OH Stretching and Libration Bands of Solitary Water in Ionic Liquids and Dipolar Solvents Share a Single Dependence on Solvent Polarity

Abstract

Ionic liquids are an emerging class of materials which are finding application in a variety of technologically important areas. Because of their hydrophilic character, at least a small concentration of water is often present when ionic liquids are used in practical applications. This study employs infrared spectroscopy in the OH stretching and libration regions together with DFT calculations to better characterize the state of dilute water in ionic liquids. Water mole fractions (x w ∼ 0.1) are chosen such that nearly all water occurs in monomeric form and spectra probe the solvation structure and dynamics of solitary water molecules. New data are reported for a series of 1-ethyl-3-methylimidazolium liquids [Im21]­[X] with X– = (C2F5)3F3P–, (CF3SO2)2N–, BF4 –, B­(CN)4 –, CF3SO3 –, C2H5SO4 –, NO3 –, SCN–, and CH3CO2 –, as well as for the two 1-hexyl-3-methylimidazolium liquids [Im61]­[Cl] and [Im61]­[I]. For comparison, spectra are also recorded in a variety of dipolar solvents, and much of the available literature data are summarized, providing a comprehensive perspective on monomeric water in homogeneous solution. Most prior studies of dilute water in ionic liquids interpreted OH stretching spectra only in terms of water being specifically bonded to two anions in A–···H–O–H···A– type solvates. The more detailed analysis presented here indicates the additional presence of asymmetrically solvated water, which in some cases includes both singly solvated (A–···H–O–H) and more subtle forms of asymmetric solvation. The same pattern of solvation also pertains to dipolar solvents capable of accepting hydrogen bonds from water. No clear distinction is found between OH spectra in high-polarity conventional solvents and ionic liquids. In all solvents, OH frequencies are strongly correlated to measures of solvent basicity or hydrogen bond accepting ability. Far-infrared spectra of the water libration band also show common trends in ionic and dipolar solvents. Despite the different character of the libration and OH modes, the frequencies of these vibrations show virtually the same solvent dependence (apart from sign) except in weakly polar or nonpolar solvents.

Rapid water dynamics structures the OH-stretching spectra of solitary water in ionic liquids and dipolar solvents

“…We recently used mid-and farinfrared spectroscopy to better characterize the solvation and dynamics of dilute water in ionic liquids. 6 In that study, we found the OH-stretching spectra of isolated (monomeric) water to be more complex and indicative of a greater diversity of water solvation states than had been appreciated in prior investigations. Similar spectral features were also found in dipolar solvents of comparable polarity.…”

“…The experimental spectra presented here are all taken from the compilation of data in Ref. 6. They are solvent-subtracted transmission spectra of 6-25 µm thick samples recorded at room temperature (21 ± 1) °C at 2 cm -1 resolution using a Bruker Vertex 70v spectrometer.…”

“…To provide perspective on the solvents studied here, Table 1 lists two experimental measures of the solvents' hydrogen bonding abilities: the observed OH stretching frequencies of dilute water, <νOH>, 6 and the Kamlet-Taft hydrogen bond accepting parameter, βKT. 39 Also shown are the charges and Lennard-Jones diameters of the atoms that accept hydrogen bonds.…”

“…In Ref. 6 we reported that between three to five Gaussian components were required to represent the OH bands of isolated water in most solvents. Likewise, at least four components are needed to fit the simulated spectra in strongly hydrogen bonding solvents.…”

“…Figure 1 shows the spectrum of dilute water in 1-hexyl-3-methylimadazolium chloride ([Im61][Cl]), and the assignments of spectral features to vibrational modes of three classes of water solvates proposed in Ref. 6. (The notation 2SD2 etc.…”