Quantum-Mechanical Calculations of Molecular Force Fields at the SCF Ab Initio Level Methyl Halides as Test Cases

The possibilities of the present quantum-chemistry methods for calculating the vibrational properties of polyatomic molecules have been investigated, taking the CH₃X series (X = F, Cl, Br, I) as bench tests. Finite-difference computations carried out with various basis sets at the SCF ab initio level on methyl fluoride bring out the performance of the LCAO-MO method as concerns the prediction of molecular force constants and integrated infrared intensities. Extension of these calculations to other halides is made possible using pseudo-potentials for the interaction between the atomic cores and the valence-shell electrons of the molecule. The atomic force field parameters defined in terms of Cartesian displacement coordinates suffer from a certain lack of transferability, apparently due to the variations of the chemical bonding effects when passing from fluorine to iodine.