Hybrid High Low Frequency Separation (HLFS) method: Harmonic force fields of methyl halides

A parameter formulation of Wilson's High Low Frequency Separation (HLFS) Method is given in conjunction with isotopic frequencies in a third order vibrational problem. A new approximation scheme called the Hybrid HLFS Method is proposed which is superior to the pure HLFS method in the sense that it enables the evaluation of all twelve force constants. This, however, furnishes two sets of force fields out of which one is seen to be in better agreement with that obtained using various additional data.One of the authors (AJV) is grateful to the CSIR, New Delhi, for the award of a Junior Research Fellowship.