Comparison of the structure and spectra of the HNO⁺ and NOH⁺ ions using ab initio SCF and CI methods

Abstract

The equilibrium geometry of the HNO⁺ molecule ion is calculated to be NH = 1·997 bohr, NO = 2·126 bohr and HNO = 130·8° on the basis of an SCF treatment, while the corresponding parameters for NOH⁺ are found to be OH = 1·864 bohr, NO = 2·245 bohr and NOH = 123·7°. The ground-state dissociation products are H + NO⁺ (¹Σ⁺), whereby a dissociation energy for HNO⁺ of 41·2 kcal/mole is determined by CI calculations; HNO⁺ is found to be more stable than NOH⁺ by 11 kcal/mole. The barrier towards interconversion between the two conformers is found to be higher than the corresponding dissociation energy for the ground state, but in the first excited state interconversion is calculated to be more likely than dissociation, in this case into H⁺ + NO (²Π). The vertical electronic excitation energies of HNO⁺ and NOH⁺ are also determined via CI calculations and the corresponding oscillator strength results are obtained. The general appearance of this spectrum is found to be wholly similar to that of isoelectronic HCO, with the lowest excited state (²Π) preferring a linear nuclear arrangement; the excitation energy to this state is indicated to be 0·5 eV lower than the analogous HCO value. Comparison with results for the isoelectronic partners HCO, COH and HN₂ is made whenever possible.