NIR Excitation-Driven Conformational Isomerizations of Thymol and Carvacrol Isolated in a Nitrogen Cryomatrix

Abstract

Thymol (2-isopropyl-4-methylphenol) and carvacrol (5-isopropyl-2-methylphenol) differ only in the relative positions of the hydroxyl and alkyl substituents, yet this subtle change reverses their torsional energy profiles: in thymol, isopropyl rotation is more hindered than OH torsion, whereas in carvacrol the opposite occurs. These features make the two compounds ideal model systems to investigate how differences in OH and isopropyl torsional barriers influence the redistribution of vibrational energy following near-infrared (NIR) excitation of the 2νOH overtone. Monomers of both compounds were isolated in a nitrogen matrix at 15 K and irradiated within the profile of the 2νOH overtone band (7110 to 7060 cm−1) by using tunable laser light. Spectroscopic monitoring revealed selective and bidirectional interconversion between rotamers differing in the OH orientation. On the contrary, no interconversion between conformers differing in the isopropyl orientation was observed, which is particularly striking for carvacrol, where its torsional barrier is lower than that of OH rotation. These findings support OH rotamerization as the main route for vibrational energy redistribution, independent of the relative barrier heights. The photogenerated higher-energy cis-OH rotamers were observed to relax spontaneously to their trans counterparts via intramolecular hydrogen-atom tunneling, whose kinetics were quantitatively determined.