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- 13 C 2 H 2 F 2 . 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 ν 7 and ν 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 ν 7 and ν 12 bands is needed. Directions 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.