Evidence for intramolecular OH center dot center dot center dot pi hydrogen bonding in unsaturated alcohols from UV photoelectron spectroscopy

The gas phase He(I) photoelectron (PE) spectra of several unsaturated alcoh ols (1-11) have been measured and analysed with respect to intramolecular O H . . . pi hydrogen bonding. Evidence for such a hydrogen bond has been det ected in the spectra of 2-allylphenol (1) and 2-phenylethan-1-ol (3). 1 exi sts as a conformational mixture of a hydrogen bonded form la and an open fo rm Ib in a composition of roughly 2:1. A strong ionization band (IPnu = 10. 01 eV; where IPnu is the vertical ionization potential) is assigned to the ethylenic C=C double bond in the major conformer (la) and a weak band (IPnu = 9.72 eV) to that of the minor conformer (Ib). The latter IP coincides wi th the corresponding ionization of allylbenzene. In the series of omega -ph enylalkan-1-ols, compound 3 exhibits an unusually low n(pi)(O) ionization i ndicating hydrogen bonding between the OH group and the pi 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 (1 0) and pent-4-yn-1-ol (11) are consistent with OH . . . pi hydrogen bonded conformers. The methanol/ethylene heterodimer has a T-shaped structure, as indicated by B3LYP/6-311++G(d) calculations, with a binding energy of 5.65 kJ mol(-1). (C) 2001 Elsevier Science B.V. All rights reserved.