The preferred all-gauche conformations in 3-fluoro-1,2-propanediol

Abstract

A competition between the terminal fluorine and hydroxyl groups by the central hydroxyl group as hydrogen bond donor in 3-fluoro-1,2-propanediol would be expected to dictate the conformational isomerism of this compound, but also the repulsion between the electronegative and bulky vicinal substituents. Indeed, an intramolecular hydrogen bond has been verified only for a local minimum using QTAIM calculations, while the most stable conformer exhibits an all-gauche conformation with a small stabilizing contribution from the n_F→σ^*_OH interaction. The preferred orientation of the OH and F substituents was confirmed from the chemical shifts and coupling constants of the diastereotopic hydrogens. This conformational preference, which is calculated to exist both in the gas phase and solution (using implicit CHCl₃ and CH₃CN solvents), is better described by predominant hyperconjugative interactions over Lewis-type interactions. The strong contribution from antiperiplanar interactions involving σ_CH and σ_CC as electron donors and σ^*_CF and σ^*_CO as electron acceptors dictates the gauche effect in 3-fluoro-1,2-propanediol rather than a hydrogen bond. The absence of J_F,H(O) and J_H(O),H(O) coupling constants confirms that any influence from a hydrogen bond to the conformational isomerism of 3-fluoro-1,2-propanediol is secondary.