Vibrational spectra and assignments for cis- and trans-1,2-difluorocyclopropane and three deuterium substituted modifications of each isomer

Infrared spectra under a variety of experimental conditions and Raman spectra for the liquid phase have been recorded for the cis and trans isomers of 1,2-difluorocyclopropane and the 1,2-d2, 3,3-d2, and d4 deuterated modifications. Almost all of the 168 fundamentals of this set of eight molecules have been observed and assigned convincingly. For cis-1,2-difluorocyclopropane-d0 the fundamentals are (a') 3105, 3063, 3023, 1450, 1365, 1224, 1135, 1047, 862, 784, 468, 209 cm-1; (a'') 3055, 1346, 1150, 1089, 1060, 993, 739, 621, 319 cm-1. For trans-1,2-difluorocyclopropane-d0 the fundamentals are (a) 3070, 3021, 1457, 1380, 1203, 1132, 1068, 961, 842, 415, 279 cm-1; (b) 3110, 3070, 1304, 1161, 1072, 1005, 937, 783, 452, 303 cm-1. Only the 1089-cm-1 mode in the cis and the 937-cm-1 mode in the trans are in doubt. Group frequency descriptions are discussed qualitatively, and attention is drawn to the fundamentals near 1450 cm-1 which are characteristic of fluorocyclopropanes and appear to be largely due to ring stretching. From previously published equilibrium data for the cis-to-trans isomerization and the fundamental frequencies of the two isomers, an electronic energy difference of -2800 +- 200 cal/mol has been calculated. In contrast, the trans isomer of 1,2-difluoroethylene has an energy 1100 cal/mol greater than the cis, and the trans rotamer of 1,2-difluoroethane has an energy about 600 cal/mol greater than the gauche. (auth)