Hydration of tetrahydrofuran derived from FTIR spectroscopy

Hydration of tetrahydrofuran (THF) has been studied in aqueous solutions by means of FTIR spectroscopy. Isotopically diluted HDO in H2O has been applied, using the OD vibration band as a probe of energetic state of hydrated water molecules. The H2O stretching vibration band has been used in turn as a source of structural information concerning THF hydration. The spectra have been analyzed in a way that led to removal of the contribution of bulk water and thus to isolation of the spectra of THF-affected water. The analysis has been performed for aqueous solutions in relation to their molality. THF-affected water spectra for infinite dilution conditions have been compared to the bulk spectra and discussed in terms of the energy of water H-bonds, interatomic O⋯O distances of hydrated water, structural states of water molecules and their population in THF hydration sphere. It has been established that in diluted aqueous solutions the hydrophobic type of hydration clearly predominates the electrophilic one. The observed difference between the hydration sphere of THF and of bulk water is very subtle and comprises different distribution of water H-bonds and interatomic O⋯O distances. In net result, slight increase of water H-bonds energy and decrease of O⋯O distances have been observed in THF surroundings. There are accompanied by an increase of water H-bond population, even for ice-like type water network, but the energy of these H-bonds is weaker than for bulk water. Molecules influenced by THF have better defined structural and energetic states, which means better segregation and resulting negative contribution to the entropy change corresponding to THF transfer into water environment.