The observation of a large gauche preference when 2-fluoroethylamine and 2-fluoroethanol become protonated

Abstract

The energies of the gauche and anti conformers of 2-fluoroethylamine, 2-fluoroethanol and their protonated analogues are calculated using density functional theory. Unlike the non protonated systems, the protonated systems show a strong gauche effect where the C-F and the C-(+)NH(3) or C-F and C-(+)OH(2) bonds are gauche rather than anti to each other. Single crystal X-ray diffraction studies of 2-fluoroethylammonium compounds identify the same conformational preference.

The fluorine-NHC gauche effect: a structural and computational study

“…This value is typical of the stabilisation imparted by antiperiplanar hyperconjugative σ C-H → σ* C-F and σ C-H → σ* C-N interactions, in preference to σ C-N → σ* C-F and σ C-F → σ* C-N . Similar stereoelectronic gauche effects in NCCF systems have previously been described for imines, [44] amines, [6,[45][46][47] and amides. [6,48] Free NHCs and their precursor cations therefore follow the pattern found in other NCCF systems as far as the gauche preference is concerned.…”

“…The magnitude of this stabilisation is characteristic of an electrostatic gauche effect, which orients the partially negatively charged fluorine preferentially towards the cationic moiety, similarly to the situation seen in N-(2-fluoroethyl)iminium, [7a] -pyridinium, [45] or -ammonium [6,46,47] systems. The Hirshfeld partial charges for 23 (which are practically independent of the torsion angle) are -0.13e (F), 0.00 (both N), and +0.24e (C carb H).…”

Towards novel difluorinated sugar mimetics; syntheses and conformational analyses of highly-functionalised difluorinated cyclooctenones

“…Both conformers A and C maximise attractive gauche interactions between C-F and C-O acceptor bonds and potential donors, whereas both bisected conformations contain a pair of acceptor bonds in an antiperiplanar arrangement. 46 It is possible that the theory used has failed to account adequately for the potential magnitude of this effect. The calculated coupling constant for 17a type C (26.4 Hz) agrees quite well with the measured value, whereas the value calculated using the observed molecular structure in the crystal is rather lower (23.5 Hz).…”

Theoretical Gas Phase Study of the Gauche and Trans Conformers of 1-Fluoro-2-Haloethanes CH2F-CH2X (X=Cl, Br, I) by Ab Initio and Density Functional Methods: Absence of Gauche Effect

“…It is generally found that the trans conformer is more stable than the gauche form and this is due to steric hindrance in the gauche conformation [2]. However fluorine has strong stereoelectronic and polar effects on the conformers of fluorine substituted molecules [3][4][5][6][7][8]. The net effect is that the gauche conformer is unusually more stable for the fluoro substituted conformers and this has been associated with the "gauche effect" [2,9].…”