Deuterium-induced primary and secondary isotope effects on C-13,H coupling constants in halomethanesстатья
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Аннотация:Primary (p) Delta(C-13,H) and secondary (s) Delta(C-13,H) isotope effects on carbon-proton coupling constants were measured in studies of proton-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 magnitudes 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 isotope effect on the C-13,H coupling constant in fluoroform are in accord 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.