Rate Constants for the Reactions t-C4H9+DX→i-C4H9D+X(X = Br,I), 295T(K) < 384: Heat of Formation of the tert-Butyl Radical

Absolute values for the rate constants of the metathesis reactions of DX (X = Br, I) with tert-butyl, generated by 351-nm photolysis of 2,2{prime}-azoisobutane, were determined in a low-pressure Knudsen cell reactor using the VLP{Phi} (very low pressure photolysis) technique. For X = Br, the values are 10{sup {minus}8}k (M{sup {minus}1} s{sup {minus}1}) = 0.9 and 2.3, and for X = I the values are 2.1 and 3.1 at 295 and 384 K. The latter are in good agreement with earlier measurements from this laboratory. The former values are lower by a factor of 50 compared to those recently reported at 295 K and fail to show the negative activation energy found in that work. Correcting by {radical}2 for the primary isotope effect and combining with the rate constants for the reverse reactions lead to {Delta}H{sub f}{degree} {sub 298}(tert-butyl) (kcal mol{sup {minus}1}) = 9.2 {plus minus}0.5 in both cases. First-order heterogeneous loss of tert-butyl radicals on the Teflon-coated surface employed is small (< 2 s{sup {minus}}1). 17 refs., 9 figs., 3 tabs.