Singlet stabilization of oxazole- and isoxazolenitrene-n-oxides by radical delocalization

Spin-flip coupled-cluster methods have been used to calculate the structures and energies of singlet and triplet electronic states of all the geometrical isomers of isoxazolyl- and oxazolylnitrene-n-oxides. At the CCSD(T)/cc-pVTZ level of theory, 2-oxazolylnitrene-n-oxide and 5-isoxazolylnitrene-n-oxide are calculated to have the smallest singlet–triplet energy splittings, at 4.7 and 6.3 kcal/mol, respectively. The results for all isomers indicate that the singlet states are most effectively stabilized by having the nitrene nitrogen adjacent to the oxygen in the ring, and being able to delocalize π-spin density onto the nitroxyl oxygen. Empirical analysis of the spin dependent stabilization finds that the extent to which singlet states are stabilized is approximately twice that found for triplet states, suggesting that radical stabilizing ability can also be applied to singlet–triplet splitting.