Comparison of theab initioforce constants of ethane, ethylene and acetylene

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Abstract
The force constants of ethane and acetylene have been calculated from Hartree-Fock-Roothaan SCF wavefunctions by the force method, using the following gaussian basis set: C : 7s,3p, H : 3s, CH midpoint : 1s, CC midpoint : 1s, 1p. Agreement with experiment is good. It is important that all significant coupling constants agree in sign and magnitude with the experimental data. The assumptions of the hybrid orbital force field are shown to be in qualitative agreement with the ab initio results. The calculations confirm the experimentally observed large bend/bend interaction constants. Localized orbitals have been calculated at some distorted nuclear conformations of ethylene and a qualitative orbital following model is given which explains the signs of the significant coupling force constants. Molecular geometries have been determined by the force relaxation method. The results agree well with experiment.