LASER FLASH-PHOTOLYSIS DETERMINATION OF ABSOLUTE RATE CONSTANTS FOR REACTIONS OF BROMINE ATOMS IN SOLUTION

The photodecomposition of vicinal dibromides at 266 nm produces bromine atoms with a quantum yield of approximately 2.0. This results from an efficient primary photocleavage of a C-Br bond, followed by rapid elimination of a second bromine atom from radicals of the type RCH-CH2Br. This cleavage occurs with a lifetime of <20 ns at room temperature. Bromine atoms react with bromine ions with a rate constant of 1.6 X 10(10) M-1 s-1 to yield Br2.-, an easily detectable and long-lived radical ion. This reaction can be used as a probe in order to determine absolute rate constants for other reactions of bromine atoms. For example, the rate constants for methanol, 2-propanol, and triethylamine are 9.3 x 10(5), 4.1 x 10(7), and 2.9 x 10(10) M-1 s-1, respectively. It is suggested that these hydrogen atom transfer reactions may involve a considerable degree of charge transfer. Alpha-bromoacetophenone also serves as a convenient bromine atom source in those cases where vicinal dibromides cannot be employed. The advantages and disadvantages of using probe techniques in the determination of absolute rate constants are discussed in some detail.