| Abstract
| - Infrared and Raman spectra are reported for cis and trans 1,2-13C2H2F2. Quantum chemical calculations atB3LYP and MP2 levels have been made for cis and trans 1,2-difluoroethylenes, using 6-311G** and6-311++G** bases. The resulting harmonic force fields for each compound were scaled with nine independentfactors, using frequency data corrected, where necessary, for Fermi resonances and for liquid/gas shifts. Thepreviously accepted assignments for ν7 and ν12 in the trans isomer are interchanged. Several scale factors forbending motions differ markedly between the cis and trans compounds. Centrifugal distortion constants observedwith significant accuracy are predicted within 6%. Harmonic contributions to the vibrational dependence ofthe rotational constants and perpendicular amplitudes are calculated. The CC and C−H bonds in the cisand trans isomers are essentially identical in respect to length, force constant, and isolated CH stretchingfrequency. However, the C−F bond is slightly stronger in the cis compound. A revised allocation of observedinfrared intensity between the overlapping trans ν7 and ν12 bands is needed. Directions of ∂p/∂Q for the transBu bands are given, which differ from those reported earlier. Magnitudes and directions of the bond dipolederivatives ∂μ/∂r for the CH and CF bonds are obtained from calculated atomic polar tensors. In both cis andtrans isomers, the vector ∂μ/∂rCF lies within 17° of the bond direction, but for the CH bond, ∂μ/∂r is roughlyperpendicular to the CH direction.
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