Mass analysed threshold ionisation spectroscopy of flexible 2-para-fluorophenylethanol conformers with and without an intramolecular OH...pi bond.

The cationic state of the prototype flexible molecule 2-para-fluorophenylethanol has been investigated by combination of mass-analysed threshold-ionisation (MATI) spectroscopy and quantum chemistry ab initio density functional theory (DFT) calculations employing two different functionals: the B3LYP functional and the new hybrid functional M05. The MATI spectra measured via vibronic bands in the S1 intermediate state belonging to the most stable gauche conformer stabilised by an intramolecular OH...pi hydrogen bond are structureless, while the spectra recorded via bands of the anti conformer feature well-resolved peaks. This result is in a good accord with our theoretical predictions showing that upon ionisation, the anti conformer retains its structure, while the lowest-energy gauche conformer undergoes a significant structural change resulting in a break of the OH...pi bond. This and the good agreement between the measured band positions and the theoretically predicted frequencies for the cationic anti conformer confirm the conformational assignment. The result for the cation provides clear evidence for the existence of a nonclassical intramolecular OH...pi hydrogen bond if the electron density in the aromatic ring is sufficiently high as is the case only for the neutral molecule.