Evaluation of hydrogen bonding ability of liquids and solids by C-13 NMR. Silica gel as a strong hydrogen bond donor

The chemical shift difference between signals of C(ß) and C(a) of unsaturated ketones, ?d, which we used before to measure acid strengths, has now been used to evaluate the hydrogen bond donor ability of solvents which are not acidic enough to hydronate the indicator. For such solvents there is no general correlation between H-bond donor ability and acid strength: hexa-fluoroisopropanol is a much weaker acid than acetic acid, but it is a stronger H-bond donor. The method can be applied to evaluate the H-bonding properties of solid surfaces, and it was thus found that silica gel has a much stronger H-bond donor ability than methanol or acetic acid.