INFLUENCE OF A SIDE-CHAIN ON DEUTERIUM-ISOTOPE EFFECTS IN C-13 NMR-SPECTRA OF SOME BENZENE-DERIVATIVES

Deuterium-induced isotope effects on C-13 chemical shifts were measure d in a series of benzene derivatives, viz, toluene, benzoic acid and b enzophenone, The effects over two, three and four bonds reflect a depe ndence on the transmission pathway and the nature of the side-chain. S teric and inductive influences of side-chain lane pair electrons locat ed at the P-position to the phenyl ring account for a lower magnitude and negative sign of some effects in the deuteriated phenyl ring. In m onodeuteriated benzophenones isotope effects are transmitted to the se cond phenyl ring, e.g. in p-deuteriobenzophenone over up to nine bonds , Although the carbonyl group allows only a minor conjugation between the phenyl rings, the extent, the magnitude and the sign of isotope ef fects resemble those in fully conjugated molecules, indicating a simil ar transmission mechanism. In polydeuteriated isotopomers, the additiv ity of isotope effects holds for the majority of carbon atoms. However , in few cases significant deviations exist owing to perturbations in hydrogen-bond dynamics and redistribution of rotamer populations cause d by deuteriation, For monodeuteriated isotopomers, a number of C-13-H -2 coupling constants through one and three bonds are also reported.