Evidence for intramolecular OH...p hydrogen bonding in unsaturated alcohols from UV photoelectron spectroscopy.
The gas phase He(I) photoelectron (PE) spectra of several unsatd. alcs. have been measured and analyzed with respect to intramol. OH...p hydrogen bonding. Evidence for such a hydrogen bond has been detected in the spectra of 2-allylphenol (I) and 2-phenylethan-1-ol (II). I exists as a conformational mixt. of a hydrogen bonded form (III) and an open form (IV) in a compn. of roughly 2:1. A strong ionization band (IPv = 10.01 eV; where IPv is the vertical ionization potential) is assigned to the ethylenic C=C double bond in the major conformer III and a weak band (IPv = 9.72 eV) to that of the minor conformer IV. The latter IP coincides with the corresponding ionization of allylbenzene. In the series of w-phenylalkan-1-ols, II exhibits an unusually low np(O) ionization indicating hydrogen bonding between the OH group and the p electron system of the Ph ring. The higher homologs prefer \"open\" conformations without such interaction. The PE spectra of alkenols such as but-3-en-1-ol and pent-4-en-1-ol as well as of alkynols such as but-3-yn-1-ol and pent-4-yn-1-ol are consistent with OH...p hydrogen bonded conformers. The methanol/ethylene heterodimer has a T-shaped structure, as indicated by B3LYP/6-311++G(d) calcns., with a binding energy of 5.65 kJ mol-1.
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