The electronic structure of nitrogen dioxide. I. Multiconfiguration self-consistent-field calculation of the low-lying electronic states

Abstract

Traditional spectroscopic analysis of the complex and irregular absorption spectrum of NO₂ has provided a relatively small amount of information concerning the nature of the excited states. An extensive ab initio investigation has been undertaken, therefore, to provide a basis for interpretation of the experimental results. Multiconfiguration self‐consistent‐field (MC–SCF) wavefunctions have been computed for the low‐lying X̃ ²A₁, à ²B₂, B̃ ²B₁, C̃ ²A₂, ⁴B₂, ⁴A₂, and ²Σ⁺_g electronic states of NO₂. The minima of the à ²B₂, B̃ ²B₁, and C̃ ²A₂ states have all been found to be within 2 eV of the minimum of the X̃ ²A₁ ground state; for these states, C_2v potential surfaces have been constructed for purposes of a spectral interpretation. The ⁴B₂, ⁴A₂, and ²Σ⁺_g states are all more than 4 eV above the minimum of the ground state and have been examined in less detail. The study described here significantly improves on previous NO₂ ab initio calculations in three important areas: (1) The double‐zeta‐plus‐polarization quality basis set is larger and more flexible; (2) the treatment of molecular correlation is more extensive; and (3) the electronic energies have been calculated for several different bond lengths and bond angles in each state. For the four lowest doublet states the following spectral data have been obtained: The ground state experimental constants are included in parentheses. The estimated accuracy of the various parameters is ±0.02 Å for bond length, ±2° for bond angle, ±10% for the vibrational frequencies, ±0.10 D for dipole moments, and ±0.3 eV for the adiabatic excitation energies. An unusual feature has been found for the ²Σ⁺_g state. The equilibrium geometry of this linear state has two unequal bond lengths of 1.20 Å and 1.42 Å and the inversion barrier is approximately 800 cm⁻¹.