Ground state multiplicity of acylnitrenes: computational and experimental studies

The singlet—triplet energy splitting (ΔE_ST = E_S − E_T) was calculated for formylnitrene (5) and for the syn- and anti-rotamers of carboxynitrene HOC(O)N (6) by the CCSD(T) method. Extrapolation of ΔE_ST to a complete basis set was calculated to be negative for 5 and strongly positive for 6. Similar results were obtained by the G2 procedure. The reason for the dramatic stabilization of the singlet state appeared to be a special bonding interaction between the nitrogen and oxygen atoms, which results in the structure intermediate between those of nitrene and oxazirene. It was found that the B3LYP/6-31G(d) method overestimates ΔE_ST by ∼9 kcal mol⁻¹ for 5 and by ∼7 kcal mol⁻¹ for 6. Taking into account this overestimation and the results of DFT calculations, it was concluded that benzoylnitrene has a singlet ground state. It was proved experimentally using photolysis of benzoyl azide in an argon matrix at 12 K that benzoylnitrene has a singlet ground state and its structure is similar to that of oxazirene. Nevertheless, these singlet intermediates have low barrier to the aziridine formation, which is traditionally considered to be a typical singlet nitrene reaction.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 519–526, March, 2005.