Ab initio rotation-vibration energies and intensities for the HNC + molecule

Abstract

Accurate ab initio calculations at the multi-reference configuration interaction level of theory were carried out to determine the potential energy surface of the X ̃ 2 Σ + electronic ground state of the HNC + molecular ion. The electronic energies and the electronically averaged dipole moment components were calculated at a total of 44 geometries with energies up to 14 000 cm −1 above equilibrium. Additional calculations were done to determine the barrier for isomerization to the X ̃ 2 Π electronic ground state of HCN + ( H = 1.83 eV), the adiabatic ionization potential of the parent neutral HNC (IP = 11.88 eV), the dissociation energy for the process HNC + ← H + + CN ( X ̃ 2 Σ +)(D 0 = 5.36 eV) , and the energy separation between the electronic ground state and the first excited state ( 2Π) of HNC + ( T e = 1.70 eV). An analytical function was fitted through the X ̃ 2 Σ + ab initio points of HNC +, and relatively large stretch-bend interaction terms were obtained in the fitted function. The analytical potential function was used with the Morse Oscillator Rigid Bender Internal Dynamics Hamiltonian [ P. Jensen, J. Mol. Spectrosc. 128, 478–501 (1988); J. Chem. Soc., Faraday Trans. 2 84, 1315–1340 (1988)] in a variational calculation of the rotation-vibration energies with N ≤ 6 of the HNC + and DNC + molecules. This calculation neglects the possibility of the molecule tunneling into the HCN + minimum of the potential energy surface and the effects of the non-zero electronic spin in the X ̃ 2 Σ + state of HNC +. We obtain ν 1 = 3404 cm −1, ν 2 = 523 cm −1, and ν 3 = 2163 cm −1 for HNC +. We also calculate the vibrational transition moments using the dipole moment functions obtained from fits to the ab initio dipole moment data and obtain for HNC + that the vibrationally averaged dipole moment is 0.66 Debye in the vibrational ground state, and that the vibrational transition moments for the fundamental transitions ν 1, ν 2, and ν 3 are 0.24, 0.48, and 0.02 Debye, respectively.