The gas phase He(I) photoelectron (PE) spectra of several unsaturated alcohols (1–11) have been measured and analysed with respect to intramolecular OH⋯π hydrogen bonding. Evidence for such a hydrogen bond has been detected in the spectra of 2-allylphenol (1) and 2-phenylethan-1-ol (3). 1 exists as a conformational mixture of a hydrogen bonded form 1a and an open form 1b in a composition of roughly 2:1. A strong ionization band (IPv=10.01eV; where IPv is the vertical ionization potential) is assigned to the ethylenic CC double bond in the major conformer (1a) and a weak band (IPv=9.72eV) to that of the minor conformer (1b). The latter IP coincides with the corresponding ionization of allylbenzene. In the series of ω-phenylalkan-1-ols, compound 3 exhibits an unusually low nπ(O) ionization indicating hydrogen bonding between the OH group and the π electron system of the phenyl ring. The higher homologs 4 and 5 prefer ‘open’ conformations without such interaction. The PE spectra of alkenols such as but-3-en-1-ol (7) and pent-4-en-1-ol (8) as well as of alkynols such as but-3-yn-1-ol (10) and pent-4-yn-1-ol (11) are consistent with OH⋯π hydrogen bonded conformers. The methanol/ethylene hetero-dimer has a T-shaped structure, as indicated by B3LYP/6-311++G(d) calculations, with a binding energy of 5.65kJmol−1.
