1H NMR study of self-diffusion and molecular association of caffeine, theophylline and theobromine in the binary solvent methanol d4-carbon tetrachloride

The chemical shifts and self-diffusion coefficients of methylxanthines (caffeine, theophylline and theobromine) and tetramethylsilane in the binary solvent methanol d(4)-carbon tetrachloride have been investigated by H-1 NMR and H-1 PGSE NMR methods at the temperatures of 298, 303 and 313 K. The concentration dependences of tetramethylsilane self-diffusion coefficients have an inflection point and the concentration dependences of methylxanthines self-diffusion coefficients have a minimum in the region of 02-0.4 mol fraction of methanol d(4). The self-diffusion coefficient values for theophylline and theobromine are the same and lower than the corresponding value for caffeine. It has been found that rise in the methanol concentration leads to an increase in the chemical shifts of the imidazole ring protons (CH) in the methylxanthine molecule, which indicates a change in the methylxanthines molecular association degree. The differences in the concentration dependences of the self-diffusion coefficients were interpreted by taking into account the molecular association within the framework of the Stokes-Einstein equation. (C) 2018 Elsevier B.V. All rights reserved.