Manifestation of intermolecular interactions in FTIR spectra of methylene blue molecules

Fourier-transform infrared (FTIR) spectra of cationic dye methylene blue (MB) were analyzed in its free form (gas-phase), hydrated crystalline state, and dried MB solution. A good agreement of experimental FTIR spectra with the results of theoretical calculations of MB vibration modes was observed. The calculations were performed by means of the density functional theory (DFT/B3LYP/6-31 + G(d)), using Gaussian-03 software package. Vibrational bands of the=N+(CH3)(2) (1640-1650 cm(-1)) and C-S-C (625-615,1095 cm(-1)) functional groups are characteristic of the presence of MB monomers in the studied samples. The participation of the =N+(CH3)(2) functional group in the formation of one bridging H-bond with OH groups of the solvent has been confirmed. The decrease in intensity of these bands with the increase of the fraction of dimers and H-aggregates of MB was observed. It was concluded that the configuration and primary localization of the charge in the heterocycle of ionic form MB+ are changed in the case of formation of dimers and H-aggregates. For samples with a dominant fraction of dimers and H-aggregates, the occurrence of intermolecular H-bonds of the type N-het center dot center dot center dot H-O center dot center dot center dot S-het(+) was found. The occurrence of peaks at 1339-1356,1481-1491, and 885 cm(-1) was observed. It was concluded that there is formation of dimers and H-aggregates due to interaction of sulfur and nitrogen atoms with OH groups of water molecules. Moreover, the manifestation of pi-pi stacking interactions of aromatic rings of the heterocycle of ionic form MB+ during self-association was determined on the basis of data on the behavior of the peaks of heterocyclic of CH (3050-3015 cm(-1)), C=C, C=N (1600-1595 cm(-1)), C-C, and C-N (1564-1546 cm(-1)) vibrations. (C) 2016 Elsevier B.V. All rights reserved.