Ground States of Conjugated Molecules. VII. Compounds Containing Nitrogen and Oxygen

Abstract

The semiempirical SCF—MO procedures described in previous papers of this series have now been extended to compounds containing heteroatoms, in particular nitrogen and oxygen. This is a treatment based on the Hückel σ/π approximation, in which the total energy of the σ electrons is approximated by assuming additivity of bond energies (allowance being made for variations of these with length by using Morse functions) and in which the π energy is calculated by a Pople-type SCF—LCAO—MO treatment with neglect of differential overlap and penetration integrals. The problem of estimating the remaining integrals is discussed in detail; valence-state ionization potentials and one-center repulsion integrals are determined empirically by standard methods from spectroscopic data, but a new procedure is used for estimating effective nuclear charges. Two sets of values are used for the remaining repulsion integrals, one closely similar to that recommended by Pariser and Parr, the other corresponding to a modified SPO treatment. Calculations are reported for a wide range of conjugated and aromatic compounds of nitrogen and oxygen, including azines, azoles, furans, pyrylium, and quinones. Calculated heats of formation and bond lengths seem to be in satisfactory agreement with experiment in the cases where such data are available and the results seem in general to be much superior to those given by previous methods.