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J. Phys. Chem. B 2009, 113, 6426–6433
Effect of Cation Symmetry and Alkyl Chain Length on the Structure and Intermolecular Dynamics of 1,3-Dialkylimidazolium Bis(trifluoromethanesulfonyl)amide Ionic Liquids Dong Xiao, Larry G. Hines, Jr., Shengfu Li, Richard A. Bartsch, and Edward L. Quitevis* Department of Chemistry and Biochemistry, Texas Tech UniVersity, Lubbock, Texas 79409-1061
Olga Russina and Alessandro Triolo* Istituto per i Processi Chimico-Fisici-CNR, Salita Sperone, C. da Papardo, 98158 Faro Superiore, Messina, Italy, and Helmholtz Zentrum Berlin, SF-1 BENSC, Glienicker Strasse, D-14109 Berlin, Germany ReceiVed: NoVember 21, 2008; ReVised Manuscript ReceiVed: February 19, 2009
In this article, the structure and intermolecular dynamics of 1,3-alkylmethylimidazolium bis(trifluoromethanesulfonyl)amides [Cnmim][NTf2] with n ) 2-5 are compared to those of 1,3-dialkylimidazolium bis(trifluoromethanesulfonyl)amides [(Cn)2im][NTf2] with n ) 2-5. The structures of these room-temperature ionic liquids (RTILs) were studied by small-wide-angle X-ray scattering (SWAXS), and their intermolecular dynamics were studied by optical Kerr effect (OKE) spectroscopy. The SWAXS measurements indicate that, on a microscopic scale, the liquid structure of RTILs with symmetric cations is similar to that of RTILs with asymmetric cations. The OKE measurements indicate that the intermolecular dynamics of RTILs with symmetric cations are higher in frequency than those of RTILs with asymmetric cations. These results suggest that the local structure of RTILs with symmetric cations is more solid-like than that of RTILs with asymmetric cations. Further evidence for this difference in local structure on a mesoscopic spatial scale is that the width of the low-Q peak in the SWAXS data is narrower for [(C5)2im][NTf2] than for [C5mim][NTf2]. Moreover, the structure and intermolecular dynamics of the RTILs with ethyl-substituted cations appear to be quite different from those of other RTILs within a given series. This difference is evidenced by a clear change in the dependence of the spectral parameters of the intermolecular part of the OKE spectrum on the alkyl chain length in going from n ) 2 to n ) 3. The dependence of the SWAXS and OKE data on alkyl chain length is discussed within the context of the nanoscale heterogeneities of RTILs. I. Introduction Room-temperature ionic liquids (RTILs) are salts with melting points below 373 K. A typical RTIL consists of a bulky organic cation and a noncoordinating inorganic anion. Because of their low vapor pressure and wide liquid range and the ability to tune their physicochemical properties by modifying the cation and anion, RTILs were initially promoted as designer green solvents for liquid/liquid separations and as media for organic and inorganic reactions.1-4 The past several years have seen an enormous growth in interest in RTILs. This growth has led to novel applications of RTILs in chemical sensors,5,6 dyesensitized solar cells,7,8 and batteries9 and as energetic materials,10 thermal fluids,11 hydraulic fluids,12 lubricants,13 and ionogels.14 An important factor in determining the low melting points of RTILs is the unsymmetrical nature of the cation. For example, the melting points of N-butylpyridinium ([Nbupy]+) salts are 100 °C higher than those of their 1-ethyl-3-methylimidazolium ([C2mim]+) analogs because [Nbupy]+ has a mirror plane, whereas [C2mim]+, which has only C1 symmetry, does not.15 Dzyuba and Bartsch16 reported the synthesis of 10 1,3dialkylimidazolium hexafluorophosphates with dialkyl moieties ranging from dipropyl to didecyl. Surprisingly, 1,3-dialkylimidazolium hexafluorophosphates with dibutyl, dipentyl, diheptyl, dioctyl, dinonyl, and didecyl groups were found to be RTILs. * To whom correspondence should be addressed. E-mail: edward.
[email protected] (E.L.Q.),
[email protected] (A.T.).
In this work, the small-wide-angle X-ray scattering (SWAXS) and optical Kerr effect (OKE) spectra of 1,3-alkylmethylimidazolium bis(trimethanesulfonyl)amides [Cnmim][NTf2] with n ) 2-5 are compared to those of 1,3-dialkylimidazolium bis(trimethanesulfonyl)amides [(Cn)2im][NTf2] with n ) 2-5. The OKE spectra were obtained by means of optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). OHD-RIKES is a nonlinear optical time-domain technique that measures the collective polarizability anisotropy dynamics of a liquid.17-20 By use of a Fourier transform deconvolution procedure,21,22 the OHD-RIKES time-domain data can be converted to a reduced spectral density (RSD) or OKE spectrum, which is directly related to the depolarized Rayleigh/ Raman spectrum of the liquid.23 Because of its ease of use and the high quality of the resultant data, OHD-RIKES has recently become the most common method for studying the lowfrequency intermolecular modes of liquids19,20,24 and, in particular, RTILs.25-35 II. Experimental Section A. Synthesis of Ionic Liquids. The synthesis of [Cnmim][NTf2] compounds has been described previously.36 The following scheme summarizes the preparative method for [(Cn)2im][NTf2] Imidazole (125 mmol) was added dropwise to 125 mmol of NaH in distilled tetrahydrofuran (THF) under nitrogen in an ice-water bath. After addition was complete, the ice bath was removed, and the mixture was stirred for 2 h at room
10.1021/jp8102595 CCC: $40.75 2009 American Chemical Society Published on Web 04/09/2009
Structure and Dynamics of [Cnmim][NTf2] RTILs
temperature. Then, 1-bromoalkane (375 mmol) was added dropwise, and the mixture was refluxed for 8 h. [(Cn)2im]Br was obtained by filtering to remove the NaBr and evaporating the THF from the filtrate in vacuo. The excess 1-bromoalkane was removed by rinsing the ionic liquid with hexane. The known [(Cn)2im]Br compounds with n ) 2-5 were isolated in 90-95% yields. To exchange Br- for NTf2-, [(Cn)2im]Br was dissolved in water, and the solution was rinsed with dichloromethane to remove organic impurities. Lithium bis(trifluoromethanesulfonyl)amide (115 mmol) was added, and the mixture was stirred for 30 min. The lower organic layer was separated, rinsed with water three times, and dried with benzene in a Dean-Stark apparatus. Removal of the benzene in vacuo gave [(Cn)2im][NTf2] with n ) 2-5 in 97-99% yields. Although [(Cn)2im][NTf2] with n ) 2 and 4 have been reported, analogues with n ) 3 and 5 are new compounds whose structures were verified by combustion analysis. Calculated for C11H17N3S2O4F6 (i.e., n ) 3): C, 30.49; H, 3.95; N, 9.70. Found: C, 30.53; H, 3.89; N, 9.49. Calculated for C15H25N3S2O4F6 (i.e., n ) 5): C, 36.81; H, 5.15; N, 8.58. Found: C, 36.65; H, 4.82; N, 8.46. The water contents of the purified RTILs were determined by Karl Fischer titration to be
