Application of self-consistent-field ab initio calculations to organic molecules

Abstract

Self-consistent-field ab initio calculations of the equilibrium geometries, r _calc ^e , of water, methanol and dimethyl ether have been carried out with a 4-31 G basis set. Results were related to experimental r^g and r^s geometries. Around a ‘standard geometry’ based on both experimental and calculated geometries the complete General Valence Force Field of water and methanol was determined from 4-31 G energies. A previously developed scale factor method has been applied to scale down the ab initio force constants on harmonic and experimental frequencies of water and on the experimental frequencies of methanol. In the latter case, seven scale factors were adjusted on 123 vibrational frequencies of methanol and deuterated analogues. The average difference between observed and calculated frequencies amounted to 9·9 cm^-1 or 0·67 per cent. The results suggest some misassignments in the CH-stretching regions of CH₃OH, CH₃OD, CH₂DOH and CH₂DOD.