Conformational landscapes in flexible organic molecules:: 4-hydroxy phenyl ethanol (p-tyrosol) and its singly hydrated complex

The conformational structures of 4-hydroxy phenyl ethanol (p-tyrosol) and its 1:1 hydrated cluster, have been characterised in the low temperature environment of a free jet expansion through a combination of mass-selected, resonantly enhanced two-photon ionisation spectroscopy (MS-R2PI), fluorescence excitation and ultra-violet 'hole-burning' spectroscopy. These strategies, coupled with rotational band contour analysis (using a cross-correlation fitting procedure) and ab initio computation at the HF/6-31G*, MP2/6-31G* and CIS levels of theory, indicate a strong preference for a folded, gauche conformation of the flexible side chain, stabilised by hydrogen-bonded interaction between the terminal OH group and the aromatic ring. Folded conformations that do not allow this are not detected experimentally. It has also been possible to assign the structures of both the gauche cis and gauche a ans OH rotamers, as well as the extended anti conformer and their 1:1 hydrated clusters. The 'tagging' of the phenolic OH group by the hydrogen-bonded water molecule also allows the unambiguous assignment of the cis and traits gauche conformers through rotational band contour analysis. Although the OH substituent strongly attenuates the degree of rotation, promoted by side-chain folding, of the electronic transition moment in the molecular frame, the residual angle continues to be modulated by the cis-trans rotation of the phenolic OH group.