Solid state and theoretical evaluation of beta-fluoroethyl esters indicate a fluorine-ester gauche effect

Single crystal X-ray diffraction studies and a theoretical analysis indicate a preferred conformation for O-beta-fluoroethyl esters, where the C-F and C-O(CO) bonds are gauche rather than anti to each other. The O-C-C-F dihedral angles for three compounds and five independent structures indicate a range of only 63.4-69.6degrees. Evaluation of a rotational energy profile around this bond in a model system (beta-fluoroethyl acetate) predicted a similar dihedral angle and the gauche conformation to be the minimum on the rotational energy profile. High level ab initio calculations measured the gauche conformer to be 0.95 kcal mol(-1) lower in energy than the anti conformer and application of a solvation model further increased this differential to 1.6 kcal mol(-1), consistent with a previous solution state (NMR) evaluation of this system. (C) 2003 Elsevier B.V. All rights reserved.