engleski

Deuterium-Induced Isotope Effects on H-1 Chemical Shifts in cis- and trans-Stilbene

Isotopic substitution affects NMR spectral parameters: chemical shifts, coupling constants and relaxation times. Isotope effects are important since they give valuable informations about molecular structure and dynamics in solution. Deuterium-induced isotope effects on C-13 chemical shifts have frequently been studied, while this is not the case with the effects on H-1 shifts. We have, therefore, prepared a series of specifically deuteriated cis- and trans-stilbenes and recorded their H-1 NMR spectra at 360 MHz in order to study changes in H-1 spectra caused by the isotopic substitution. The long range isotope effects have shown to be a sensitive probe of molecular conformation and its changes, as found for similar molecules of the Ph-Z-Ph type (where Ph refers to phenyl and Z to the bridging group, like C=C, C=O, C=N, N=N etc.). Of special interest is the opposite trend found here for three bond effects on H-alpha in respect to that observed for one bond effects on C-alpha, in alpha-deuteriated ethene, styrene, cis- and trans-stilbene. It has been demonstrated that the sign, magnitude and slight nonadditivity of isotope effects on H-1 chemical shifts depend on the site of the isotopic substitution, steric and lone-pair interactions, planarity and redistribution of rotamer population.

nije evidentirano

nije evidentirano