KINETICS AND THERMOCHEMISTRY OF THE REVERSIBLE COMBINATION REACTIONS OF THE ALLYL AND BENZYL RADICALS WITH NO

The equilibrium kinetics of the reversible additions of NO to the benzyl (C6H5CH2) and allyl (CH2CHCH2) radicals have been studied at atmospheric pressure and over the temperature range 400 < T < 520 K. Experiments were performed using a flash photolysis/UV absorption technique with kinetic data being derived from numerical simulation of real-time decays of benzyl and allyl radicals, recorded both with and without the addition of excess nitric oxide. The equilibrium constants were therefore deduced as the ratio of the rate coefficients (k(1)/k(-1) and k(3)/k(-3)) for the forward and reverse processes: C6H5CH2 + NO + N-2 <----> C6H5CH2NO + N-2 (reactions 1, -1) and CH2CHCH2 + NO + N-2 <----> CH2CHCH2NO + N-2 (reactions 3, -3). Thermodynamic treatment of the data by both second law and third law methods of analysis yielded values for the enthalpy and entropy of reactions 1 and 3 which were self-consistent and hence the following average values: reaction 1, Delta H-298(0) = -123 +/- 5 kJ mol(-1) (thus Delta H-0(0) = -117 +/- 6 kJ mol(-1) and Delta H-f298(0) (C6H5CH2-NO) = 176 +/- 7 kJ mol(-1), Delta S-298(0) = -159 +/- 9 J K-1 mol(-1)); reaction 3, Delta H-298(0) = -110 +/- 5 kJ mol(-1) (thus Delta H-0(0) = -105 +/- 4 kT mol(-1) and Delta H-f298(0) (CH2CHCH2NO) = 148 +/- 8 kJ mol(-1), Delta S-298(0) = -154 +/- 9 kJ mol(-1)). The bond dissociation energies for these adducts (= -Delta H-298(0)) are compared and discussed in relation to those for other alkyl nitrosos. Also as part of the study, some measurements of the rate coefficients for the benzyl and allyl association reactions with NO (reactions 1 and 3) and their radical self-recombinations (reaction 4, C6H5CH2 + C6H5CH2, and reaction;5, CH2CHCH2 + CH2CHCH2) were made under similar experimental conditions, yielding (in units 10(-11) cm(3) molecule(-1): s(-1)): k(1)(415, 443, 466 K) = 0.91 +/- 0.08, 0.81 +/- 0.05, and 0.62 +/- 0.09, k(3)(403 K) = 0.71 +/- 0.04, k(4)(435-519 K) = 2.9 +/- 0.3, k(5)(403-540 K) = 2.6 +/- 0.2.