Isotope effects in nuclear shielding

Abstract

Publisher Summary Isotope effects have been known for a long time and an excellent review covering the early reports exists. The magnitude of the isotope shift is generally dependent on how remote the isotope substitution is from the nucleus under observation. the magnitude of the shift is also a function of the shielding range of the resonant nucleus and the isotope shift is largest where the fractional change in mass upon substitution is largest. In general, the isotope nuclear shielding is approximately proportional to the number of atoms in the molecule that has been substituted by isotopes. The chapter proposes theories explaining one-bond isotope effects. It also reviews empirical trends and practical uses of mostly secondary isotope effects, both of the intrinsic and equilibrium types. The chapter discusses basic trends of intrinsic isotope effects, such as additivity solvent effects, temperature effects, steric effects, substituent effects, and hyperconjugation. The two-bond isotope effect of acetone was the first negative isotope effect to be reported. The negative value has been ascribed to hyperconjugation. Negative values, although smaller, have also been observed in substituted camphors.