Ab Initio Study of the Phosphorescence of Nitrite Ions

In order to interpret the phosphorescence spectra of NaNO₂ and similar single crystals, we performed MCSCF geometry optimization in the ground singlet (X ¹ A ₁) and in the first excited triplet (a ³ B ₁) states of the NO ₂ ^- ion and MCSCF quadratic response (QR) calculation of the a ³ B ₁–X ¹ A ₁ transition probability at different bending angles and asymmetric stretch modes. The complete form of the spin–orbit coupling (SOC) operator is accounted for in the QR procedure. Dunning's correlation-consistent polarized valence double- (cc-pVDZ) and triple- (cc-pVTZ) basis sets are imployed. The electric-dipole transition moment from the T ^z spin sublevel (z is the C ₂ axis) oriented along the y direction (the other in-plane axis) is found to be 5 times higher than that from the T ^y sublevel in the ground-state geometry. This is in agreement with polarization measurements and with optical detection of ESR spectra. The T ^z–S ₀ transition moment decreases almost linearly with an increase in the ONO bond angle. The so-called non-Condon effects in the phosphorescence spectra of NaNO₂ crystals are explained on these backgrounds. The long progression of the bending vibrations (v ₂, a ₁) with an anomolous intensity distribution in the T ^z–S ₀ transition and additional involvement of the asymmetric stretch mode (v ₃, b ₂) in the T ^y–S ₀ transition are interpreted by force field and SOC calculations in the MCSCF-response technique. Configuration interaction (CI) calculations of the spin-allowed electric dipole transitions in NO ₂ ^- ions with effective one-electron SOC operator matrix element estimations were done for comparison with the results of the quadratic and linear response methods. Other T _n–S ₀ transitions are also studied. Finally, a short discussion of nonradiative processes is presented.