Density Functional and Multireference Perturbation Theory Calculations of Bis-nitronyl Nitroxide Biradical

Abstract

The electronic structure of bis-nitronyl nitroxide biradical has been investigated by using density functional theory (DFT) and ab initio multireference (MCSCF and MCQDPT2) methods. The broken-symmetry DFT approach has been applied to the open-shell singlet biradical at the BLYP, B3LYP, and BHandHLYP levels. DFT and MCSCF calculations show that the singlet biradical state is slightly more stable than the triplet state, which is refined at the MCQDPT2 level. The dependence of the singlet-triplet energy gap on the conformational change between two monoradicals is discussed. The results indicate that the choice of the theoretical model plays an important role in understanding the electronic nature of the organic biradicals.