Gas-phase, conformer-specific infrared spectra of 3-chlorophenol and 3-fluorophenol†

Conformational isomerism of phenol derivatives has been a subject of extensive spectroscopic study. Combining the capabilities of the widely tuneable infrared free-electron laser FELIX with molecular beam technologies allows for revisiting existing data and gaining additional insights into far-IR spectroscopy of halogenated phenols. Here we present conformer-resolved infrared spectra of the syn and anti conformers of 3-chlorophenol and 3-fluorophenol recorded via IR-UV ion-dip spectroscopy. The experimental work is complemented by density functional theory calculations to aid in assignment of the observed bands. The experimental spectra for the two conformers of each molecule show overall a great similarity, but also include some distinct conformer-specific bands in the spectral range investigated. Our spectra confirm previously reported OH torsional mode frequencies for the syn and anti conformers of 3-chlorophenol (3CP) at 315 cm⁻¹, (Manocha et al., J. Phys. Chem., 1973, 77, 2094) but reverse their assignment of the 311 and 319 cm⁻¹ bands for 3-fluorophenol. 1D torsional mode calculations were performed for 3CP to help assign possible OH torsion overtones.