The preferred conformation of N-β-fluoroethylamides. Observation of the fluorine amide gauche effect

Abstract

Experimental and theoretical data indicate a preferred conformation for N-β-fluoroethylamides in which the C–F and C–N(CO) bonds are gauche rather than anti to each other. Theoretical calculations on a model system predict the gauche conformation to be 1.8 kcal mol⁻¹ lower in energy than the anti conformation. This compares with a value of 0.7 kcal mol⁻¹ for the gauche effect in 1,2-difluoroethane.