SOLVENT EFFECTS ON THE CONFORMATIONAL EQUILIBRIUM OF 1,1,2-TRICHLOROETHANE

Proton coupling constants, J, and proton chemical shifts, delta-(CH) and delta-(CH2), are reported for 1,1,2-trichloroethane in 32 protic and aprotic solvents, and in the gas phase. These have been analysed in terms of a conformational equilibrium between conformers I and II using multiple linear regression analysis through the Abraham-Kamlet-Taft equation. The main solvent influence on all three NMR parameters arises through reaction field effects as modelled by the Kamlet-Taft solvent dipolarity parameter pi-1x. There is a small effect of solvent hydrogen-bond basicity on delta-(CH) but not on delta-(CH2) indicating that the CHCl2 proton is slightly acidic. A similar effect of solvent basicity on the coupling constant J shows that not only is conformer II more dipolar than conformer I but that the CHCl2 proton in II is more acidic than the CHCl2 proton in I. The gas phase values of J, delta-(CH) and delta-(CH2) are more comparable with a suggested value of -0.4 for pi-1x than with the directly measured value of - 1.1 units.