A molecular orbital theory of hydrocarbons

Abstract

An independent electron molecular orbital theory is applied to all valence electrons of saturated hydrocarbons, using the 2s and 2p atomic orbitals of carbon and the 1s orbitals of hydrogen as a basis. It is shown that certain results about uniformity of charge distribution, already known for π-electrons, apply to all valence electrons in paraffins under conditions more general than those which lead to localized bonding. An attempt is then made to estimate the extent of electron delocalization in paraffins by calculating long-range molecular orbital bond orders and the associated contributions to energy stabilization. This is done by a perturbation method, the features causing delocalization being introduced as a perturbation of a theory of completely localized bonds. It is concluded that delocalization may arise from (1) the difference between atomic 2s and 2p energies, (2) bonding between non-neighbouring atoms and (3) partial π-bonding in carbon-carbon bonds. The first two causes lead to geminal interactions, but tend to cancel each other. Partial π-bonding leads to vicinal delocalization which may be considerable and is greatest in the trans configuration. The total energy correction due to delocalization is calculated and found to be quite large, but this is shown to be consistent with the established approximate additivity of bond energies.