HNO⁺ and NOH⁺ Potential energy surfaces for the lowest two electronic states. including the barrier to isomerization

Abstract

Ab initio quantum-mechanical calculations on the triatomic ions HNO⁺ and NOH⁺ in their lowest two electronic states are reported, which approximate complete configuration interaction results in a double-zeta plus polarization quality basis. Relative energies of key points on the potential energy surfaces are determined as a function of wavefunction accuracy and used to give best estimated values. Ground-state HNO⁺ has an equilibrium geometry with r _NH = 1·063 Å, r _NO = 1·127 Å, < HNO = 125·9°, and an energy 54 kJ/mol lower than isomeric NOH⁺, whose ground-state equilibrium geometry is r _OH = 1·006 Å, r _NO = 1·180 Å and < NOH = 116·4°. The lowest barrier to isomerization on the ground-state surface is 247 kJ/mol. The HNO⁺ dissociation energy to H + NO⁺ is 147 kJ/mol. The NOH⁺ dissociation energy to the same dissociation limit is 88 kJ/mol. Vertical excitation from the ground-state equilibrium HNO⁺ molecule is at an energy of 1·78 eV, in good agreement with the 7200 Å (1·73 eV) absorption system reported by Herzberg. In NOH⁺ the equivalent excitation is at 1·39 eV. Excited state HNO⁺ has a linear equilibrium geometry with r _NH = 1·040 Å, r _NO = 1·130 Å, and is 71 kJ/mol below excited state NOH⁺ whose equilibrium geometry is also linear, with r _OH = 0·998 Å, r _NO = 1·171 Å. In the excited state the lowest barrier to isomerization is 251 kJ/mol. Rotational constants are reported.