Revisiting the visible B2A2 ← X2B1 spectrum of the phenoxy radical: Multireference energetics and finite-temperature Franck–Condon simulations

The visible B̃2A2 ← X̃2B1 absorption system of the phenoxy radical is characterized by a pronounced ∼500 cm-1 vibronic progression and broad, asymmetric band profiles. The assignment of the lowest-wavenumber component to the 0-0 band is supported by the 7 K Ar-matrix spectra, whereas the progression and room-temperature asymmetry have been assigned to the Wilson 6a vibration and sequence structure involving the low-frequency 10b vibration, respectively. Reference SA-CASSCF and TD-B3LYP Franck-Condon simulations show complementary limitations: CASSCF gives an overly compact 6a progression, whereas TD-B3LYP gives an overly extended cold-band envelope. The X̃2B1 and B̃2A2 states are therefore treated by second-order multireference Rayleigh-Schrödinger perturbation theory using a common state-averaged CASSCF reference. The resulting structures and harmonic force fields define a Duschinsky transformation involving all 30 normal coordinates. The finite-temperature vibrational basis was then expanded across all four C2v symmetry species and converged using mode-specific vibrational limits and state-energy cutoffs. The converged 30-mode model is compared directly with the room-temperature cavity-ringdown spectra of Araki et al. after one common wavenumber shift and an explicitly defined local normalization. It reproduces the principal 6a progression and parts of the asymmetric local envelopes qualitatively, but overestimates several wings and the high-wavenumber congestion. A calculation differing only in population temperature shows that this global high-wavenumber rise disappears at 7 K, identifying it as an accumulation of transitions from thermally populated lower states. The room-temperature spectrum is better described as a congested 6a progression with contributions from 10b and other modes than as a single 10b sequence.