Weak Hydrogen Bonds in Some Six- and Five-atom Interactions: An AIM Topological Analysis

In the 1H NMR spectra of several thiazole derivatives we have found that some of them present downfield shifts (to higher frequency). These can be attributed, in a broad sense, to intramolecular hydrogen bonding. However, we present an AIM study that shows there are two types of atomic interactions in these compounds: some originate bond critical points, but others do not. It is interesting to note that intramolecular hydrogen bonds were formed when a six-member ring and an electronegative atom, such as oxygen or chlorine, were involved. However, interactions involving five-member rings and a nitrogen atom as electron donor showed only three of the six required theoretical properties for hydrogen bonding. These special interactions can be considered as very weak hydrogen bonds. Nevertheless, both types of atomic interactions caused similar downfield shifts in the 1H NMR spectra. Thus, this study reveals differences in the nature of the atomic interactions that gave rise to downfield shifts in the 1H NMR spectra.