Selective vibrational excitation of the ethylene–fluorine reaction in a nitrogen matrix. II

The product branching between 1,2-difluoroethane and vinyl fluoride (plus HF) of the selective vibrationally stimulated reaction of molecular fluorine with C2H4 has been studied in a nitrogen matrix at 12 K and found to be the same for five different vibrational transitions of C2H4 between 1896 and 4209 cm−1. The HF/DF branching ratio of the reaction of F2 with CH2CD2, trans-CHDCHD, and cis-CHDCHD was determined to be 1.1, independent of precursor C2H2D2 isomer and particular mode which excited the reaction. These results, as well as the analysis of the mixtures of partially deuterated vinyl fluoride molecules produced by each C2H2D2 isomer indicate that the product branching occurs by αβ elimination of HF(DF) from a vibrationally excited, electronic ground state 1,2-difluoroethane intermediate. Selective vibrational excitation of fluorine reactions in isotopically mixed matrices t-CHDCHD/C2H4/F2/N2 and CH2CD2/C2H4/F2/N2, and in matrices C2H2/C2H4/F2/N2 revealed a high degree of isotopic and molecular selectivity. The extent to which intermolecular energy transfer occurred is qualitatively explained in terms of dipole coupled vibrational energy transfer. A study of the loss of absorbance of the C2H4 ⋅ F2 pairs in case of ν9 as a function of both the laser irradiation frequency within the absorption profile, and the ethylene concentration showed that the C2H4 ⋅ F2 absorption is inhomogeneously broadened. Substantial depletion of reactive pairs which did not absorb laser light is interpreted in terms of Forster transfer.