Attempts have been made to induce under cryogenic conditions reactions between O3and C2H2, C2H4, NO2, and NO, and between F2and C2H4, by selective vibrational excitation of nearest neighbor reactant pairs. Samples in inert gas or nitrogen matrices at 12 K were irradiated with gratinghyphen;tuned cw CO2and CO lasers. Only linewidth narrowing due to thermal effects was observed in the first three systems with laser excitation of ngr;3of ozone, ngr;7and ngr;7+ngr;8of ethylene, or ngr;3of NO2. However, the dark reaction NO+O3rarr;NO2+O2in nitrogen matrices could be significantly enhanced by exciting the NO stretch of NOsdot;O3pairs at 1874 cmminus;1. No products were detected upon prolonged excitation of ngr;7of C2H4sdot;F2reactive pairs at 953 cmminus;1whereas excitation of ngr;7+ngr;8of ethylene at 1896 cmminus;1did promote reaction with an approximate quantum yield of 10minus;4. The products aretranshyphen; andgauchehyphen;1,2hyphen;difluorethane, vinyl fluoride, and HF. Kinetic studies revealed first order kinetics with linear dependence of the rate constant on laser intensity, which indicates a single photon photochemical process. The observed products are consistent with a molecular four center reaction mechanism, rather than the gas phase free radical mechanism. Isolated C2H4sdot;F2pairs produce predominantly the elimination products vinyl fluoride and HF, whereas an additional ethylene molecule in the reaction cage increases 100hyphen;fold the stabilization of the reaction adduct CH2Fndash;CH2Fdeg;. Thegauche/transratio of 1,2hyphen;difluorethane is near 2, close to the high temperature limit.
展开▼
