Theoretical and infrared studies on the conformations of monofluorophenols

Abstract

Theoretical calculations for the isolated state indicate the presence of two conformers (A and B) for 2- and 3-fluorophenols, while a single form exists for 4-fluorophenol, as it is well known. The conformer with the hydroxyl hydrogen directed toward the fluorine atom (A) in 2-fluorophenol is calculated to be significantly more stable than the second form, while similar behavior is not obtained for the meta isomer. The infrared C–F stretching intensities confirm these findings in cyclohexane solution, where internal hydrogen bonding in 2-fluorophenol is supposed to be a stabilizing effect of A, but the figure changes in acetonitrile solution and neat liquid, where the most polar conformer B is preferred and intermolecular hydrogen bonding seems to take place, as confirmed by the broadening and shifting of the O–H stretching band. In fact, the strong solvent effect in 2-fluorophenol suggests that conformer A is the most stable form in the isolated state and nonpolar solution predominantly due to dipolar repulsion in B.