Spin-state dependent radical stabilization in nitrenes: the unusually small singlet-triplet splitting in 2-furanylnitrene

Geometries and energies of the triplet and singlet states of 2-furanylnitrene and 3-furanylnitrene have been calculated by using spin-flip coupled-cluster methods. Calculations with triple-ζ basis sets predict a singlet-triplet splitting of 10.9 kcal/mol for 2-furanylnitrene, 4.5 kcal/mol smaller than that in phenylnitrene. In contrast, the singlet-triplet splitting in 3-furanylnitrene is computed to be 1.9 kcal/mol larger than that in phenylnitrene. The differences in the singlet-triplet splittings for the furanylnitrenes are attributed to the differences in the radical stabilizing abilities of the 2-furanyl- and 3-furanyl-groups compared to a phenyl ring. Comparison of the singlet-triplet splittings of more than 20 substituted aromatic nitrenes and the radical stabilizing ability of the aromatic systems reveals a high degree of correlation between the singlet-triplet splitting and the radical stabilizing ability, indicating that singlet states of aromatic nitrenes are preferentially stabilized by radical stabilizing substituents. The preferential stabilization of the singlet states is attributed to the decrease in electron pair repulsion resulting from increased delocalization of the radical electron.