KINETICS OF THE REACTIONS OF ALKYL RADICALS WITH HBR AND DBR

Time-resolved resonance fluorescence detection of Br atom appearance following laser flash photolysis of RI (R = CH3, CD3, C2H5, t-C4H9) or Cl2/RH (R = CH3, C2H5) has been employed to study the kinetics of the reactions CH3 + HBr (1), CD3 + HBr (2), CH3 + DBr (3), C2H5 + HBr (4), C2H5 + DBr (5), t-C4H9 + HBr (6), and t-C4H9 + DBr (7) as a function of temperature (257-430 K) and pressure (10-300 Torr of N2). The rates of all reactions are found to increase with decreasing temperature; i.e., activation energies are negative, and 298 K rate coefficients for reactions 1 and 3-7 are found to be significantly faster than previously thought. Arrhenius expressions for reactions 1, 3, 4, and 6 in units of 10(-12) cm3 molecule-1 s-1 are k1 = (1.36 +/- 0.10) exp[(233 +/- 23)/T], k3 = (1.07 +/- 0.17) exp[(130 +/- 55)/T], k4 = (1.33 +/- 0.33) exp[(539 +/- 78)/T], and k6 = (1.07 +/- 0.34) exp[(963 +/- 152)/T]; errors are 2-sigma and represent precision only. Normal kinetic isotope effects are observed (k(HBr) > k(DBr), although the ratio k(HBr/k(DBr) decreases in magnitude with decreasing activation energy; i.e., k(HBr)/k(DBr) is largest for R = CH3 and smallest for R = t-C4H9. Combining our results with the best available kinetic data for the reverse reactions (Br + RH) yields the following 298 K alkyl radical heats of formation in units of kcal mol-1: CH3, 35.3 +/- 0.6; C2H5, 29.1 +/- 0.6; t-C4H9, 12.1 +/- 0.8; errors are 2-sigma and represent estimates of absolute accuracy.