Manifestation of intermolecular interactions in FTIR spectra of methylene blue molecules

• FTIR spectra of Methylene Blue cation (MB+) were analyzed in their free form (gas-phase), hydrated crystal form and in solution.
• The complications, emerging during the interpretation are due to the high activity of this molecule to the intermolecular interactions.
• FTIR spectra of aqueous solution and hydrate show manifestation of dimerization and H-aggregation.
• Intermolecular hydrogen bonds and π–π interaction of MB+ aromatic rings were observed during self-association.
• The analysis of FTIR spectra indicates the identical nature of emerging intermolecular interactions for H-aggregates and hydrates.

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 Nhet…Н-O⋯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 π-π 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 СН (3050–3015 cm−1), СС, СN (1600–1595 cm−1), СС, and СN (1564–1546 cm−1) vibrations.