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 benz yl (C6H5CH2) and allyl (CH2CHCH2) radicals have been studied at atmosp heric pressure and over the temperature range 400 < T < 520 K. Experim ents were performed using a flash photolysis/UV absorption technique w ith kinetic data being derived from numerical simulation of real-time decays of benzyl and allyl radicals, recorded both with and without th e addition of excess nitric oxide. The equilibrium constants were ther efore 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 dat a 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-cons istent 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-f2 98(0) (CH2CHCH2NO) = 148 +/- 8 kJ mol(-1), Delta S-298(0) = -154 +/- 9 kJ mol(-1)). The bond dissociation energies for these adducts (= -Del ta 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 r ate coefficients for the benzyl and allyl association reactions with N O (reactions 1 and 3) and their radical self-recombinations (reaction 4, C6H5CH2 + C6H5CH2, and reaction;5, CH2CHCH2 + CH2CHCH2) were made u nder 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.