CUMULATIVE DEUTERIUM-ISOTOPE EFFECTS ON THE CONFORMATIONAL EQUILIBRIA OF TRIBENZYLAMINE

Conformational equilibrium isotope effects (CEIE) were measured for several alpha-deuteriated isotopomers of tribenzylamine via NMR isotope shifts. Unlabeled benzyl groups give an AB system of two doublets in the benzylic region of the H-1 NMR spectrum when the conformational equilibria are perturbed by labeling in another benzyl group. The benzylic proton signal of a monolabeled benzyl group is shifted upfield by an intrinsic deuterium isotope effect and may be additionally shifted by an equilibrium isotope effect. An equilibrium constant for the isotope effect, K(iso), derived from the isotope shift for a labeled benzyl group is the same as one derived from an unlabeled benzyl group of the same isotopomer. The CEIEs are cumulative for placing additional deuteriums to create additional chiral benzyl groups of the same configuration. Thus, a deuterium in one benzyl group appears to influence equally the average conformations of all three benzyl groups. The results are consistent with the expectations for CEIEs for a structure of C3 symmetry, but a substantial contribution from a C1 structure is also possible.