There has been a recent interest in electrostatic interactions in organic chemistry and in their influence on the conformation and reactivity of organic molecules. [1,2] It emerges that organic cations containing oxygen and fluorine find stabilization through such interactions: polarized C À OH and C À F bonds orient towards charged centers when the molecular conformation allows. This effect has been particularly noted by Snyder, Lankin, and co-workers, who reported the conformational preferences of 3-fluoropiperidinium (1) and related ring systems, such as 3-fluoro-N,N-dimethylpiperidinium (2). [3][4][5][6] They established that there is a strong preference (calculated energy difference of 4.0-5.4 kcal mol À1 between the gas-phase anti and gauche conformations) for the structures with the fluorine atom in the axial position (1 a/ 2 a) over those with the fluorine atom in the equatorial position (1 b/2 b; Scheme 1). [5] In our computational studies, we have explored the conformational preferences of bfluoroethylamine (3) and its protonated counterpart bfluoroethylammonium (4).[7] Our density functional theory (DFT) calculations indicated that there is no intrinsic gauche effect for the neutral amine 3; the neutral molecule only prefers a gauche conformation because there is a weakly stabilizing intramolecular NÀH···FÀC hydrogen bond. When the nitrogen atom is formally charged in 4, however, the bfluoroethylammonium cation displays a very strong preference (5.8 kcal mol À1 ) for the gauche conformation (4 a). This phenomenon extends to 2-fluoroethanol (5) and to protonated 2-fluoroethanol (6).[7] The slight gauche preference in the neutral molecule 5 is almost entirely attributable to a weak bridging hydrogen bond. In contrast, there is a significant intrinsic gauche preference in 6 of about 4.4 kcal mol À1 , which increases to 7.2 kcal mol À1 when one hydrogen atom of the H 2 O + group is in a bridging endo position. These charged systems exhibit a much larger gauche preference than do neutral molecules such as 1,2-difluoroethane (7), for which the energy difference was calculated to be in the range 0.5-1.0 kcal mol À1 .[7]More recently, we studied the conformation of 3-fluoroazetidinium (8).[8] Owing to the ring constraint, it was not possible to establish gauche and anti conformations, and the energy stabilization of the electrostatic interaction could not be calculated directly. We, therefore, instead investigated the influence of the interaction on the conformation, by comparing structures both with and without a positive charge. For the cation 8, our DFT calculations were consistent with the singlecrystal X-ray structure, indicating a puckered conformation that brings the C À F and N + À H bonds into proximity. Note that the positive Mulliken charge density on the hydrogen atoms is significant. When an additional electron was added to 8 to give the neutral (but sterically identical) azetidine 9, however, the ring puckered in the opposite direction, consistent with the removal of a favorable C À F···H À N + i...

The structures of 3-fluoroazetidinium hydrochloride and 3-fluoro-1,5-diazacyclooctane hydrobromide are explored both by X-ray diffraction analysis and DFT calculations, and the conformations of these molecules are shown to be significantly influenced by the through space C-F...N+ interaction.

There has been a recent interest in electrostatic interactions in organic chemistry and in their influence on the conformation and reactivity of organic molecules. [1,2] It emerges that organic cations containing oxygen and fluorine find stabilization through such interactions: polarized C À OH and C À F bonds orient towards charged centers when the molecular conformation allows. This effect has been particularly noted by Snyder, Lankin, and co-workers, who reported the conformational preferences of 3-fluoropiperidinium (1) and related ring systems, such as 3-fluoro-N,N-dimethylpiperidinium (2). [3][4][5][6] They established that there is a strong preference (calculated energy difference of 4.0-5.4 kcal mol À1 between the gas-phase anti and gauche conformations) for the structures with the fluorine atom in the axial position (1 a/ 2 a) over those with the fluorine atom in the equatorial position (1 b/2 b; Scheme 1). [5] In our computational studies, we have explored the conformational preferences of bfluoroethylamine (3) and its protonated counterpart bfluoroethylammonium (4).[7] Our density functional theory (DFT) calculations indicated that there is no intrinsic gauche effect for the neutral amine 3; the neutral molecule only prefers a gauche conformation because there is a weakly stabilizing intramolecular NÀH···FÀC hydrogen bond. When the nitrogen atom is formally charged in 4, however, the bfluoroethylammonium cation displays a very strong preference (5.8 kcal mol À1 ) for the gauche conformation (4 a). This phenomenon extends to 2-fluoroethanol (5) and to protonated 2-fluoroethanol (6).[7] The slight gauche preference in the neutral molecule 5 is almost entirely attributable to a weak bridging hydrogen bond. In contrast, there is a significant intrinsic gauche preference in 6 of about 4.4 kcal mol À1 , which increases to 7.2 kcal mol À1 when one hydrogen atom of the H 2 O + group is in a bridging endo position. These charged systems exhibit a much larger gauche preference than do neutral molecules such as 1,2-difluoroethane (7), for which the energy difference was calculated to be in the range 0.5-1.0 kcal mol À1 .[7]More recently, we studied the conformation of 3-fluoroazetidinium (8).[8] Owing to the ring constraint, it was not possible to establish gauche and anti conformations, and the energy stabilization of the electrostatic interaction could not be calculated directly. We, therefore, instead investigated the influence of the interaction on the conformation, by comparing structures both with and without a positive charge. For the cation 8, our DFT calculations were consistent with the singlecrystal X-ray structure, indicating a puckered conformation that brings the C À F and N + À H bonds into proximity. Note that the positive Mulliken charge density on the hydrogen atoms is significant. When an additional electron was added to 8 to give the neutral (but sterically identical) azetidine 9, however, the ring puckered in the opposite direction, consistent with the removal of a favorable C À F···H À N + i...