Directionality of Intrinsic Deuterium Isotope Effects in 13C NMR Spectra of Molecules Containing One or Two Phenyl Groups

Intrinsic deuterium-induced isotope effects have been measured and analyzed for compounds having one or two phenyl groups specifically labelled at different positions in a molecule. Some typical examples of directional and orientational dependence of the effects have been given. Different magnitudes of isotope effects that are transmitted in opposite directions have been observed. Thus, in p-2H-cis-stilbene, the effect over six bonds at C-a of 8.7 ppb is found, while the corresponding effect in a-2H-cis-stilbene at C-4 is only 1.4 ppb. Similarly, in o'-2H-trans-N-benzylideneaniline, the effect over six bonds at C-2,6 is 1.7 ppb, whereas that in o-2H-trans-N-benzylideneaniline at C-2',6' is equal to zero. Similar behaviour was observed in other molecules and at other positions as well. Additional factors contributing to the magnitude and sign of isotope effects, such as -electron delocalization, conformation, steric hindrance and lone pair electrons are also discussed.