Ionic liquids vibrational spectroscopy

Even the lowest amounts of organic side products from this reaction lead to intense colourations. Most times their concentration is beyond the detection limit of, for example, vibrational spectroscopy or NMR techniques, and their identity is often unclear. The most efficient way to get rid of these impurities is by stirring the ionic liquids with activated charcoal and subsequently passing the reaction product through a column of (acidic) alumina [2], In the case of iodide salts attention should be paid to their light sensitive character. 

In contrast to the increasing information on the bulk properties of ionic liquids, very little is known about the interfacial structure of ionic liquids with other phases. Examinations of gas-ionic liquids inter ces of 1,3-dialkylimidazolium-based liquids using direct recoil spectroscopy [41] or sur ce vibrational spectroscopy [42-44] have provided some general indicative information about the sur ce composition... 

In another study which used a support more relevant to supported ionic liquid catalysts, sum-frequency vibrational spectroscopy (SFVS) was used to study the structure of ionic liquid cations present at the interface between a silica support and imidazolium ionic liquids [RMIM][X], with R = hex)d, oct)4 or decyl, and X = (CF3(CF2) S02)2N with = 0 or 1 [46]. The measurements were performed by introducing the ionic liquids in a thoroughly cleaned fused silica prism. 

These surface-specific techniques provide information on the ionic liquid structure and composition at the surface. They probe the interfacial region at various technique-specific probing depths for analysis of surface concentration and structural identification. Vibrational spectroscopy, SFG in particular, is a widely used tool for orientational analysis of ionic Uquids at interfaces [1]. 

One important use of SFG vibrational spectroscopy is the orientational analysis of ionic liquids at gas-liquid interfaces. For example, the study of the structural orientation ofionic liquids using common cation types, that is, [BMIM], combined with different anions, gives information on the effects of both cation and anion types [3, 22, 26-28]. Additional surface analytical work includes SFG studies under vacuum conditions for probing the second-order susceptibility tensor that depends on the polar orientation of the molecule and can be correlated to the measured SFG signal intensities. Supporting information is frequently obtained by complementary bulk spectroscopic techniques, such as Raman and Fourier transform infrared (FTIR) analysis, for the analysis of the pure ionic liquids. 

Ionic liquids at the gas-liquid and solid-liquid interface have been extensively studied by a variety of surface analytical techniques. The most prominent technique for surface orientational analysis proves to be SFG. Other vibrational spectroscopic and surface-sensitive techniques such as surface-enhanced Raman spectroscopy (SERS) and total internal reflection Raman spectroscopy (TIR Raman) have been employed for studying surface processes these techniques, however, have not been applied yet specifically for the study of ionic hquids. 

H.-O., and Ouchi, Y. (2007) Local structure at the air/liquid interface of room-temperature ionic liquids probed by infrared-visible sum frequency generation vibrational spectroscopy ... 

Fitchett, B.D. and Conboy, ).C. (2004) Stmcture of the room-temperature ionic liquid/SiOj interface studied by sum-frequency vibrational spectroscopy. J. Phys. Chem. B, 108, 20255-20262. 

Vibrational cooling rates in room temperature ionic liquids were measured with picosecond time-resolved Raman spectroscopy [63]. The 1570-cm Raman band of the first excited singlet (Sj) state of frans-stilbene was used. The recorded vibrational cooling rates in l-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (emimTf2N) and l-butyl-3-methylimidazolium bis(trifluoromethylsu]fonyl)imide (bmimTf2N) were close to those in ordinary molecular solvents despite a large difference in thermal diffusivity. 

Abstract Vibrational sum-frequency spectroscopy in conjunction with interfacial pressure measurements are used to provide the first direct spectroscopic information about the structure of amphiphillic molecules adsorbed to the interface between two immiscible liquids by total internal reflection sum-frequency vibrational spectroscopy (TIR SFVS). The effect of the ionic head group on the conformational order of sodium dodecyl sulfate (SDS), sodium dodecylsulfonate (DDS), dodecyltrimethylammonium chloride (DTAC), and dodecylamine hydrochloride (DAC) adsorbed at the D2O/CCI4 interface has been examined. In addition, the effect of the length of the alkyl chain on the conformation and orientation of sodium hexylsulfonate (HS), sodium undecylsulfonate (UDS), and sodium dodecylsulfonate (DDS) is also presented. SF vibrational spectra indicate the presence of gauche conformations in the hydrocarbon chains of all the surfactants examined. An increase in the surface coverage results in the reduction of gauche defects in the hydrocarbon chains as determined from the intensity ratio of the methyl to methylene symmetric... 

Presented here is a brief review of the vibrational spectroscopic techniques used to characterise ionic liquids on surfaces. Specifically, this review includes results obtained from sum-frequency generation spectroscopy (SFG), infrared reflection-absorption spectroscopy (IRAS), surface-enhanced Raman scattering (SERS) and high-resolution electron energy loss spectroscopy (HREELS). 

Mbrational spectroscopy is well suited to elucidating the details on the surface of ionic liquids. This is due to the high degree of chemical information that is provided in the vibrational spectrum The various techniques mentioned earlier each have advantages and limitations, but the level of information is clearly unique and highly informative. From the vibrational spectrum, information as to the identity of the molecules is available. However, more specifically, the chemical functional groups are identified as they each have unique... 

Overall, vibrational spectroscopy provides very useM chemical information on the surface of ionic liquids, and some specific exanples are given in the following. 

Iwahashi, T., Sakai, Y, Kanai, K., Kim, D. and Ouchi, Y, Allq l-chain dividing layer at an alcohol/ionic liquid buried interface studied by sum-frequency generation vibrational spectroscopy, Phys. Chem. Chem. Phys. 12,12943-12946 (2010). 

Iwata, K., Yoshida, K., Takada, Y. and Hamaguchi, H., Vibrational cooling process of trans-stilbene in ionic liquids observed with picosecond time-resolved Raman spectroscopy, Chem. Lett. 36, 504-505 (2007). 

Berg, R.W., Riisager, A., Van Buu, O.N., Fehrmann, R., Harris, P, Tomaszowska, A.A. and Seddon, K.R., Crystal structure, vibrational spectroscopy and ab initio density functional theory calculations on the ionic liquid forming 1,1,3,3-tetramethylguanidinium... 

Fig. 2 Suggested orientations of the imidazolium ring on the ionic liquid surface. The black line illustrate the surface plane a, b ion recoil spectrometry first classical molecular dynamics simulation d sum frequency generation vibrational spectroscopy ...

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