A-Band Absorption Spectrum of the ClSO Radical: Electronic Structure of the Sulfinyl Group

Sulfur oxide species (RSO_x) play a critical role in many fields, ranging from biology to atmospheric chemistry. Chlorine-containing sulfur oxides may play a key role in sulfate aerosol formation in Venus' cloud layer by catalyzing the oxidation of SO to SO₂ via sulfinyl radicals (RSO). We present results from the gas-phase UV-vis transient absorption spectroscopy study of the simplest sulfinyl radical, ClSO, generated from the pulsed-laser photolysis of thionyl chloride at 248 nm (at 40 Torr of N₂ and 292 K). A weak absorption spectrum from 350 to 480 nm with a peak at 385 nm was observed, with partially resolved vibronic bands (spacing = 226 cm^-1), and a peak cross section σ(385 nm) = (7.6 ± 1.9) × 10^-20 cm². From ab initio calculations at the EOMEE-CCSD/ano-pVQZ level, we assigned this band to 1²A' ← X²A″ and 2²A' ← X²A″ transitions. The spectrum was modeled as a sum of a bound-to-free transition to the 1²A' state and a bound-to-bound transition to the 2²A' state with similar oscillator strengths; the prediction agreed well with the observed spectrum. We attributed the vibronic structure to a progression in the bending vibration of the 2²A' state. Further calculations at the XDW-CASPT2 level predicted a conical intersection between the excited 1²A' and 2²A' potential energy surfaces near the Franck-Condon region. The geometry of the minimum-energy conical intersection was similar to that of the ground-state geometry. The lack of structure at shorter wavelengths could be evidence of a short excited-state lifetime arising from strong vibronic coupling. From simplified molecular orbital analysis, we attributed the ClSO spectrum to transitions involving the out-of-plane π/π orbitals along the S-O bond and the in-plane orbital possessing a σ/σ character along the S-Cl bond. We hypothesize that these orbitals are common to other sulfinyl radicals, RSO, which would share a combination of a strong and a weak transition in the UV (near 300 nm) and visible (400-600 nm) regions.