Hydration of tetraethylammonium tetrafluoroborate derived from FTIR spectroscopy

Hydration of tetraethylammonium tetrafluoroborate (Et4NBF4) has been studied in aqueous solutions by means of FTIR spectroscopy. OD vibration of isotopically diluted HDO in H2O has been used as a probe of energetic states of hydrating water molecules. The H2O stretching vibration band has served as a source of structural information concerning solute hydration. The spectra have been analyzed in a way that led to removal of the contribution of bulk water and thus to the isolation of the spectra of ion-affected water. Et4N+-affected water spectrum has been discussed in terms of the energy of water H-bonds, interatomic O⋯O distances of hydration water, structural states of water molecules and their populations in the ion hydration sphere. Spectra evidence that water in the hydration sphere of the cation is in two different structural states: of ice-like and distorted tetrahedral water structures. The population of the first type is markedly increased when compared with the bulk phase, at the expense of the second type. The energy of ice-like H-bonds is, however, clearly lower than that of bulk water. Correspondingly, the interatomic O⋯O distances for water hydrating the cation are longer. Thus, water in the Et4N+ ion hydration shell appears to have a weakly bonded ice-like structure. The BF4− ion, as was previously known, is a very strong structure breaker. It has been established that water molecules interacting with the anion point their OH groups towards two fluorine atoms of BF4− but this orientation is not symmetric.