It is generally expected that the hydrogen bond strength in a D-H-A adduct
is predicted by the difference between the proton affinities of D and A,
measured by the adduct stabilization, and demonstrated by the 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 delta-PA 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.