TEMPERATURE-DEPENDENT KINETICS STUDIES OF THE REACTIONS BR(2P3/2) + H2S REVERSIBLE SH + HBR AND BR(2P3/2) + CH3SH REVERSIBLE CH3S + HBR - HEATS OF FORMATION OF SH AND CH3S RADICALS

Time-resolved resonance fluorescence detection of Br(2P3/2) atom disappearance or appearance following 266-nm laser flash photolysis of CF2Br2/H2S/H2/N2, CF2Br2/CH3SH/H2/N2, Cl2CO/H2S/HBr/N2, and CH3SSCH3/HBr/H2/N2 mixtures has been employed to study the kinetics of the reactions Br(2P3/2) H2S reversible SH + HBr (1, -1) and Br(2P3/2) + CH3SH reversible CH3S + HBr (2, -2) as a function of temperature over the range 273-431K. Arrhenius expressions in units of 10(-12) cm3 molecule-1 s-1 which describe the results are k1 = (14.2 +/- 3.4) exp[(-2752 +/- 90)/T], k-1 = (4.40 +/- 0.92) exp[(-971 +/- 73)/T], k2 = (9.24 +/- 1.15) exp[(-386 +/- 41)/T], and K-2 = (1.46 +/- 0.21) exp[(-399 +/- 41)/T]; errors are 2-sigma and represent precision only. By examining Br(2P3/2) equilibration kinetics following 355-nm laser flash photolysis of Br2/CH3SH/H2/N2 mixtures, a 298 K rate coefficient of (1.7 +/- 0.5) x 10(-10) cm3 molecule-1 s-1 has been obtained for the reaction CH3S + Br2 --> CH3SBr + Br. To our knowledge, these are the first kinetic data reported for each of the reactions studied. Measured rate coefficients, along with known rate coefficients for similar radical + H2S, CH3SH, HBr, Br2 reactions are considered in terms of possible correlations of reactivity with reaction thermochemistry and with IP - EA, the difference between the ionization potential of the electron donor and the electron affinity of the electron acceptor. Both thermochemical and charge-transfer effects appear to be important in controlling observed reactivities. Second and third law analyses of the equilibrium data for reactions 1 and 2 have been employed to obtain the following enthalpies of reaction in units of kcal mol-1: for reaction 1, DELTA-H298 = 3.64 +/- 0.43 and DELTA-H0 = 3.26 +/- 0.45; for reaction 2, DELTA-H298 = -0.14 +/- 0.28 and DELTA-H0 = -0.65 +/- 0.36. Combining the above enthalpies of reaction with the well-known heats of formation of Br, HBr, H2S, and CH3SH gives the following heats of formation for the RS radicals in units of kcal mol-1: DELTA-H(f)-degrees 0(SH) = 34.07 +/- 0.72, DELTA-H(f)-degrees 298(SH) = 34.18 +/- 0.68, DELTA-H(f)-degrees 0(CH3S) = 31.44 +/- 0.54, DELTA-H(f)-degrees 298(CH3S) = 29.78 +/- 0.44; errors are 2-sigma and represent estimates of absolute accuracy. The SH heat of formation determined from our data agrees well with literature values but has reduced error limits compared to other available values. The CH3S heat of formation determined from our data is near the low end of the range of previous estimates and is 3-4 kcal mol-1 lower values derived from recent molecular beam photofragmentation studies.