High-pressure infrared spectroscopy was applied to study the hydrogen-bonding structures of 1-butyl-3-methylimidazolium halides/D2O mixtures. No drastic changes were observed in the concentration dependence of the alkyl C-H band frequency at high concentration of 1-butyl-3-methylimidazolium chloride. Nevertheless, the alkyl C-H exhibits an increase in frequency upon dilution at low concentration. These observations may indicate a clustering of the alkyl groups at high concentration and the formation of a certain water structure around alkyl C-H groups in the water-rich region. The imidazolium C-H band at ca. 3051 cm(-1) displays a monotonic blue-shift in frequency as the sample was diluted at high concentration of 1-butyl-3-methylimidazolium chloride. That is, water can be added to change the structural organization of 1-butyl-3-methylimidazolium chloride in the ionic liquid-rich composition region by introducing water-imidazolium C-H interactions. Analyzing the pressure dependence of the imidazolium C-H stretches yielded anomalous nonmonotonic pressure-induced frequency shifts. This result may reflect the strengthening of C-H-O interactions between imidazolium C-H groups and the water clusters. Density functional theory calculations also revealed that the characteristic bonded C2-H vibration may be shifted via the modification of C2-H-Cl- associations.

The structural organization in mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF(4)])/water or methanol was studied by infrared spectroscopy. No drastic change in the concentration dependence of the alkyl C-H band frequency was observed at high concentration of the ionic liquid. This behavior indicates a clustering of the ionic liquid in alkyl regions. Nevertheless, the presence of methanol significantly perturbs the ionic liquid-ionic liquid associations in the imidazolium region. On the basis of the responses to change in pressure and concentration, two different types of O-H species, i.e., free O-H and bonded O-H, were observed in the O-H stretching region. For [bmim][BF(4)]/water mixtures, the compression leads to loss of the free O-H band intensity. It is likely that free O-H is switched to bonded O-H as high pressures are applied. For [bmim][BF(4)]/methanol mixtures, the free O-H is still stable under high pressures.

We have investigated the aggregation behaviors of water molecules in 1-butyl-3-methylimidazolium tetrafluoroborate/ water mixtures using high-pressure methods. Under ambient pressure, the IR spectra indicate that two types of O-H species:freeO-HandbondedO-H,existinginionicliquid/watermixtures.Assampleswerecompressed,acontinuous lossofthefreeO-Hbandintensitywasobserved.Thisobservationmayhavearisenfromchangesinthelocalstructures of water molecules, and the geometrical properties of the hydrogen-bond network are likely to be perturbed as the pressureiselevated.AcomplementaryinsightoftheO-Hspectralfeaturesisobtainedbymeasuringtheconcentrationdependent variation in the mid-infrared spectra under high pressure. A sharp O-H stretching band was observed in a dilutedmixturecorrespondingtothehighorderinaniceVII-likestructure.Nevertheless,averybroadO-Hfeaturewas measuredinaconcentratedmixture,whichmaybeattributedtothepresenceofmorethanonestablecluster.