A b i n i t i o computation of force constants. IV. The theoretical anharmonic force fields and vibrational frequencies of methylamine, methanol, and methanethiol

The anharmonic force fields of methylamine, methanol, and methanethiol have been computed ab initio. The harmonic force constants agree fairly well with experiment. Better agreement is found between the experimental frequencies and the theoretical anharmonic frequencies. The stretching modes are predicted especially well. The calculated ν13 and ν14 for CH3NH2 agree with the solid phase but not the gas phase experimental results. Methylamine, methanol, and methanethiol exhibit Bohlmann bands, i.e., a lowering of the stretching frequency of a C–H bond antiperiplanar to a lone pair. The computations reveal that these bands are linked to a smaller force constant for the anti CH bond and to Fermi resonance between the symmetric C–H stretch and CH bending overtones.