A theoretical study of the infrared vibrational intensities of CH3F

The atomic polar tensors, vibrational frequencies, and infrared intensities were calculated for methyl fluoride using ab initio molecular orbital methods. Calculations were carried out using several split-valence basis sets (3-21G, D95, 6-31G, 6-311G) augmented with various numbers of diffuse and polarization functions. The CID/6-31G(d) optimized geometry was found to agree well with the experimental geometry and was used for all of the calculations. Atomic polar tensors were computed by analytical differentiation of the dipole moment at the Hartree–Fock level, and by numerical differentiation at the CISD level. The Hartree–Fock limit of the dipole moment is estimated to be 2.06±0.02 D at the CID/6-31G(d) optimized geometry. Compared with the observed, anharmonic vibrational frequencies, the calculated frequencies are an average of 8±4% too high at the HF/6-311++G(3d,3p) level, with larger errors for bending modes than for stretches. When correlation corrections are included the errors are significantly smaller and more uniform, 4.4±1.0%. Infrared intensities are much more sensitive to basis set effects than frequencies, requiring multiple sets of polarization functions, diffuse functions, and correlation corrections. A useful relation has been found between the calculated molecular dipole moment and the intensity of the CF stretching mode.