Diode laser probes of tert-butyl radical reaction kinetics: Reaction of C(CH₃)₃^• with HBr, DBr, and HI

The rate constants for reaction of tert-butyl radical with HBr, DBr, and HI have been measured by use of a tunable infrared diode laser probe. The measured rate constants at room temperature are 1.0 × 10^-11, 8 X 10^-12, and 2.5 × 10^-11 cm³ molecule^-1 s^-1, respectively. The reaction with HI exhibits a negative activation energy of 1.5 kcal mol^-1. The reaction rates of HBr and DBr with tert-butyl radical at elevated temperatures are also slower than the corresponding rates at room temperature. The rate constant for reaction of tert-butyl radicals with HBr is in excellent agreement with the faster of the two conflicting previously reported measurements for the tert-butyl-HBr system. From this study the heat of formation of the tert-butyl radical has been calculated to be 11.5 ± 0.8 kcal mol^-1. The negative activation energy is consistent with the reaction proceeding through a weakly bound complex.