KINETICS AND THERMOCHEMISTRY OF THE R-REVERSIBLE-ARROW-RH+BR (R=N-C3H7, ISOC(3)H(7), N-C4H9, N-C4H9, ISOC(4)H(9), SEC-C4H9 OR TERT-C4H9) EQUILIBRIUM(HBR)

The kinetics of the reactions of n-C3H7, isoC(3)H(7), n-C4H9, isoC(4)H (9), sec-C4H9 and tert-C4H9 radicals, R, with HBr have been investigat ed in a heatable tubular reactor coupled to a photoionization mass spe ctrometer. The reactions were studied by a time-resolved technique und er pseudo-first-order conditions, where the rate constants of R + HBr reactions were obtained by monitoring the decay of the radical as a fu nction of time. The radical was photogenerated in situ in the how reac tor by pulsed 248 nm exciplex laser radiation. All six reactions were studied separately over a wide range of temperatures and, in these tem perature ranges, the rate constants determined were fitted to an Arrhe nius expression (error limits stated are 1 sigma + Students t values, units cm(3) molecule(-1) s(-1)): k(n-C3H7)=(1.6+/-0.2) x 10(-12) exp[(5.4+/-0.2) kJ mol(-1)/RT], k(isoC(3)H(7))=(1.4+/-0.2) x 10(-12) exp[(6.9+/-0.2) kJ mol(-1)/RT], k(n-C4H9)=(1.3+/-0.2) x 10(-12) exp[+(6.4/-0.4) kJ mol(-1)/RT], k(isoC(4)H(9))=(1.4+/-0.2) x 10(-12) exp[+(6.1/-0.2) kJ mol(-1)/RT], k(sec-C4H9)=(1.4+/-0.3) x 10(-12) exp[+(7.5+/-0 .3) kJ mol(-1)/RT] and k(tert-C4H9)=(1.2+/-0.3) x 10(-12) exp[+(8.3+/- 0.3) kJ mol(-1/RT].) The kinetic information was combined with the kin etics of the Br + RH reactions to calculate the entropy and the heat o f formation values for the radicals studied. The thermodynamic values were obtained at 298 K using a second-law procedure. The entropy value s and enthalpies of formation are (entropy in J K-1 mol(-1) and enthal py in kJ mol(-1)): 284+/-5, 100.8+/-2.1 (n-C3H7); 281+/-5, 86.6+/-2.0 (isoC(3)H(7)); 329+/-5, 80.9+/-2.2 (n-C4H9); 316+/-5, 72.7+/-2.2 (isoC (4)H(9)); 330+/-5, 66.7+/-2.1 (sec-C4H9) and 315+/-4, 51.8+/-1.3 (tert -C4H9). The C-H bond strength of analogous saturated hydrocarbons deri ved from the enthalpy of reaction values are (in kJ mol(-1)): 423.3+/- 2.1 (primary C-H bond in propane), 409.1+/-2.0 (secondary C-H bond in propane), 425.4+/-2.1 (primary C-H bond in n-butane), 425.2+/-2.1 (pri mary C-H bond in isobutane), 411.2+/-2.0 (secondary C-H bond in n-buta ne) and 404.3+/-1.3 (tertiary C-H bond in isobutane). The enthalpy of formation values are used in group additivity calculations to estimate Delta(f)H degrees(298) values of six pentyl and four hexyl free radic al isomers.