Spectroscopic investigations on intramolecular hydrogen bonds

The Mannich reaction of aldehydes and secondary amines like piperidine or morpholine with substituted phenols or naphtols yields products which contain a proton donor and a proton acceptor in the same molecule. A favorable geometry allows the formation of an intramolecular hydrogen bond. Syntheses of derivatives of these Mannichbases with various aromatic substituents or different nitrogen bases allow to modify the donor and acceptor properties of the atoms belonging to the hydrogen bond. Such compounds are excellent model systems for the study of hydrogen bonding as a function of the electronic properties of the atoms in the bond. In addition, these compounds are useful for investigations of environmental effect. It was shown that increase of proton donor strength and increase of the basicity of the nitrogen base favor proton transfer. The properties of hydrogen bonds in different structures, solvents and at different temperatures were investigated by new spectroscopic techniques (UV, 1H-NMR, X-ray). The donor and acceptor subunits as well as the substituent at the methylen bridge determine the chemical shift of the proton which is a measure of the hydrogen bond strength. The chemical shift is dependent on the distance between the two heavy atoms of the hydrogen bond. It was shown that changes of environment and temperature changes have great influence on the structure and the properties of the hydrogen bond. (Author)