Vibrational excitation of ozone and molecular fluorine reactions in cryogenic matrices

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Attempts have been made to induce under cryogenic conditions reactions between O3 and C2H2, C2H4, NO2, and NO, and between F2 and C2H4, by selective vibrational excitation of nearest neighbor reactant pairs. Samples in inert gas or nitrogen matrices at 12 K were irradiated with grating-tuned cw CO2 and CO lasers. Only linewidth narrowing due to thermal effects was observed in the first three systems with laser excitation of ν3 of ozone, ν7 and ν7+ν8 of ethylene, or ν3 of NO2. However, the dark reaction NO+O3→NO2+O2 in nitrogen matrices could be significantly enhanced by exciting the NO stretch of NO⋅O3 pairs at 1874 cm−1. No products were detected upon prolonged excitation of ν7 of C2H4⋅F2 reactive pairs at 953 cm−1 whereas excitation of ν7+ν8 of ethylene at 1896 cm−1 did promote reaction with an approximate quantum yield of 10−4. The products are trans- and gauche-1,2-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 C2H4⋅F2 pairs produce predominantly the elimination products vinyl fluoride and HF, whereas an additional ethylene molecule in the reaction cage increases 100-fold the stabilization of the reaction adduct CH2F–CH2F°. The gauche/trans ratio of 1,2-difluorethane is near 2, close to the high temperature limit.