Molecular Orbital Theory of the Electronic Structure of Organic Compounds

Abstract

The semiempirical molecular orbital CNDO/S-CI spectral parameterization has been used to elucidate the lower electronic states of a series of dioxodiazacycloalkanes. The a 1 , b 1 , and a' occupied molecular orbitals lie predominantly on the oxygen, while a 2 , b 2 , and a" are largerly nonbonding orbitals delocalized on the nitrogen and oxygen atoms. The two lowest unoccupied virtual orbitals are predicted to be of b 2 , a 2 , and a" symmetry. These orbitals are strongly localized on the C = 0 group. The resulting 1 B 2 (u 0 π*) and 1 A 2 (n 0 π*) spectroscopic states involve intramolecular charge transfer from the oxygen to the carbon atom of the carbonyl group, which is supported by electron density calculations of these excited states. Although the calculated transition energies may not allow for absolute comparisons with experimental values, it appears that the introduction of self-consistency together with solvation energy and configuration interaction, when the elements of the interaction matrix are properly evaluated, lead to a fairly good interpretation of the singlet-singlet transitions. The lowest energy singlet excited state calculated for each structure is comprised almost entirely of the nπ* configuration.