Kinetics and photochemistry of transient free-radicals

Abstract

We report here several experimental and theoretical studies of free-radicals in the gas phase. Bimolecular rate constants for the reactions BH + NO → products k = 1.26 →!± 0.07 x 10^10 cm^3 s^-1 BH + C2H4 → products k = 1.41 →!± 0.09 x 10^10 cm^3 s^-1 CN + NH3 → HCN + NH2 k = 3.0 →!± 0.4 x 10^11 cm^3 s^-1 CN + ND2 → DCN + ND2 k = 1.5 →!± 0.3 x 10^11 cm^3 s^-1 have been measured and upper limits to the rate constants for the reactions BH + CO → products k < 1.1 x 10^11 cm^3 s^-1 BH + CH2H4 → products k < 1.6 x 10^12 cm^3 s^-1 BH + CxH6 → products k < 9.1 x 10^13 cm^3 s^-1 BH + O2 → products k < 1.3 x 10^13 cm^3 s^-1 have been set. BH (1𝚺) radicals were generated by excimer-laser photolysis of diborane at 193 nm and detected by time-resolved laser-induced fluorescence. The effects of varying the temperature and the nature of the buffer gas on the reactions of BH with NO and C2H4 have been investigated. CN (2𝚺+) radicals were generated by excimer-laser photolysis of C2N2 at 193 nm and their time evolution was monitored by absorption spectroscopy. In addition to the rate constant measurements, the products of the reactions of CN with NH3and ND3 have been determined using mass spectrometry and infrared absorption spectroscopy. The methods of ab initio molecular orbital theory have been used to determine structures and relative energies of species which could conceivably be involved in these reactions. The results of all studies have been interpreted in relation to the mechanisms of the reactions concerned. Prompt emission observed during the 193 nm excimer-laser photolyses of diborane and stannane has been characterised and the identity of the probable emitting species for each band observed has been assigned on the basis of band structure, known thermochemistry and the dependence of band intensity on photolysis laser power. The results obtained have been interpreted in terms of the nature of the photodissociation processes.