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

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 rNH = 1∙063 Å, rNO= 1 ∙ 127 Å, <HNO = 125∙9°, and an energy 54 kJ/mol lower than isomeric NOH+, whose ground-state equilibrium geometry is roH = 1∙006A, rNO = l∙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, rno = 1∙171 Å. In the excited state the lowest barrier to isomerization is 251 kJ/mol. Rotational constants are reported. © 1978 Taylor & Francis Group, LLC.