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 1,2-¹³C₂H₂F₂. Quantum chemical calculations at B3LYP and MP2 levels have been made for cis and trans 1,2-difluoroethylenes, 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 v₇ and v₁₂ 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 with 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 v₇ and v₁₂ bands is needed. Direction of ∂p/∂Q for the trans B_u bands are given, which differ from those reported earlier. Magnitudes and directions of the bond dipole derivatives ∂μ/∂r for the CH and CF bonds are obtained from calculated atomic polar tensors. In both cis and trans isomers, the vector ∂μ/∂r_CF lies within 17° of the bond direction, but for the CH bond, ∂μ/∂r is roughly perpendicular to the CH direction.