DIRECTIONALITY OF INTRINSIC DEUTERIUM-ISOTOPE EFFECTS IN C-13 NMR-SPECTRA OF MOLECULES CONTAINING ONE OR 2 PHENYL GROUPS

Intrinsic deuterium-induced isotope effects have been measured and ana lyzed for compounds having one or two phenyl groups specifically label led at different positions in a molecule. Some typical examples of dir ectional and orientational dependence of the effects have been given. Different magnitudes of isotope effects that are transmitted in opposi te directions have been observed. Thus, in p-H-2-cis-stilbene, the eff ect over six bonds at C-alpha of 8.7 ppb is found, while the correspon ding effect in alpha-2H-cis-stilbene at C-4 is only 1.4 ppb. Similarly , in o'-H-2-trans-N-benzylideneaniline, the effect over six bonds at C -2,6 is 1.7 ppb, whereas that in o-H-2-trans-N-benzylideneaniline at C -2',6' is equal to zero Similar behaviour was observed in other molecu les and at other positions as well. Additional factors contributing to the magnitude and sign of isotope effects, such as pi-electron deloca lization, conformation, steric hindrance and lone pair electrons are a lso discussed.