Redshift or adduct stabilization--a computational study of hydrogen bonding in adducts of protonated carboxylic acids.

It is generally expected that the hydrogen bond strength in a D-H(***)A adduct is predicted by the difference between the proton affinities (DeltaPA) 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 DeltaPA 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.