Ab initio calculation of nitrogen oxide dimer structure and its anion-radical

Abstract

A comparative quantum chemical analysis has been made for the most stable dimer of nitrogen oxide with the structure cis-ONNO in a singlet state ¹ A ₁ by ab initio method of SCF MO LCAO, allowing for electron correlation according to Meller-Plesset perturbation theory of the second order (MP2), and density functional technique (DFT). The computations by MP2 method show anion-radical (ONNO)⁻ to have a strong bond between nitrogen atoms (N-N 1.44 Å) in contrast to molecular weakly bound cis-dimer with equilibrium distance N-N 2.23 Å. Molecular orbital structure of the dimer and its anions was examined that made it possible to suggest a reason of preferable stabilization of nitrogen oxide dimer in the cis-form. Calculated high affinity to electron (E _a = −1.55-−1.69 eV) for the molecular dimer ONNO (¹ A ₁) explains an intense strengthening of N-N bond in anion-radical and confirms the experimental data on a possibility of surface anion-π-radical formation on electron donor centers. The DFT computations indicate that this technique poorly reproduces the experimental geometry and electron structure of the cis-dimer ONNO having predicted a triplet ground state with the equilibrium distance N-N ≈2 Å instead of a singlet one with N-N 2.26 Å. The comparison between MP2 and DFT calculations for complex dimer ONNO with copper cation reveals the energy state of the complex (Cu-O₂N₂)⁺ corresponding to stabilization of anion-π-radical (N₂O₂)⁻ {term-³ A ₂, Cu(d)⁹-(ONNO)⁻¹} to be highly overestimated by DFT.