Evidence of the weakness of the OH?F hydrogen bond from a conformational study of 3-fluoro-1-propanol by microwave spectroscopy

The rotational spectra of the OH and OD isotopic species have been observed for three rotamers of 3-fluoro-1-propanol. One of them (HBC form) displays an internal hydrogen bond with a distorted chair conformation of the six-membered ring. The other two rotamers have the oxygen atom gauche with respect to the C₂C₃ bond, the hydroxyl hydrogen trans with respect to the C₁C₂ bond and the fluorine atom gauche (GGT form) and trans (TGT form), respectively, with respect to the C₂C₁ bond. The energies of the vibrational ground states of the HBC and TGT forms are ～0.4 and 1.0 kcal/mole higher than that of the GGT form, respectively (from relative intensity measurements). The hydrogen bond is therefore rather weak in this compound. With compounds capable of forming OH?O or OH?N bonds, the conformation appropriate for hydrogen bonding is normally the most stable form. Several excited states have been analyzed for the TGT and GGT rotamers in order to have additional data with respect to the potential function for the internal rotation about the C₃C₂ bond.