Deuterium Isotope Effects on13C Chemical Shifts incis-Stilbene

Over 100 isotope effects on 13C 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 atoms in the molecule. The variations in two- and three-bond effects were 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—CξC dihedral angle, as in other binuclear aromatic molecules. In [para-2H]-cis-stilbene the isotope effect over ten bonds was observed, amounting to 2.0 ppb. In mono- and dideuterio-cis-stilbenes, effects over up to three bonds from the site of the deuteriation have a positive sign and decrease monotonously in magnitude. In contrast, effects through more than three bonds generally show an alternation in sign and oscillations in magnitude. In polydeuteriated cis-stilbenes, changes in sign and magnitude are governed by the additivity rule, which reproduces the observed values of isotope effects within a few percent.