LASER FLASH-PHOTOLYSIS STUDY OF THE BR + I-C4H10 REVERSIBLE HBR + T-C4H9 REACTION - HEAT OF FORMATION OF T-C4H9

The heat of formation of the tert-butyl radical has been determined in two independent ways using laser flash photolysis coupled with detection of Br by resonance fluorescence. The t-C4H9 + HBr reaction was studied by monitoring the rise of the Br product at room temperature following photolysis of (t-C4H9)2N2 at 351 nm. Combination of the rate constant with k(Br+i-C4H10) gives DELTA-H formal f,298(t-C4H9) = 50.9 +/- 3.5 kJ mol-1. The measured value of k(t-C4H9 + HBr) ((3.2 +/- 1.0) x 10(-11) cm3 molecule-1 s-1) is in reasonable agreement with the measurements of Russell et al. and of Richards et al. Equilibration in the reaction system Br + i-C4H10 reversible HBr + t-C4H9 was studied by generating Br by photolysis of CF2Br2 at 248 nm in the presence of i-C4H10 and HBr and observing the relaxation of [Br]. These experiments were conducted at 573 and 641 K. The measurements yield data for both forward and reverse rate constants, for the equilibrium constant, and hence for DELTA-H formal f,298(t-C4H9), giving values of 44.2 +/- 4.0 and 48.1 +/- 4.0 kJ mol-1 at the respective temperatures. Combination of the three independent estimates gives DELTA-H formal f,298(t-C4H9) = 47.3 +/- 3.5 kJ mol-1. The values for k(t-C4H9 + HBr) at these elevated temperatures agree with the data of Russell et al. and demonstrate a negative temperature dependence in the rate constant.