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

Abstract

AbstractThe 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.