Ab Initio Study of the Phosphorescence of Nitrite Ions

Abstract

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-ζ (cc-pVDZ) and triple-ζ (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.