Self-association of caffeine in aqueous solution: an FT-IR study

Spectra of caffeine in D 2 O solution (0.003 to 0.129 M) were studied by Fourier-transform infrared spectroscopy. Spectral changes in the carbonyl stretching region were interpreted in terms of the self-association equilibria resulting from the stacking of caffeine molecules. Absence of isosbestic points indicated the occurrence of more than two spectroscopically distinct caffeine species. A simple monomer–dimer–polymer model was employed, in which the successive equilibria are governed by two association constants, K d for the formation of dimers and K p for the formation of trimers and larger polymers. The value K d = 158.1 L mol −1 was taken from a recent ultraviolet study by Iza etal. and the value K p = 27 L mol −1 was derived from the analysis of infrared band profiles sharpened by Fourier self-deconvolution. From these values of K d and K p , the weight fractions of monomers, dimers, and polymers were calculated for every caffeine concentration and their individual molar absorptivity spectra were derived. In agreement with the earlier work of Maevsky and Sukhorukov, it was found that stacking association causes shifts of both C=O stretching fundamentals to higher wavenumbers. The monomer-dimer shifts are smaller than the dimer-polymer shifts. Analogous shifts were observed for the stacking self-association of 1,3-dimethyluracil in D 2 O. The shifts to higher wavenumbers are probably caused by the diminished hydrogen bonding of water to C=O groups of the stacked bases.