An ab initio study of the vibrational frequencies and infrared intensities of CH2F2

The infrared vibrational frequencies and intensities were calculated for methylene fluoride using ab initio molecular orbital theory. Calculations were carried out using several split valance basis sets (3-21G, 4-31G, 6-31G, 6-311G) augmented with diffuse functions and several sets of polarization functions. The CID/6-31G(d) optimized geometry was found to agree well with the experimental Re structure and was used for all of the frequency calculations. The atomic polar tensors were calculated by differentiation of the dipole moment at the Hartree–Fock and MP2 levels. The HF/6-311++G(3d,p) dipole moment is 2.20 D; the dipole moment is 2.12 D at the MP2/6-311+G(d,p) level and is estimated to be 1.90 D at the MP2/6-311++G(3d,3p) level, compared to the experimental value of 1.9785±.0021 D. The average error in the calculated harmonic frequencies is 5% at HF/6-311++G(3d,p), 1.9% at MP2/6-311+G(d,p), and 1.3% at the estimated MP2/6-311++G(3d,3p) level (compared to the experimental harmonic frequencies). The infrared intensities are more sensitive to the size of the basis set than the frequencies. The average error in the calculated double harmonic IR intensities is 26 km/mol at HF/6-311++G(3d,p), 7.8 km/mol at MP2/6-311+G(d,p) and 3.1 km/mol at the estimated MP2/6-311++G(3d,3p) level (compared to the observed values for A2, A4, A8, A1+A6, and A3+A7+A9 ).