Carbon-13 and proton chemical shifts of cyclohexanes in relation to interaction energy

A comparison of nuclear magnetic resonance spectra for isomeric cyclohexane derivatives (methyl, hydroxyl, halogen) shows that net shielding of carbon-13 nuclei of these compounds increases additively with an increase in repulsive non-bonding interactions, and thus with decreasing enthalpy in the series. By contrast, an inverse shielding pattern is found for the appended protons. Hence, a destabilizing interaction in these compounds alters polarization of the C—H bond, placing greater electron density on carbon, and its impact appears to be delocalized over many C—H bonds of the molecule.