Mass-Analyzed Threshold Ionization Spectroscopy of Ortho Fluorinated 2-Phenylethanol: Identification of an Additional Gauche Conformer

The cationic ground state of the ortho fluorinated 2-phenylethanol has been investigated by combination of mass-analyzed threshold ionization (MATT) spectroscopy and quantum chemistry ab initio density functional theory (DFT) calculations employing the hybrid functional M05 with cc-pVDZ basis set. The MATI spectra measured via vibronic bands in the S-1 intermediate state of the most stable gauche conformer stabilized by an intramolecular OH center dot center dot center dot pi hydrogen bond are structureless. The MATI spectrum recorded via a small band blueshifted by 3 cm(-1) from the 0(0)(0) electronic origin of the gauche conformer features well-resolved peaks and is assigned to a cationic gauche structure without an OH center dot center dot center dot pi bond. The ab initio calculations are qualitatively consistent with the experimental observations and show that the presumable conformer giving rise to the observed MATI spectrum retains its structure during ionization, whereas the lowest-energy gauche conformer undergoes a significant structural change resulting in a break of the OH center dot center dot center dot pi bond, thus leading to unfavorable Franck-Condon factors for the threshold ionization.