Conformational Structure, Infrared Spectra and Light-Induced Transformations of Thymol Isolated in Noble Gas Cryomatrices

The conformational space of the natural product thymol (2-isopropyl-5-methylphenol) was investigated using quantum chemical calculations at B3LYP and MP2 levels, which revealed existence four types conformers differing in orientation isopropyl hydroxyl groups. Thymol monomers were isolated noble gas (Ar Xe) matrices (at 15 K) characterized by IR spectroscopy. With support harmonic vibrational calculations, two most stable trans-OH-conformers, orientation, identified cryomatrices. less cis-OH not detected as they shall undergo fast tunneling to ones. Annealing experiments a Xe matrix up 75 K did lead any conversion between conformers, is accordance with significative energy barrier computed for rotamerization bulky group (~24 kJ mol?1). Vibrational excitation promoted broadband or narrowband irradiation, 2?(OH) frequencies either, interpreted terms more efficient transfer (associated lower light H-atom) than coordinate. Broadband UV irradiation (? > 200 nm) led prompt transformation thymol, spectroscopic evidence suggesting formation isomeric alkyl-substituted cyclohexadienones, Dewar isomers open-chain conjugated ketenes. photochemical mechanism interpretation concords that reported analogous phenol derivatives.