Solvent Effects in Nuclear Magnetic Resonance Spectra

Abstract

Contributions to nuclear screening (chemical shifts) arising from molecular interactions with solvent molecules (excluding hydrogen bonding) are discussed in terms of appropriate theoretical models. These include contributions from van der Waals interactions σ_w, from the magnetic anisotropy of the solvent molecule σ_a, and from polar effects σ_E. By a suitable choice of solute‐solvent systems it has been possible to demonstrate each of these effects experimentally for proton resonances. For CH₄ as a solute, σ_w was in all cases negative, its magnitude varying with the nature of the solvent and amounting to as much as 0.6 ppm for high molecular weight solvents. In agreement with the theoretical models, σ_a was found to be positive for disk‐shaped solvent molecules and negative for cylindrically symmetrical rod‐shaped molecules, its magnitude in extreme cases reaching 0.75 ppm. For CH₃CN as a solute, σ_E was negative and showed the expected dependence on the dielectric constant of the solvent.