ R-H + I equilibria and those obtainedfrominvestigation of dissociation-recombination equilibria has been identified. It is the difference between the assumed generic activation energy of R + HI rate constants (4 kJ mol-1) that had beenused in all prior thermochemical calculations and the actual values of these activation energies. Acomplex mechanism for R + HI reactions that is consistent with the observed kinetic behavior of these reactions is discussed. © 1990, American Chemical Society. All rights reserved.