ROTATIONAL ISOMERIZATION OF 2,3-DIFLUOROPROPENE IN CRYOGENIC MATRICES - QUANTUM YIELDS, MATRIX EFFECTS, AND MODE SELECTIVITY

Selective interconversion of the cis and gauche isomers of 2,3-difluoropropene and 2,3-difluoropropene-d4 using IR lasers in Ar, Kr, and N2 matrices at 12 K has been demonstrated. This interconversion greatly facilitates assignment of the vibrational spectra of individual isomers. The quantum efficiencies for isomerization range from 10(-7) to 10(-4) and are dependent on photon energy and host matrix. In general, the interconversion quantum yields are ordered phi-N2 < phi-Ar < phi-Kr. The anomalously high isomerization yields associated with excitation of the C-F stretches near 1080 cm-1 suggest that these vibrations are more closely coupled to the reaction coordinate: the CH2F torsion rather than higher energy C = C stretches. In addition, the rates for the cis-2,3-difluoropropene --> gauche-2,3-difluoropropene conversion are generally higher than for the reverse process. This result is surprising in light of gas-phase measurements that indicate that the cis isomer is more stable than the gauche form by 145 cm-1.