Magnetic Anisotropy of the C-C Single Bond

Abstract

Anisotropic effects are broadly used in NMR spectroscopy for structure elucidation. With the development of computational methods it has become possible to quantify the effects and obtain further insight into their origin. Some classical interpretations have been questioned. Herein, we show that the classical "anisotropy cone" representing the anisotropic effect of the C-C single bond should be revised: deshielding at its side and shielding along its end are observed. Consequently, methyl, methylene, and methyne hydrogen atoms are not de-shielded by C-C bonds as is conventionally explained in NMR spectroscopy textbooks. They are just less shielded than by the C-H bonds attached at the same carbon. In addition, this anisotropic effect is dependent on the environment and care should be taken when drawing conclusions based on it. For example, it differs for the staggered and eclipsed conformations of ethane in HCCH planes, as well as for cyclohexane. In fact, it is not the anisotropy of the C2-C3/C5-C6 bonds that determines the chemical shift difference of axial and equatorial protons of a rigid cyclohexane ring, but magnetic contributions from all bonds.