Molecular states of water in room temperature ionic liquids

ATR and transmission IR spectroscopy have been used to investigate the stat e of water in room temperature ionic liquids (RTILs) based on the 1-alkyl-3 -methylimidazolium cation with the anions: [PF6](-), [SbF6](-), [BF4](-), [ ClO4](-), [CF3SO3](-), [(CF3SO2)(2)N](-), [NO3](-) and [CF3CO2](-). It has been shown that in these RTILs water molecules absorbed from the air are pr esent mostly in the "free" (not self-associated) state, bound via H-bonding with [PF6](-), [BF4](-), [SbF6](-), [ClO4](-), [CF3SO3](-), [(CF3SO2)(2)N] (-) with the concentrations of dissolved water in the range 0.2-1.0 mol dm( -3). It has been concluded that most of the water molecules at these concen trations exist in symmetric 1:2 type H-bonded complexes: anion . . . HOH . . . anion. Additional evidence that the preferred sites of interaction with water molecules are the anions has been obtained from the experiments with RTILs of the 1-butyl-2,3-dimethylimidazolium and 1-butyl-2,3,4,5-tetrameth ylimidazolium cations. Water molecules can also form associated liquid-like formations in RTILs with anions of stronger basicity such as [NO3](-) and [CF3CO2](-). When these RTILs are exposed to air the water concentrations e xceed 1.0 mol dm(-3). The strength of H-bonding between water molecules and anions increases in the order [PF6](-) <[SbF6](-) <[BF4](-) <[(CF3SO2)(2)N ](-) <[ClO4](-) <[CF3SO3](-) <[NO3](-) <[CF3CO2](-). The energies of this H -bonding were estimated from spectral shifts, with the resulting enthalpies being in the range 8-13 kJ mol(-1). ATR-IR spectroscopy has also been used to study H-bonding between methanol and RTILs.