Calculation of the Anharmonicity in the Vibrational Frequencies of Alkane Molecules by the CFF Functions

The CFF method is extended to include all the first five terms of the Taylor expansion of the molecular potential functions. A scheme for anharmonicity calculations in any molecule, in every equilibrium conformation is suggested. The actual anharmonicity calculation for ethane, n-butane, and cyclohexane is described. It is shown that a mixed function of bond stretching and bond angle bending, with Morse type dependence on the bond length is suitable for such calculations. The advantage of using potential functions instead of a force constant field, which is a central idea of the CFF philosophy, is strongly supported by the calculations.