DEUTERIUM-ISOTOPE EFFECTS ON C-13 CHEMICAL-SHIFTS IN CIS-STILBENE

Over 100 isotope effects on C-13 chemical shifts were determined in a series of deuteriated cis-stilbene isotopomers. The magnitude and sign of these effects depend on the position and the number of deuterium a toms in the molecule, The variations in two- and three-bond effects we re rationalized by steric interactions between the phenyl ring and the olefinic group, The six-bond effects in cis-stilbene isotopomers are related to the molecular Ph-C= C dihedral angle, as in other binuclear aromatic molecules, In [para-H-2]-cis-stilbene the isotope effect ove r ten bonds was observed, amounting to 2.0 ppb. In mono- and dideuteri o-cis-stilbenes, effects over up to three bonds from the site of the d euteriation have a positive sign and decrease monotonously in magnitud e, In contrast, effects through more than three bonds generally show a n alternation in sign and oscillations in magnitude, In polydeuteriate d cis-stilbenes, changes in sign and magnitude are governed by the add itivity rule, which reproduces the observed values of isotope effects within a few percent.