Vibrational Assignments and Potential Constants for cis- and trans-1,2-Difluoroethylenes and Their Deuterated Modifications

Infrared and Raman spectra and a complete assignment of vibrational fundamentals are presented for cis‐ and trans‐1,2‐difluoroethylenes and their deuterated modifications. For cis‐CFHCFH the fundamentals are: ${\mathrm{(a}}_1)$ 3122, 1716, 1263, 1015, 237; ${\mathrm{(a}}_2)$ 839, 495; ${\mathrm{(b}}_1)$ 3136, 1374, 1130, 769; and ${\mathrm{(b}}_2)$ 756 cm⁻¹. For trans‐CFHCFH the fundamentals are: ${\mathrm{(a}}_g)$ 3111, 1694, 1286, 1123, 548; ${\mathrm{(a}}_u)$ 875, 329; ${\mathrm{(b}}_g)$ 788; and ${\mathrm{(b}}_u)$ 3114, 1274, 1159, 341 cm⁻¹. The near degeneracy in ${\nu }_7$ and ${\nu }_{12}$ of the trans isomer is shown from low‐temperature crystal and matrix studies and zero‐order normal coordinate calculations. Analysis of the rotational structure in the region of the overlapped bands proves that these fundamentals are strongly perturbed by Coriolis coupling. From the vibrational fundamentals and a previous measurement of ${\mathrm{\Delta H°}}_{650}$ , the cis isomer is shown to have 1086 cal/mole less electronic energy than the trans isomer. For the in‐plane vibrations refined Urey–Bradley potential constants are presented. These constants are obtained from an overlay calculation involving the full set of six molecules. For the out‐of‐plane vibrations, refined general valence constants, obtained for the separate isomers, are presented.