Isotope Effects on nmr Spectra of Equilibrating Systems

Studies based on deuterium isotope effect on 13C chemical shifts

“…As expected from the theoretical studies of the deuterium effect on the equilibrium constant ðK H =K D Þ; the differences in the zero-point energies of the respective bonds AH and AD bring a major contribution to the isotope effect [64]. These differences are known to increase with increasing vibrational force constant, which in turn depends on the shape of the potential (Fig.…”

“…The zero-point energy differences between the AD and AH oscillators are the greater the deeper the well. Thus, the heavier isotope prefers the bond with a higher force constant [64].…”

“…As has been shown above, the deuterium atom prefers to be bonded to the C atom, for which the force constant is greater and the C -H bond is shorter. The electronic and steric effects that influence the respective force constants, and as a consequence lead to the equilibrium isotope effects, have been discussed in numerous papers [12,64,68,69].…”

Solvent Effects on Nitrogen Chemical Shifts

“…29) both in moderately polar dichloromethane-d 2 (ε ¼ 9.1) [69,150] and in polar, aprotic acetonitrile-d 3 (ε ¼ 37.5) [68] solution. The symmetry of these complexes was elucidate using the isotopic perturbation of equilibrium NMR technique [151], and was confirmed by DFT calculations. Diffusion NMR indicates tight coordination of OTf À to the [N-X-N] + complexes in dichloromethane-d 2 , but separate solvation of the cationic and the anionic parts of these complexes in the polar acetonitrile-d 3 [68,69].…”

Halogen Bonding in Solution

“…10) [80,81]. As standard spectroscopic methods are unable to distinguish between a static symmetric and a rapidly equilibrating mixture of asymmetric structures, the latter providing time averaged signals, Saunder's isotopic perturbation of equilibrium technique [83,84] was utilized with 13 . Importantly, the charge distribution of the complexes remains unaffected by polarity change of the environment, suggesting high stabilization by formation of a truly symmetric threecenter-four-electron complex.…”