N-15 AND O-18 KINETIC ISOTOPE EFFECTS IN THE THERMAL-DECOMPOSITION OFN2O CATALYZED BY BROMINE

N-15 and O-18 kinetic isotope effects (KIEs) in the thermal decomposit ion of N2O catalyzed by bromine were experimentally determined in the temperature range 773-873 K, resulting in KIE (N-15) = -2.07 + 4020/T and KIE (O-18) = -0.41+3290/T. For theoretical interpretation, based o n the Bigeleisen formalism, the following planar transition states wer e taken into account: trans (N-N-O-Br)double dagger, trans (Br-N-N-O)d ouble dagger, and branched (N-N < O/Br)double dagger. In addition to K IE, activation energy according to the Sanderson bond-energy-bond-orde r relationship and pre-exponential factor were calculated as subsidiar y parameters in selecting an appropriate transition state among all pr obable configurations. The result reveals that it is meaningless to sp eculate whether the Br atom approaches the central or the terminal N a tom of the N2O molecule, since both transition states could generate a cceptable values of the kinetic isotope effects, activation energy, an d pre-exponential factor.