Application of self-consistent-field <i>ab initio</i> calculations to organic molecules

Abstract

Self-consistent-field ab initio calculations of the equilibrium geometries, r calc e, and quadratic force constants for methane, ethane, propane, ethene, cyclopropane and cyclopropene have been carried out using an extended basis of contracted gaussian functions (4–31G). In the case of unstrained saturated molecules or parts of molecules a simple (anharmonicity) correction yields calculated structures for which the average deviation with ‘best’ experimental bond lengths and angles is 0·003 Å and 0·4° respectively. Calculated carbon-carbon double bonds are typically too short by a constant amount (0·022 Å). The average deviation of calculated and experimental bond angles about CH2 groups in three membered rings and those about sp2 hybridized carbons are 1·5° and 1·0° respectively. Quadratic force constants for symmetric stretchings are compared with force constants deduced from vibrational analysis. Trends and differences are discussed.