Abinitio MO predictions for the geometries and energies of the Σ and II states of some conjugated free radicals

Abstract

Abinitio MO calculations are reported for the lowest Σ and Π doublet states of the radicals 1–10. Configuration interaction is included, using the restricted open-shell MO's as a base. The important geometric variables are optimized in calculations using the STO-3G basis set; in some cases energies and geometries are redetermined using the 4-31G basis. In most cases the energy separation between the Σ and Π doublet states is calculated to be small; the results are very dependent upon the basis set whenever the radical is localized upon a nitrogen atom. For radicals XYZ with different terminal groups, a valid prediction of the structure is obtained only if the basis set properly distinguishes between the π energies of X=Y and Y=Z bonds. Small basis sets appear to underestimate the π energy of carbonyl bonds, and thus are overbiased in favour of X=C—O••structures. Unsymmetrical structures are predicted for the Σ states of some radicals, even when the terminal groups are identical.