Vicinal Deuterium Perturbations on Hydrogen NMR Chemical Shifts in Cyclohexanes
journal contributionposted on 15.10.2008, 00:00 by Daniel J. O’Leary, Damian G. Allis, Bruce S. Hudson, Shelly James, Katherine B. Morgera, John E. Baldwin
The substitution of a deuterium for a hydrogen is known to perturb the NMR chemical shift of a neighboring hydrogen atom. The magnitude of such a perturbation may depend on the specifics of bonding and stereochemical relationships within a molecule. For deuterium-labeled cyclohexanes held in a chair conformation at −80 °C or lower, all four possible perturbations of H by D as H−C−C−H is changed to D−C−C−H have been determined experimentally, and the variations seen, ranging from 6.9 to 10.4 ppb, have been calculated from theory and computational methods. The predominant physical origins of the NMR chemical shift perturbations in deuterium-labeled cyclohexanes have been identified and quantified. The trends defined by the Δδ perturbation values obtained through spectroscopic experiments and by theory agree satisfactorily. They do not match the variations typically observed in vicinal JH−H coupling constants as a function of dihedral angles.