Does Tetrahydrofuran (THF) Behave like a Solvent or a Reactant in the Photolysis of Thionyl Chloride (Cl₂SO) in Cyclohexane? A Transient Infrared Difference Study

The photolysis of thionyl chloride (Cl₂SO) in pure cyclohexane (cHex) and in cHex with a small amount of tetrahydrofuran (THF) irradiated with 266 nm pulsed laser was investigated using time-resolved step-scan Fourier-transform spectroscopy. The density functional theory B3LYP, with the conductor-like polarizable continuum model to account for the effects of solvents, was employed to predict the molecular parameters of the relevant species. Monitoring the wavenumbers and infrared absorbances attributed to the [S,O] species and accounting for the stoichiometry revealed SO₂ to be the major oxygen-containing end product for the thermal decomposition of Cl₂SO. Upon successive irradiation with 266 nm pulsed laser, the major product, as detected by IR absorption, was S₂O with minor SO₃, which could be generated from the secondary reactions of the photolytic intermediate ClSO. The majority of the transient vibrational features upon 266 nm irradiation of the mixture of Cl₂SO/cHex was attributed to ClSO, characterized at 1155 cm^–1, coupled with a minor contribution of (ClSO)₂ at 1212 and 1173 cm^–1. For the mixture of Cl₂SO/THF/cHex, the transient population of ClSO was retained, but the amount of (ClSO)₂ was slightly reduced, coupled with a new upward feature at 1054 cm^–1 that was plausibly attributed to the C–O–C asymmetric stretching mode of ClSO–THF complex. Upon the successive irradiation of the Cl₂SO/THF/cHex mixture, the amount of S₂O was also decreased. The observed complexes of THF with solutes suggested that THF should not be merely treated as a solvent but regarded as a coordination molecule in organic synthesis. The formation of the intermediate–THF complexes altered the reaction pathways, as well as the types and populations of the end products.