Vibrational and quantum chemical studies of 1,2-difluoroethylenes:: Spectra of 1,2-13C2H2F2 species, scaled force fields, and dipole derivatives

Infrared and Raman spectra are reported for cis and trans 1,2-(C2H2F2)-C-13. Quantum chemical calculations at B3LYP and MP2 levels have been made for cis and trans 1,2-difluoroethvlenes, using 6-311G** and 6-311++G** bases. The resulting harmonic force fields for each compound were scaled with nine independent factors, using frequency data corrected, where necessary, for Fermi resonances and for liquid/gas shifts. The previously accepted assignments for nu(7) and nu(12) in the trans isomer are interchanged. Several scale factors for bending motions differ markedly between the cis and trans compounds. Centrifugal distortion constants observed with significant accuracy are predicted within 6%. Harmonic contributions to the vibrational dependence of the rotational constants and perpendicular amplitudes are calculated. The C=C and C-H bonds in the cis and trans isomers are essentially identical in respect to length, force constant, and isolated CH stretching frequency. However, the C-F bond is slightly stronger in the cis compound. A revised allocation of observed infrared intensity between the overlapping trans nu(7) and nu(12) bands is needed. Directions of partial derivativep/partial derivativeQ for the trans B-u bands are given, which differ from those reported earlier. Magnitudes and directions of the bond dipole derivatives partial derivativemu/partial derivativer for the CH and CF bonds are obtained from calculated atomic polar tensors. In both cis and trans isomers, the vector partial derivativemu/partial derivativer(CF) lies within 17degrees of the bond direction, but for the CH bond, partial derivativep/partial derivativer is roughly perpendicular to the CH direction.