On the relationship between the preferred site of hydrogen bonding and protonation

Ab initio molecular orbital calculations have been employed to investigate the interactions between a set of basic substrates (B) with H+ and HF, and the interaction between acids of varying strength (AH+) with two bases, vinylamine and furan. The preferred site for protonation of the substrates appears to be determined primarily by the ability of the protonated species (BH+) to delocalize the acquired positive charge. On the other hand, localization of a pair of electrons at a proton-acceptor site of B tends to be more important in determining the preferred site for hydrogen bonding with HF. The behavior of acids stronger than HF lies between these extremes. Consistent with a previously proposed Hammond postulate for complexes, when a substrate (B) interacts with a range of acids (AH+), proton transfer is generally found to occur when the proton affinity of A is significantly less than that of B. When the proton affinity of A is greater than that of B, a hydrogen-bonded complex is generally formed without proton transfer. Strongest binding (relative to the lowest energy components) occurs when the proton affinities of A and B are comparable. Proton transfer from AH+ is found to take place in some cases when this would not be predicted on the basis of protonation energies alone, because of specific interactions in the resulting complexes.