Application of self-consistent-fieldab initiocalculations to organic molecules

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Abstract
The complete GVFF of ethane, propane and cyclopropane has been calculated from ab initio energies using an extended (4–31G) basis set. A scale factor method was developed to scale down the ab initio force constants in order to reproduce the experimental vibrational frequencies. The latter were taken from the literature. It was found that scale factors were transferable in the present series of molecules. A total of 180 vibrational frequencies of ethane, propane and cyclopropane and of a series of deuterated analogues were calculated using six scale factors. The average difference between the observed and the calculated frequencies amounted to 10·4 cm-1 or 0·74 per cent. A comparison was made with force constants obtained from minimal basis set (STO-3G) and INDO calculations.