DEUTERIUM-INDUCED PRIMARY AND SECONDARY ISOTOPE EFFECTS ON C-13,H COUPLING-CONSTANTS IN HALOMETHANES

Primary (p) Delta(C-13,H) and secondary (s) Delta(C-13,H) isotope effe cts on carbon-proton coupling constants were measured in studies of pr oton-and deuteron-coupled C-13 NMR spectra of the halomethanes CHnDmX4 -n-m, where X is Cl, Br or I. Both effects are always negative. The ma gnitudes of p Delta(C-13,H) increase with the number of halogen atoms whereas those of (s) Delta J(C-13,H) decrease. (p) Delta J(C-13,H) is almost linearly dependent on the number of halogen atoms whereas for ( s) Delta J(C-13,H) the dependence is non-linear. New results for the i sotope effect on the C-13,H coupling constant in fluoroform are in acc ord with data for the other members of the haloform series. For CH,Br and CH3I estimates of the main features of the surfaces for the C-13,H coupling constants were made using known molecular force fields. (C) 1998 John Wiley & Sons, Ltd.