It is generally expected that the hydrogen bond strength in a D–H^•••A adduct is predicted by the difference between the proton affinities (ΔPA) of D and A, measured by the adduct stabilization and demonstrated by the infrared (IR) redshift of the D–H bond stretching vibrational frequency. These criteria do not always yield consistent predictions, as illustrated by the hydrogen bonds formed by the E and Z OH groups of protonated carboxylic acids. The ΔPA and the stabilization of a series of hydrogen bonded adducts indicate that the E OH group forms the stronger hydrogen bonds, whereas the bond length changes and the redshift favor the Z OH group, matching the results of NBO and AIM calculations. This reflects that the thermochemistry of adduct formation is not a good measure of the hydrogen bond strength in charged adducts, and that the ionic interactions in the E and Z adducts of protonated carboxylic acids are different. The OH bond length and IR redshift afford the better measure of hydrogen bond strength.

Keywords: hydrogen bond, ab initio calculations, thermochemistry, carboxylic acids, redshift, deformation energy, charge transfer, proton affinity