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 nF→σ 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 σCH and σCC as electron donors and σ and σ as electron acceptors dictates the gauche effect in 3-fluoro-1,2-propanediol rather than a hydrogen bond. The absence of JF,H(O) and JH(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.

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Salicylic acid is the precursor of an important analgesic drug, and fluorination may modify its properties, including pK a in solution, viscosity, polarity and molecular conformation. The intramolecular hydrogen bond is an interaction that modifies the conformational equilibrium of molecules and, therefore, it can modify their macroscopic properties, such as bioactivity. Competition between F and N as proton acceptors for the acidic COOH hydrogen in 6-fluoroanthranilic acid could not be experimentally probed earlier because of, among other factors, its tendency to form a zwitterionic species due to the higher basicity of the nitrogen. In 6-fluorosalicylic acid (1), the fluorine atom competes with oxygen for the COOH proton, thus affecting the conformational equilibrium when compared to salicylic acid itself.Indeed, 1 shows two major conformers in the gas phase and solution, while the isomer 4-fluorosalicylic acid is expected to exhibit only one among three possible conformers. Theoretical calculations indicate that the second most stable conformer of 1 exhibits intramolecular F/H(OOC) hydrogen bonds, which is corroborated by the small, but detectable correlation between 19 F and 1 H(OOC) in the HETCOR NMR spectrum in benzene-d 6 solution.

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