Consequences of accidental degeneracy within density functional theory: the enigmatic structure of boron nitrosyl

Boron nitrosyl was examined with DFT, MP2, CCSD(T), and CASSCF. Poor overlap of the atomic orbitals on boron with the molecular orbitals of NO causes an accidental near degeneracy of the highest occupied σ- and π-orbitals. A near degeneracy of three electronic states results. DFT locates all three electronic states but finds unstable solutions for the 3Σ− and 3Π linear isomers. CASSCF/6-311+G(3df) and MP2/6-311+G(2df) predict 3Σ− and 3Π isomers but do not locate the 3A″ bent structure. CCSD(T)/6-311+G(d), predicts the 3A″ bent isomer to be 1.7 and 4.2 kcal/mol lower than the 3Σ− and 3Π linear isomers, respectively.