Semi-empirical Unrestricted SCF-MO Treatment for Valence Electron Systems. I. Application to Small Doublet Radicals

Abstract

Abstract A semi-empirical SCF method for valence electron systems including a differential overlap previously proposed by the present authors is here extended to molecules with an open-shell structure. The unrestricted Hartree-Fock method is applied, and the one-centre and part of two-centre σ-π-type exchange repulsion integrals, playing an essential role in spin-density calculations, are included in the calculations. The calculated spin densities are divided into the mechanistic (spin-polarization and spin-delocalization) contributions. The spin-polarization mechanism is shown to be important even in cases where the spin delocalization mechanism has usually been considered to be dominant (e.g., ethyl and vinyl radicals). The calculated spin densities of π-electron radicals (methyl, ethyl, allyl, and trans-butadienyl anion radicals) and of σ-electron radicals (vinyl, formyl, NO2, CO2−, CN) are discussed. Their hfs constants and mechanistic contributions are calculated; for the methyl and vinyl radicals these are shown to be strongly angular-dependent. The calculated potential curve and the hfs constants of the vinyl radical lead to the CCHa angle, θ\simeq135°; furthermore, the Hα and Cβ hfs constants are shown to be negative. Generally, the calculated proton hfs constants agree satisfactorily with the experimental data and with other calculations except in the case of the formyl radical. The atomic dipoles of some σ-electron radicals are calculated, and some interesting features common to all the σ-electron radicals studied are found.