Infrared study of the interaction between caffeine and hydroxylic derivatives.

The interaction between caffeine and hydroxylic derivatives (phenols, water) has been studied by infrared (IR) spectroscopy in 1,2-dichloroethane. The formation constants, enthalpies, and entropies of complex formation have been determined. The IR spectra in the vOH and vc=o ranges indicate that hydrogen bond formation occurs at the carbonyl functions, approximately 70% of the complexes being formed at the O6 atom. These results are compared with 1,3-dimethyluracil complexes and discussed as a function of the ionization potential of the two oxygen atoms. In the solid hydrate, the spectroscopic results show that the interaction of water takes place on the N9 atom of the imidazole ring in agreement with earlier X-ray results. The different interaction sites in solution and in the solid state are briefly discussed.