Theoretical studies of spin density populations on nitroxide and nitronylnitroxide derivatives

Spin densities for nitroxide and nitronylnitroxide derivatives were calculated by the UHF, UMP2, QCID, QCISD and UKS B(S)-LYP methods. Ab initio QCISD and UKS B-LYP methods provided similar spin populations for alkyl nitroxides, whereas the former overestimated a negative spin density induced on the α-carbon atom of nitronylnitroxides by the spin polarization effect. The approximately spin-projected UHF(APUHF)/6-31G* method provided a reasonable negative spin density and almost equal distributions of the spin densities on nitrogen and oxygen in phenyl nitronylnitroxide, in accord with experiment. The spin populations on the benzene ring in phenyl nitronylnitroxides by the APUHF and UKS B-LYP/6-31G* methods were similar to the experimental values.