CuNO2 and Cu+NO2 revisited:: A comparative ab initio and DFT study

We have reinvestigated CuNO2 and Cu+NO2 at ab initio as well as at pure and hybrid DFT levels of approximation employing large ANO basis sets. The systems were fully optimized using the CCSD(T), QCISD(T), BPW91, PBE, PBE0, and B3LYP methods. Several stationary points ( minima and transition structures) were found on the related potential energy surfaces (PES). The C-2v bidentate eta(2)-O, O isomer is calculated to be the most stable species on the CuNO2 PES, followed by two monodentate isomers-the C-s eta(1)-O and C-2v eta(1)-N species which are higher in energy by 12 and 14 kcal/mol, respectively, at CCSD(T)/Basis-II (where Basis-II is 21s15p10d6f4g/8s7p5d3f2g for Cu; 14s9p4d3f/5s4p3d2f for O and N). On the Cu+NO2 PES, the C-s monodentate eta(1)-O trans (0 kcal/mol) and cis (+ 3 kcal/mol at CCSD(T)/Basis-II) isomers are found, followed by the C-2v monodentate eta(1)-N isomer (+ 14 kcal/mol at CCSD/Basis-II). In contrast to the pure DFT, the hybrid DFT methods perform reasonably well for predicting the relative stabilities (except for eta(1)-N of CuNO2) and structures; however, their predictions of the bond dissociation energies are less reliable (for CuNO2 the difference is as much as 10 kcal/mol compared to the CCSD( T) values). The performance of the QCISD( T) method was analyzed, and, furthermore, the issue of symmetry breaking was investigated.