Slightly Viscous Amino Acid Ionic Liquids ... - ACS Publications

Oct 26, 2009 - ... glycine ethyl ester (DMPFGlyET+), and N-butyl-N,N-dimethyl glycine ethyl ester (DMBGlyET+) cations have been investigated and analy...
0 downloads 0 Views 3MB Size
Download PDF
15162

J. Phys. Chem. B 2009, 113, 15162–15169

Slightly Viscous Amino Acid Ionic Liquids: Synthesis, Properties, and Calculations Ling He,† Guo-Hong Tao,† Damon A. Parrish,‡ and Jean’ne M. Shreeve*,† Department of Chemistry, UniVersity of Idaho, Moscow, Idaho, 83844-2343, and NaVal Research Laboratory, 4555 OVerlook AVenue, Washington, DC 20375 ReceiVed: May 30, 2009; ReVised Manuscript ReceiVed: September 22, 2009

Slightly viscous N-alkyl-substituted glycine ester ionic liquids were prepared via alkylation of glycine ethyl esters with appropriate haloalkanes followed by anion exchange with lithium bis(trifluoromethanesulfonyl)amide. These ionic liquids have been characterized by IR, NMR, elemental analysis, thermal stability, phase behavior, viscosity, and density. Compound 9 crystallizes in the monoclinic system, P21/c. The viscosities of the N,N,N-trialkyl-substituted glycine ester ionic liquids (6, 8, 10) are in the ∼200 to 400 cP range at 25 °C. While exhibiting liquid characteristics analogous to the traditional heterocyclic ionic liquids, these new liquids are much less viscous than known amino acid ionic liquids. The correlation of viscosity and temperature was determined. To understand the influence of the alkyl and ester-substituted groups on viscosity, the electronic distributions and the electrostatic potential surfaces of the glycine-based cations, including glycine (Gly+), glycine ethyl ester (GlyET+), N,N-dimethyl glycine (DMGly+), N,N-dimethyl glycine ethyl ester (DMGlyET+), N,N-dimethyl-N-propyl glycine ethyl ester (DMPGlyET+), N,N-dimethyl-N-3-fluoropropyl glycine ethyl ester (DMPFGlyET+), and N-butyl-N,N-dimethyl glycine ethyl ester (DMBGlyET+) cations have been investigated and analyzed. The possible effects on the viscosity coming from the intermolecular interactions arising from Coulomb interactions, hydrogen bonding, polarizability effects, and van der Waals interactions are considered. Introduction In recent years, room temperature ionic liquids have received much attention as interesting modifiable solvents and soft materials.1-4 Because of their unique physical properties (e.g., high thermal stability, large liquid range, negligible vapour pressure), these materials are well suitable for use in a wide range of applications such as solvents,5-7 catalysts,8-11 electrolytes,11 lubricants,12 propellants,13-16 magnetics,17,18 and optical fluids.19-21 In most situations, the excellent liquid-state characteristics, for instance, good fluidity, good mass/heat transferability, etc., are required for useful applications which demand ionic liquids with low melting points and low viscosities.3 Viscosity is an intrinsic characteristic of all fluids arising from the internal friction of the fluid, and it manifests itself externally as the resistance of the fluid to flow.3 The viscosities of the common heterocyclic ionic liquids range from 10 to 10 000 cP at ambient temperature.22 In a simplified model, the high delocalization of the ion charges result in low Coulomb interactions between anions and cations, and thus low lattice energies, melting points, and viscosities. It is possible to modify the melting points and viscosities of ionic liquids by changing the type and the composition of cations and the anions.23 Some functionalized ions or groups, such as bis(trifluoromethylsulfonyl)amide (NTf2-) and dicyanamide (DCA-) anions,24,25 ether groups,26-30 and silyl- and siloxy-substituted imidazolium cations22,31 lead to reduction in the viscosity of ionic liquids. Amino acid ionic liquids are fascinating for chemists in view of their close association with chirality and biomolecules.32-35 A large number of novel amino acid ionic liquids based on amino acid cation or anion frameworks have been prepared.36-40 * To whom correspondence should be addressed. E-mail: jshreeve@ uidaho.edu. † University of Idaho. ‡ Naval Research Laboratory.

However, nearly all of the reported amino acid ionic liquids have relatively higher viscosities and melting points than the conventional imidazolium ionic liquids. Many of them are solid at room temperature. Others are room temperature liquids but with rather high viscosities from hundreds to tens of thousands cP.33 This “honeylike” characteristic of amino acid ionic liquids reduces their many potential applications as fluids and soft materials. The strong intermolecular interactions especially hydrogen-bonding interactions in the amino acid ionic liquids were supposed to be the key points for the high melting points and viscosities.22,40 Amino acid ionic liquids with ester groups have been reported with lower melting points as the result of reduced hydrogen bonding.40,41 While studies on the relationships of physicochemical properties and the structures of amino acid ionic liquids are absent, the amino acid ionic liquids with low melting points and low viscosities are of considerable interest and may have worthwhile applications. Herein, we report ionic liquids composed of N-substituted glycine cations with the bis(trifluoromethylsulfonyl)amide anion. These salts were selected as models in order to establish a general rule which would aid in predicting the impact on viscosity as different groups are substituted in the amino acid ionic liquids. For ready comparison of the effect perpetrated by the cation, the bis(trifluoromethylsulfonyl) amide (NTf2-) anion was chosen as the common anion. The ionic liquids obtained after alkylation and esterification were found to have low melting points as well as low viscosities. The intermolecular interactions in these amino acid ionic liquids may be explained based on the calculated charge distributions. Results and Discussion Synthesis. The N-unsubstituted or N,N-dialkyl-substituted glycine-based ionic liquids (1-4) were prepared by direct acidification of the corresponding glycine or glycine ester precursors by hydrochloric acid,40 followed by anion exchange

10.1021/jp905079e CCC: $40.75  2009 American Chemical Society Published on Web 10/26/2009

Slightly Viscous Amino Acid Ionic Liquids

J. Phys. Chem. B, Vol. 113, No. 46, 2009 15163

SCHEME 1: Synthesis of Glycine-Based Ionic Liquids

with lithium bis(trifluoromethanesulfonyl)amide (LiNTf2) in methanol. The solvent was removed, and the residue was then extracted with ethyl acetate. After evaporation under vacuum, white solids (1, 2) and colorless liquids (3, 4) were obtained (Scheme 1). The N-trialkyl-substituted glycine-based halide ionic liquids (5, 7, 9) were synthesized by the alkylation of glycine ester with bromo- or iodoalkanes. The corresponding NTf2-containing ionic liquids (6, 8, 10) were further prepared after the anion exchange of the halide ionic liquids with LiNTf2 in water. The colorless liquid products were obtained after washing the separated ionic liquid layer with water and drying under vacuum (Scheme 1). These new amino acid ionic liquids were characterized by IR, NMR, and elemental analysis. X-ray Structure. Since all of the N-trialkyl-substituted glycine-based bis(trifluoromethanesulfonyl)amides are room temperature liquids, to establish their structures, their iodide analogue, 9, was recrystallized from chloroform/diethyl ether solution, and structured by single crystal X-ray structure analysis. The crystallographic data are summarized in Table 1. Compound 9 crystallizes in the monoclinic system, P21/c. The structure consists of an N-butyl-N,N-dimethyl-glycine ethyl ester cation and an iodide anion. The density calculated for this single crystal is 1.54 g cm-3. Selected bond distances and angles are given in Table 2. All of the N-C bonds attached to the central nitrogen atom (C5-N6, 1.493 Å; N6-C13, 1.509 Å; N6-C12, 1.514 Å; and N6-C7, 1.522 Å) are a little longer than normal N-C single bonds (1.479 Å).42 Only the elongation of the N-C bond was observed owing to space hindrance (Figure 1). The space hindrance also results in the difference of the bond angles. The angle between the butyl group and the carboxylic group, C5-N6-C7, is 113.64°, a little larger than the other two angles (C5-N6-C13, 107.63°; C5-N6-C12, 110.27°). Moreover, the C4-O11 bond (1.198 Å) is a typical CdO double bond.42 The C4-O3 bond (1.316 Å) is a normal C-O single bond found in carboxylic esters,42 and, the C2-O3 bond (1.459 Å) is a normal C-O single bond in paraffinic compounds.42 Thus, there is no clear delocalization of the positive charge observed in the total cation. The packing structure of 9 is shown in Figure 2. It confirms that no extensive hydrogen-bond network exists involving the iodide anion. Physicochemical Properties. The melting point, decomposition temperature, density, and viscosity data of the compounds 1-10 are summarized in Table 3. They are thermally stable to about 200 °C. They have slightly higher stability than the known

TABLE 1: Crystal Data and Structure Refinement for 9 formula FW (g mol-1) size (mm3) cryst. syst. space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Fcalcd (g cm-3) T (K) µ (mm-1) F(000) λMoKR (Å) reflns Rint params S on F2 R1 (I > 2σ(I))a wR2 (I > 2σ(I))b R1 (all data) wR2 (all data)b ∆Fmin and max (e/Å3) a

C10H22INO2 315.19 0.42 × 0.28 × 0.25 monoclinic P21/c 7.1909(3) 15.5408(6) 12.3081(4) 90 97.359(1) 90 1364.13(9) 4 1.535 293(2) 2.329 632 0.71073 15047 0.0226 131 1.061 0.0265 0.0644 0.0319 0.0677 0.278 and -0.935

R1 ) ∑||Fo| - |Fc||)/∑|Fo|. b wR2 ) [∑w(Fo2 - Fc2)2/∑w(Fo2)2]1/2

TABLE 2: Selected Bond Lengths (Å) and Bond Angles (deg) for 9 C(2)-O(3) C(4)-O(11) N(6)-C(7) N(6)-C(13) C(5)-N(6)-C(13) C(13)-N(6)-C(12) C(13)-N(6)-C(7)

1.459(2) 1.198(3) 1.522(3) 1.509(2) 107.63(15) 106.88(16) 109.35(15)

O(3)-C(4) C(5)-N(6) N(6)-C(12)

1.316(2) 1.493(2) 1.514(3)

C(5)-N(6)-C(12) C(5)-N(6)-C(7) C(12)-N(6)-C(7)

110.27(16) 113.64(16) 108.84(16)

nitrates, GlyNO3 and GlyC1NO3, which decompose at 192 and 178 °C, respectively. Because their melting points are below 100 °C, all of these new compounds can be classified as ionic liquids. The structure of the anion is an important factor in determining the melting points of ionic liquids.3 In our case, the melting points of the halide ionic liquids (5, 7, 9) are 86, 71, 85 °C, respectively,

15164

J. Phys. Chem. B, Vol. 113, No. 46, 2009

He et al. TABLE 3: The Physicochemical Properties of Glycine-Based Ionic Liquids

Figure 1. Molecular structure of 9 (thermal ellipsoids shown at 30% probability). Hydrogen atoms are shown as open spheres of arbitrary radius and are unlabelled for clarity.

while the corresponding NTf2- anion ionic liquids (6, 8, 10) are room temperature liquids with glass transition temperatures at -62, -59, -64 °C, respectively. The NTf2- anion can introduce a significant decrease of more than 100 °C in the melting point compared with the corresponding halide ionic liquids. The lattice energy in the ionic liquids is reduced since the fluoro-substituted group disperses the anion charge. Ionic liquids 1-4, 6, 8, 10 have similar glycine cation frameworks with different alkyl-substituted groups, and the same NTf2- anion. Thus, the impact of the substituted cations on the melting points can be easily studied without other effects. Ionic liquid 1 without any substituted alkyl group melts at 50 °C. Its ethyl ester, 2, melts at 33 °C, but is still solid at room temperature. Compound 3, the isomeric compound of 2, with a dimethyl-substituted glycine cation is a room temperature liquid with a melting point of -31 °C. Ionic liquid, 4, is the ethyl ester derivative of 3 with a melting point of -48 °C. Ionic liquids 6, 8, 10 with N-alkyl-N,N-dimethyl-substituted glycine ethyl ester cations (alkyl ) N-propyl, N-3-fluoropropyl, N-butyl) did not display obvious melting points by DSC analysis. Each of them has a glass transition temperature at about -60 °C. It is interesting to note that the data show three details: (1) An ester group replacing a carboxylic group in the cation decreases the melting point by about 20 °C. This fact can be attributed to the reduction of the hydrogen bonding interactions which were due to the carboxylic acid group. While for ionic liquids with the NTf2- anion, this effect is not as obvious as in ionic liquids with other anions with the ability to participate in strong hydrogen bonding interactions such as nitrate.40 (2) The degree of the alkyl-substitution affects the melting points distinctly. From N-unsubstituted to the N,N-dialkyl-substituted, the melting points of the amino acid ionic liquids decrease by nearly 80 °C. The N,N,N-trialkyl-substituted amino acid ionic liquids have lower phase transition temperatures and excellent liquid characteristics. The hydrogen-bonding interaction involving the ammonium group is expected to be the most important factor in these NTf2- amino acid ionic liquids. (3) The fluorosubstituted group in the cation doesn’t cause an observable decrease in the phase transition temperature like that arising with the fluoro-substituted NTf2- anion. The melting point and the viscosity have positive correlations. The viscosity of a solid could be thought of as being infinite. Salts 1, 2, 5, 7, and 9 are unsuitable to be good fluid liquids at room temperature or even higher (60 °C). As a liquid, compound 3 was not checked at room temperature because it is too viscous for the instrument. Even at 60 °C, its viscosity is 1080 cP. Compound 3 is a very viscous ionic liquid because of the

d

ionic liquids

Tm (Tg) (°C)

Td (°C)

F (g cm-3)

1 2 3 4 5 6 7 8 9 10 GlyNO3b GlyC1NO3b TBPGlyd

50 33 -31 -48 86 (-62) 71 (-59) 85 (-64) 111 44 14

195 200 256 218 203 199 238 260 196 240 192 178 293

1.79 1.71 1.69 1.42 1.54 1.30 1.40 1.41 1.54 1.37 1.22 1.50

a Viscosity at 60 °C. Reference 33.

b

Reference 40.

c

η (cP)

(1080)a 611 (67)a 236 (36)a 405 (51)a 279 (41)a (92)c 415

Viscosity at 70 °C.

opportunity for hydrogen bonding which is still available although two methyl groups have replaced two hydrogen atoms which are present in 1. However, when the active hydrogen of the carboxyl group in the cation is replaced by an ester group, the viscosity of 4 is 611 cP at 25 °C; the ionic liquids 6, 8, and 10 having the N,N,N-trialkyl-substituted glycine ethyl ester cation, have lower viscosities than that of 4, at 236 cP, 405 cP, and 279 cP at 25 °C, respectively. It is clear that these N,N,Ntrialkyl-substituted amino acid ionic liquids have lower viscosities than other known amino acid ionic liquids, which are solid or very viscous (400∼10000 cP) at room temperature.33 The viscosities of many ionic liquids decrease rapidly upon temperature increase.3 At a higher temperature such as 60 °C, the viscosities of the N,N,N-trialkyl-substituted amino acid ionic liquids are lowered to a range between 10 and 100 cP. In Figure 3 is shown the temperature dependence of the viscosities for 4 (DMGlyET+/NTf2-), and 10 (DMBGlyET+/NTf2-). The graph of log viscosity versus temperature indicates that these amino acid ionic liquids do not display Arrhenius temperature behavior. Rather, the significant feature of glassy or supercooled liquids is observed. On approaching the glass transition temperature, there is a rapid increase in the viscosity and a slowing down of the structural relaxation. This temperature dependence of the viscosity η for glass-forming liquids can be well represented by the Vogel-Fulcher-Tammann (VFT) empirical equation (eq 1)43

(

η(T) ) η0 exp

DT0 T - T0

)

(1)

where η is the viscosity, T is the temperature, T0 corresponds to the characteristic temperature at which η is infinite, η0 is a reference viscosity, and D is a constant presenting the structural “strength” of the system. The VFT fit curves and the parameters in the equation are also shown in Figure 3. The viscosity versus temperature graphs can be fit well to the VFT model. For 4, η0 is 0.776 cP, D is 2.13, and T0 is 226 K. The ratio Tg/T0 is about 1. For 10, η0 is 0.012 cP, D is 9.94, and T0 is 150 K. The ratio Tg/T0 is about 1.4. However, our experimental temperature range from 298 to 343 K is much higher than the strongly supercooled liquid range. Although apparently excellent fits to the VFT model are obtained, the T0 values cannot be regarded to be highly accurate. Density Functional Theory Calculations. To obtain a better understanding of the relation between the charge distributions

Slightly Viscous Amino Acid Ionic Liquids

J. Phys. Chem. B, Vol. 113, No. 46, 2009 15165

Figure 2. Packing diagram of 9 viewed down the a-axis.

Figure 3. Temperature-dependent viscosities of 4 (DMGlyET+/NTf2-, and 10 (DMBGlyET+/NTf2-), VFT fit curves are also shown.

and the viscosities of the glycine-based ionic liquids, density functional theory calculations were performed at the level of Becke three Lee-Yan-Parr (B3LYP) parameters44-46 up to 6-311+G(d,p) basis sets using Gaussian 03.47,48 For all of the NTf2- anion ionic liquids (1-4, 6, 8, 10), we focused on the optimized geometries and the charge distribution analysis of the glycine-based cations. The differences of the substituted groups directly dominate the differences of the interactions in these ionic liquids, and then affect the melting points and the viscosities. The optimized geometries of the Gly+, GlyET+, DMGly+, DMGlyET+, DMPGlyET+, DMPFGlyET+, and DMBGlyET+ cations and the NTf2- anion, and the corresponding electrostatic potential surfaces for them are shown in Figure 4. The calculated volume in the gas phase and some selected charge distribution data of the cations are summarized in Table 4. The origin of microcosmic viscosity may be explained coming from the internal friction of molecules. The most important interaction in ionic liquids is the Coulomb interactions between ions.3 For NTf2- anion ionic liquids in this paper, the charges of ions are always +1 and -1. The volumes of the cations increase rapidly from 97 to 300 Å3 when introducing more substituted groups onto the glycine cation framework. The trend

Figure 4. Optimized geometries and electrostatic potential surfaces of the isolated Gly+, GlyET+, DMGly+, DMGlyET+, DMPGlyET+, DMPFGlyET+, DMBGlyET+ cations and NTf2- anion. The red and blue indicate regions of more negative and positive charges, respectively. The isodensity contours are 0.0004 e/bohr.3

that the bigger cation brings a lower viscosity can be supposed because the greater distance may weaken the Coulomb interactions between ions. In addition to increasing ion-ion separations, larger cations can allow greater charge delocalization. It has been shown that increasing the “charge-arm” of an ion may lead to increased mobility and lower viscosity.49 However, we find that the viscosity of ionic liquids 6 and 10, whose cation volumes are 265 and 300 Å3, respectively, are 236 and 279 cP.

15166

J. Phys. Chem. B, Vol. 113, No. 46, 2009

He et al.

TABLE 4: The Cation Volume in Gas Phase (Å3), and the Selected NBO Charge Distributions (e) of the Gly+, GlyET+, DMGly+, DMGlyET+ DMPGlyET+, DMPFGlyET+, and DMBGlyET+ cations at the B3LYP/6-311+G(d,p) Level of theory cations

V (Å3)

QN (e)

+

97.03 134.56 144.98 213.63 265.37 272.18 299.93

-0.695 -0.699 -0.447 -0.452 -0.345 -0.344 -0.345

Gly GlyET+ DMGly+ DMGlyET+ DMPGlyET+ DMPFGlyET+ DMBGlyET+

QO (e) -0.634, -0.605, -0.643, -0.605, -0.594, -0.595, -0.594,

-0.579 -0.519 -0.581 -0.526 -0.544 -0.543 -0.544

five highest QH (e) 0.521, 0.466, 0.517, 0.456, 0.251, 0.250, 0.251,

0.466, 0.442, 0.453, 0.251, 0.250, 0.249, 0.251,

0.446, 0.442, 0.256, 0.250, 0.248, 0.249, 0.247,

0.446, 0.255, 0.254, 0.232, 0.246, 0.248, 0.246,

0.262 0.255 0.233 0.231 0.228 0.231 0.228

TABLE 5: Total Energies EB3LYP and Binding Energies EB3LYPbin of Ion-Pair Structures of 1-4, 6, 8, and 10, Along with the Calculated Energies EB3LYP for the Cations and Anion Obtained at the B3LYP/6-31+G Level of Theory ionic liquids

cationa

EB3LYP, Cation (hartree)

EB3LYP, IL (hartree)

EB3LYPbin (hartree)

EB3LYPbin (kcal mol-1)

1 2 3 4 6 8 10

Gly+ GlyET+ DMGly+ DMGlyET+ DMPGlyET+ DMPFGlyET+ DMBGlyET+

-284.6965192 -363.3123315 -363.3083764 -441.9226011 -559.8318723 -659.0453632 -599.1375989

-2111.5407359 -2190.1494630 -2190.1282880 -2268.7368554 -2386.6289695 -2485.8410749 -2425.9493404

-0.2922133 -0.2851281 -0.2679082 -0.2622509 -0.2450938 -0.2437083 -0.2597381

-183.4 -178.9 -168.1 -164.6 -153.8 -152.9 -163.0

a

EB3LYP(NTf2-), -1826.5520034 hartree.

This result suggests strongly that the interaction energies between ions in these ionic liquids could be dominated not only by Coulomb interactions, but also by some other interactions such as polarizability effects and substantial effective dipoles.50 The negative charge on the nitrogen atom mainly reflects the influence of the substituted groups on the ammonium portion. The negative charges on the nitrogen atoms of these glycinebased cations are very different and can be separated into three levels. In the -NH3+-type cations, the central nitrogen atoms have the highest negative charge value (-0.695e for Gly+ and -0.699e for GlyET+). The moderate negative charges belong to the nitrogen atoms in -NMe2H+ cations. The values are -0.447e for DMGly+ and -0.452e for DMGlyET+. In the N,N,N-trialkyl-substituted cations, the nitrogen atoms have the lowest negative charge of about -0.345e. The negative charge on the oxygen atom can reflect the influence of the substituted groups on the carboxylic group portion. The negative charges on the oxygen atoms can also be separated into three levels. Without the ester group, the charges on the oxygen atoms are -0.634e, -0.579e in Gly+, and -0.643e, -0.581e in DMGly+. Esterification can disperse electronic density on the oxygen atoms. The ethyl ester group helps reduce the charge to -0.605e, -0.519e in GlyET+, and -0.605e, -0.526e in DMGlyET+. After the nitrogen atom was substituted by three alkyl groups, the charges on the oxygen atoms have the smallest negative charge (-0.594e, -0.544e in DMPGlyET+, -0.595e, -0.543e in DMPFGlyET+, -0.594e, -0.544e in DMBGlyET+). For an N-unsubstituted cation, nitrogen and oxygen atoms have the highest negative charge, and there are a sufficient number of active hydrogen atoms in the system to form a large number of hydrogen bonds, which is consistent with the high melting points and viscosities. Because N,N-dialkyl-substituted cations have fewer active hydrogen atoms, the nitrogen and oxygen atoms have smaller negative charges, thus the formation of hydrogen bonds is limited. The melting points and viscosities will be lower. For N,N,N-trialkyl-substituted cations, there is no active hydrogen atom in the system. The negative charges on the nitrogen and oxygen atoms are further reduced. The effect of hydrogen bonding in these amino acid ionic liquids can be ignored. The X-ray structure of 9 also reveals little or no hydrogen-bond network among the cations. The melting points and viscosities may reach the lowest level. Combining the

substituted groups and the charges on the nitrogen and oxygen atoms in the cations, it appears that hydrogen bonding still plays an important role in determining the melting points and viscosities of the amino acid ionic liquids. The phenomenon of hydrogen-bonding helping lower the viscosity of some imidazolium ionic liquids was not observed in such systems.51 The electrostatics from DFT calculations imply that polarizability effects will also make substantial contributions to the gas-phase pair interaction energies. The deep color on the calculated electrostatic potential surfaces (0.0004 e/bohr3) indicates a higher charge (Figure 4). The formation of the ionic liquid between the glycine-based cation and NTf2- anion should occur in those regions possessing more positive charges and more negative charges. As shown in Figure 4, the highest negative charge region in NTf2- anion appears on the central nitrogen atom. Thus, the favorable sites for glycine-based cation attack on this anion are concentrated on the regions around the nitrogen atom. For example, the high positive charge region in GlyET+ cation is around the -NH3+ group. The cation-anion electrical interactions could be concentrated between this region and the nitrogen region in NTf2- anion. There is one more high positive charge surface around the carboxylic acid group in Gly+ than in GlyET+. This results in the strongest intermolecular interactions including the hydrogen bonding with N-H and O-H. The calculated binding energy of 1 is -183.4 kcal mol-1 (Table 5). Accompanying alkylation and esterification, the area of the high positive charge region in the cations decreases, and the intermolecular interactions become weaker. Hence, the intermolecular interactions between the N,N,N-trialkyl-substituted glycine ester cations and NTf2- anion are the weakest. The calculated binding energy of 6, 8, and 10 are -153.8, -152.9, and -163.0 kcal mol-1, respectively. Meanwhile, an analogous influence also exists between two cations. The decrease of the charges on the surfaces will weaken the polarization ability. This will have some impact on the “friction” between the ions. However, ionic liquid 8 has smaller binding energy (-152.9 kcal mol-1) and higher viscosity (405 cP) than 6 (-153.8 kcal mol-1, 236 cP). For a larger cation, especially for a fluoro-substituted cation, van der Waals interactions show significant effect.24 The fluoro-substituted cation doesn’t play a dominant role on the decrease of viscosity compared with the fluoro-substituent anions.

Slightly Viscous Amino Acid Ionic Liquids Conclusion Glycine-based ionic liquids were prepared and characterized. Their physicochemical properties including melting points, decomposition temperatures, densities, and viscosities were investigated. The N,N,N-trialkyl-substituted glycine ethyl ester NTf2- ionic liquids (6, 8, 10) have the lowest melting points and viscosities. On the basis of theoretical calculations, the charge distributions and the electrostatic potential surfaces of these amino acid ionic liquids were analyzed. The possible changes of the interactions caused by different substituted groups on Coulomb interactions, hydrogen bonding, polarizability effects, and van der Waals interactions were considered. Combining the experimental and the calculated results, it concluded that the alkylation and esterification of the amino acid cations can effectively decrease the melting points and the viscosities. On the basis of our results, many low viscous amino acid ionic liquids may be designed for potential applications as solvents, electrolytes, lubricants, soft materials, etc. Experimental Details General Methods. All chemicals were obtained commercially as analytical grade materials and used as received. Solvents were dried by standard procedures. IR spectra were recorded by using KBr plates for neat liquids and KBr pellets for solids on a Biorad model 3000 FTS spectrometer. 1H, 13C, and 19F NMR spectra were recorded on a Bruker 300 MHz nuclear magnetic resonance spectrometer operating at 300, 75, and 282 MHz, respectively, with d6-DMSO as locking solvent unless otherwise stated. 1H and 13C chemical shifts are reported in ppm relative to TMS; 19 F reported relative to CCl3F. The densities of each ionic liquid were measured at 25 °C on a Micromeritics Accupyc 1330 gas pycnometer. Differential scanning calorimetry (DSC) measurements were performed on a TA DSC Q10 calorimeter equipped with an Autocool accessory and calibrated using indium. Measurements were carried out by heating from 40 °C (for solids) or -100 °C (for liquids) to 400 °C at 10 °C min-1. Thermogravimetric analysis (TGA) measurements were accomplished on a TA TGA Q50 by heating samples at 10 °C min-1 from 25 to 500 °C in a dynamic nitrogen atmosphere (flow rate 70 mL min-1). Elemental analyses (H, C, N) were performed on a CE-440 Elemental Analyzer. The viscosities were measured with a Grabner MINIVIS II Portable Micro viscometer. Computations were performed by using the Gaussian 03 (revision D.01) suites of programs.47,48 X-ray Crystallography. A colorless prism of 9 was mounted on a MiteGen MicroMesh using a small amount of Cargille Immersion Oil. Data were collected on a Bruker three-circle platform diffractometer equipped with a SMART APEX II CCD detector. The crystals were irradiated using graphite monochromated MoKR radiation (λ ) 0.71073). An MSC X-Stream low temperature device was used to keep the crystals at a constant 293(2) K during data collection. Data collection was performed and the unit cell was initially refined using APEX2 [v2.1-0].52 Data reduction was performed using SAINT [v7.34A]53 and XPREP [v2005/2].54 Corrections were applied for Lorentz, polarization, and absorption effects using SADABS [v2004/1].55 The structure was solved and refined with the aid of the programs in the SHELXTL-plus [v6.12] system of programs.56 The full-matrix least-squares refinement on F2 included atomic coordinates and anisotropic thermal parameters for all non-H atoms. The H atoms were included using a riding model. Details of the data collection and refinement are given in the Supporting Information.

J. Phys. Chem. B, Vol. 113, No. 46, 2009 15167 Glycine hydrochloride, glycine ethyl ester hydrochloride, N,Ndimethyl glycine hydrochloride, and N,N-dimethyl glycine ethyl ester hydrochloride were synthesized according to literature procedure by direct acidification of the corresponding glycine precursors.40 The corresponding NTf2- salts (1-4) were obtained via anion exchange of these chlorides with LiNTf2.40 Glycine Bis(trifluoromethanesulfonyl)amide (1). White solid. Yield: 90%. 1H NMR: δ ) 3.68 (s, 2H, CH2), 8.03 (s, 3H, NH3+) ppm. 13C NMR: δ ) 40.64, 117.38, 121.64, 169.10 ppm. 19F NMR: δ ) -78.70 ppm. IR (KBr): νmax ) 3530, 3158, 1750, 1632, 1572, 1435, 1350, 1330, 1272, 1195, 1124, 1058, 797, 743, 617, 577, 516 cm-1. Anal. Calcd for C4H6F6N2O6S2 (356.22): C, 13.49; H, 1.70; N, 7.86. Found: C, 13.41; H, 1.60; N, 7.55. Glycine Ethyl Ester Bis(trifluoromethanesulfonyl)amide (2). White solid. Yield: 92%. 1H NMR: δ ) 1.25 (t, 3H, J ) 7.2 Hz, CH2CH3), 3.82 (s, 2H, CH2), 4.22 (q, 2H, J ) 7.2 Hz, CH2CH3), 8.13 (s, 3H, NH3+) ppm. 13C NMR: δ ) 14.77, 40.01, 62.63, 118.66, 122.65, 168.69 ppm. 19F NMR: δ ) -78.70 ppm. IR (KBr): νmax ) 3509, 3229, 1746, 1634, 1502, 1437, 1421, 1350, 1329, 1304, 1203, 1123, 1054, 904, 856, 797, 770, 745, 575, 522 cm-1. Anal. Calcd for C6H10F6N2O6S2 (384.27): C, 18.75; H, 2.62; N, 7.29. Found: C, 18.79; H, 2.49; N, 6.75. N,N-Dimethyl Glycine Bis(trifluoromethanesulfonyl)amide (3). Colorless liquid. Yield: 90%. 1H NMR: δ ) 2.80 (s, 6H, CH3), 3.97 (s, 2H, CH2) ppm. 13C NMR: δ ) 44.24, 57.97, 118.38, 122.63, 168.45 ppm. 19F NMR: δ ) -78.70 ppm. IR (film): νmax ) 3546, 3146, 1744, 1635, 1471, 1418, 1348, 1331, 1196, 1140, 1059, 961, 901, 857, 796, 742, 613, 574, 513 cm-1. Anal. Calcd for C6H10F6N2O6S2 (384.27): C, 18.75; H, 2.62; N, 7.29. Found: C, 18.72; H, 2.54; N, 6.70. N,N-Dimethyl Glycine Ethyl Ester Bis(trifluoromethanesulfonyl)amide (4). Colorless liquid. Yield: 94%. 1H NMR: δ ) 1.22 (t, 3H, J ) 7.2 Hz, CH2CH3), 2.70 (s, 6H, CH3), 3.94 (s, 2H, CH2), 4.20 (q, 2H, J ) 7.2 Hz, CH2CH3) ppm. 13C NMR: δ ) 13.84, 43.56, 56.47, 62.00, 117.53, 121.79, 166.28 ppm. 19 F NMR: δ ) -78.71 ppm. IR (film): νmax ) 3543, 3144, 2994, 1744, 1635, 1454, 1412, 1329, 1197, 1053, 961, 874, 835, 793, 742, 577, 517 cm-1. Anal. Calcd for C8H14F6N2O6S2 (412.33): C, 23.30; H, 3.42; N, 6.79. Found: C, 23.45; H, 3.29; N, 6.45. N,N-Dimethyl-N-propyl Glycine Ethyl Ester Iodide (5). This was prepared by alkylation of N,N-dimethyl glycine ethyl ester with 1-iodopropane at 40 °C. N,N-dimethyl glycine ethyl ester (1.0 mmol, 131 mg) and 1-iodopropane (1.1 mmol, 187 mg) were stirred at 40 °C for 24 h. The crude product was washed with diethyl ether (3 × 5 mL) and dried under vacuum. A white solid was obtained (yield, 98%). 1H NMR: δ ) 0.86 (t, 3H, J ) 7.2 Hz, CH2CH3), 1.24 (t, 3H, J ) 7.2 Hz, CH2CH2CH3), 1.68 (m, 2H, CH2CH2CH3), 3.18 (s, 6H, CH3), 3.43 (t, 2H, J ) 8.4 Hz, CH2CH2CH3), 4.23 (q, 2H, J ) 7.2 Hz, CH2CH3), 4.40 (s, 2H, CH2) ppm. 13C NMR: δ ) 10.30, 13.77, 15.51, 51.01, 60.58, 61.94, 65.75, 164.59 ppm. IR (KBr): νmax ) 2971, 2936, 2882, 1744, 1651, 1456, 1386, 1271, 1225, 1204, 1116, 1054, 1028, 934, 879, 824, 761, 621 cm-1. Anal. Calcd for C9H20INO2 (301.17): C, 35.89; H, 6.69; N, 4.65; Found: C, 36.02; H, 6.82; N, 4.69. N,N-Dimethyl-N-propyl Glycine Ethyl Ester Bis(trifluoromethanesulfonyl)amide (6). This was prepared by anion exchange of salt 5 with LiNTf2 solution (H2O) at room temperature. The salts 5 (1.0 mmol, 301 mg) and LiNTf2 (1.1 mmol, 316 mg) were mixed with stirring at room temperature. The crude product was washed with water (3 × 5 mL) and dried under vacuum. A slightly yellow liquid was obtained (yield, 97%). 1H NMR: δ ) 0.90 (t, 3H, J ) 7.2 Hz, CH2CH3), 1.25

15168

J. Phys. Chem. B, Vol. 113, No. 46, 2009

(t, 3H, J ) 7.2 Hz, CH2CH2CH3), 1.72 (m, 2H, CH2CH2CH3), 3.18 (s, 6H, CH3), 3.43 (t, 2H, J ) 8.4 Hz, CH2CH2CH3), 4.25 (q, 2H, J ) 7.2 Hz, CH2CH3), 4.39 (s, 2H CH2) ppm. 13C NMR: δ ) 10.04, 13.46, 15.65, 51.06, 60.82, 62.13, 66.36, 117.58, 121.84, 164.75 ppm. 19F NMR: δ ) -78.70 ppm. IR (film): νmax ) 2984, 2890, 1749, 1479, 1414, 1352, 1195, 1138, 1057, 935, 884, 790, 615 cm-1. Anal. Calcd for C11H20F6N2O2S2 (454.41): C, 29.07; H, 4.44; N, 6.16. Found: C, 29.28; H, 4.43; N, 6.34. N,N-Dimethyl-N-3-fluoropropyl Glycine Ethyl Ester Bromide (7). The same procedure was followed as that described above for 5. N,N-dimethyl glycine ethyl ester (1.0 mmol, 131 mg) and 1-bromo-3-fluoropropane (1.1 mmol, 155 mg) was used. A slightly yellow liquid, 7, was obtained (yield, 95%). 1 H NMR: δ ) 1.24 (t, 3H, J ) 7.2 Hz, CH2CH3), 2.20 (m, 2H, CH2CH2CH2F), 3.25 (s, 6H, CH3), 3.67 (t, 2H, J ) 8.1 Hz, CH2CH2CH2F), 4.22 (q, 2H, J ) 7.2 Hz, CH2CH3), 4.46 (t, 1H, J ) 5.4 Hz, CH2CH2CH2F), 4.62 (t, 1H, J ) 5.1 Hz, CH2CH2CH2F) ppm. 13C NMR: δ ) 13.73, 23.64, 50.89, 60.64, 61.00, 61.95, 82.08, 164.72 ppm.19F NMR: δ ) -219.57 ppm. IR (KBr): νmax ) 2990, 2983, 2934, 1740, 1637, 1489, 1464, 1407, 1387, 1235, 1202, 1165, 1041, 1028, 994, 943, 899, 543 cm-1. Anal. Calcd for C9H19BrFNO2 (272.16): C, 39.72; H, 7.04; N, 5.15. Found: C, 40.02; H, 7.15; N, 5.26. N,N-Dimethyl-N-3-fluoropropyl Glycine Ethyl Ester Bis(trifluoromethanesulfonyl)amide (8). The same procedure was followed as that described above for 6. Salt 7 (1.0 mmol, 272 mg) and LiNTf2 solution (1.1 mmol, H2O) were used. A colorless liquid was obtained (8, yield, 93%). 1H NMR: δ ) 1.25 (t, 3H, J ) 7.2 Hz, CH2CH3), 2.19 (m, 2H, CH2CH2CH2F), 3.21 (s, 6H, CH3), 3.63 (t, 2H, J ) 8.1 Hz, CH2CH2CH2F), 4.24 (q, 2H, J ) 7.2 Hz, CH2CH3), 4.46 (t, 1H, J ) 5.4 Hz, CH2CH2CH2F), 4.62 (t, 1H, J ) 5.1 Hz, CH2CH2CH2F) ppm. 13 C NMR: δ ) 13.61, 23.60, 51.16, 60.91, 61.65, 62.12, 82.05, 117.49, 121.75, 164.72 ppm. 19F NMR: δ ) -78.71, -219.64 ppm. IR (film): νmax ) 2989, 2916, 1749, 1481, 1410, 1351, 1187, 1137, 1057, 903, 790, 741, 654, 615, 571 cm-1. Anal. Calcd for C11H19F7N2O6S2 (472.40): C, 27.97; H, 4.05; N, 5.93. Found: C, 27.97; H, 3.96; N, 5.89. N-Butyl-N,N-dimethyl Glycine Ethyl Ester Iodide (9). The same procedure was followed as that described above for 5. N,N-dimethyl glycine ethyl ester (1.0 mmol, 131 mg) and 1-iodo-butane (1.1 mmol, 202 mg) were used. A white solid was obtained (9, yield, 99%). 1H NMR: δ ) 0.91 (t, 3H, J ) 7.2 Hz, CH2CH3), 1.26 (m, 5H, CH2CH2CH2CH3), 1.66 (m, 2H, CH2CH2CH2CH3), 3.20 (s, 6H, CH3), 3.48 (t, 2H, J ) 8.4 Hz, CH2CH2CH2CH3), 4.24 (q, 2H, J ) 7.2 Hz, CH2CH3), 4.42 (s, 2H, CH2) ppm. 13C NMR: δ ) 13.36, 13.78, 18.96, 23.74, 51.01, 60.52, 61.87, 64.20, 164.62 ppm. IR (KBr): νmax ) 2994, 2968, 2938, 2876, 1740, 1465, 1461, 1403, 1378, 1238, 1211, 1171, 1124, 1027, 980, 889, 735, 592 cm-1. Anal. Calcd for C10H22INO2 (315.19): C, 38.11; H, 7.04; N, 4.44. Found: C, 38.09; H, 7.14; N, 4.55. N-Butyl-N,N-dimethyl Glycine Ethyl Ester Bis(trifluoromethanesulfonyl)amide (10). The same procedure was followed as that described above for 6. Salt 9 (1.0 mmol, 315 mg) and LiNTf2 solution (1.1 mmol, H2O) were used. A colorless liquid was obtained (10, yield, 97%). 1H NMR: δ ) 0.93 (t, 3H, J ) 7.2 Hz, CH2CH3), 1.28 (m, 5H, CH2CH2CH2CH3), 1.67 (m, 2H, CH2CH2CH2CH3), 3.18 (s, 6H, CH3), 3.47 (t, 2H, J ) 8.4 Hz, CH2CH2CH2CH3), 4.25 (q, 2H, J ) 7.2 Hz, CH2CH3), 4.39 (s, 2H, CH2) ppm. 13C NMR: δ ) 13.07, 13.44, 19.12, 23.94, 51.03, 60.75, 62.11, 64.80, 117.58, 121.84, 164.76 ppm 19F NMR: δ ) -78.71 ppm. IR (film):

He et al. νmax ) 3050, 2971, 2882, 1748, 1470, 1409, 1386, 1354, 1188, 1140, 1057, 934, 896, 790, 740, 655, 618, 571, 513 cm-1. Anal. Calcd for C12H22F6N2O6S2 (468.43): C, 30.77; H, 4.73; N, 5.98. Found: C, 30.63; H, 4.76; N, 6.17. Acknowledgment. The authors gratefully acknowledge the support of DTRA (HDTRA1-07-1-0024), NSF (CHE-0315275), and ONR (N00014-06-1-1032). Supporting Information Available: Ab initio computational data and X-ray crystallographic data of 9. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice; Oxford University Press: New York, 1998. (2) Rogers, R. D.; Seddon, K. R. Ionic Liquids: Industrial Applications to Green Chemistry; ACS Symposium, Series 818; American Chemical Society: Washington, DC, 2002. (3) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis; WileyVCH: Weinheim, Germany, 2003. (4) Rogers, R. D.; Seddon, K. R. Ionic Liquids IIIA-B: Fundamentals, Progress, Challenges, and Opportunities: Properties and Structure; ACS Symposium Series 901-902; American Chemical Society: Washington, DC, 2005. (5) Sheldon, R. A. Green Chem. 2005, 7, 267–278. (6) Wasserscheid, P.; Keim, W. Angew. Chem. Int. Ed. 2000, 39, 3772– 3789. (7) Rogers, R. D.; Seddon, K. R.; Ionic Liquids as Green SolVents: Progress and Prospects; ACS Symposium Series 856; Washington, DC, 2003. (8) Sheldon, R. A. Chem. Commun. 2001, 2399–2407. (9) Zhao, D. B.; Wu, M.; Kou, Y.; Min, E. Z. Catal. Today 2002, 74, 157–189. (10) Welton, T. Coord. Chem. ReV. 2004, 248, 2459–2477. (11) MacFarlane, D. R.; Forsyth, M.; Howlett, P. C.; Pringle, J. M.; Sun, J.; Annat, G.; Neil, W.; Izgorodina, E. I. Acc. Chem. Res. 2007, 40, 1165–1173. (12) Ye, C.-F.; Liu, W.-M.; Chen, Y.-X.; Yu, L.-G. Chem. Commun. 2001, 2244–2245. (13) Singh, R. P.; Verma, R. D.; Meshri, D. T.; Shreeve, J. M. Angew. Chem., Int. Ed. 2006, 45, 3584–3601. (14) Schneider, S.; Hawkins, T.; Rosander, M.; Vaghjiani, G.; Chambreau, S.; Drake, G. Energy Fuels 2008, 22, 2871–2872. (15) Tao, G. H.; Huang, Y.-G.; Boatz, J. A.; Shreeve, J. M. Chem.sEur. J. 2008, 14, 11167–11173. (16) Gao, H.; Joo, Y.-H.; Twamley, B.; Zhou, Z.; Shreeve, J. M. Angew. Chem. 2009, 48, 2792–2795. (17) Del Sesto, R. E.; McCleskey, T. M.; Burrell, A. K.; Baker, G. A.; Thompson, J. D.; Scott, B. L.; Wilkes, J. S.; Williams, P. Chem. Commun. 2008, 447–449. (18) Mallick, B.; Balke, B.; Felser, C.; Mudring, A.-V. Angew. Chem., Int. Ed. 2008, 47, 7635–7638. (19) Parker, S. T.; Slinker, J. D.; Lowry, M. S.; Cox, M. P.; Bernhard, S.; Malliaras, G. G. Chem. Mater. 2005, 17, 3187–3190. (20) Lunstroot, K.; Driesen, K.; Nockemann, P.; Grller-Walrand, C.; Binnemans, K.; Bellayer, S.; Le Bideau, J.; Vioux, A. Chem. Mater. 2006, 18, 5711–5715. (21) Tang, S.; Babai, A.; Mudring, A.-V. Angew. Chem., Int. Ed. 2008, 47, 7631–7634. (22) Shirota, H.; Castner, E. W., Jr. J. Phys. Chem. B 2005, 109, 21576– 21585. (23) Davis, J. H. Chem. Lett. 2004, 33, 1072–1077. (24) Bonhoˆte, P.; Sias, A.-P.; Papageorgiou, N.; Kalyanasundaram, K.; Gra¨tzel, M. Inorg. Chem. 1996, 35, 1168–1178. (25) MacFarlane, D. R.; Forsyth, S. A.; Golding, J.; Deacon, G. B. Green Chem. 2002, 4, 444–448. (26) Cooper, E. I.; Angell, C. A. Solid State Ionics 1983, 9&10, 617– 622. (27) Matsumoto, H.; Yanagida, M.; Tanimoto, K.; Nomura, N.; Kitagawa, Y.; Miyazaki, Y. Chem. Lett. 2000, 31, 922–923. (28) Funston, A. M.; Wishart, J. F. Dynamics of Fast Reactions in Ionic Liquids. In Ionic Liquids IIIA: Properties and Structure; Rogers, R. D., Seddon, K. R., Ed.; ACS Symposium Series 901; American Chemical Society: Washington, DC, 2005, 102-116. (29) Zhou, Z.-B.; Matsumoto, H.; Tatsumi, K. Chem.sEur. J. 2005, 11, 752–766. (30) Fei, Z.; Ang, W. H.; Zhao, D. B.; Scopelliti, R.; Zvereva, E. E.; Katsyuba, S. A.; Dyson, P. J. J. Phys. Chem. B 2007, 111, 10095–10108.

Slightly Viscous Amino Acid Ionic Liquids (31) Shirota, H.; Wishart, J. F.; Castner, E. W., Jr. J. Phys. Chem. B 2007, 111, 4819–4829. (32) Baudequin, C.; Bre´geon, D.; Levillain, J.; Guillen, F.; Plaquevent, J.-C.; Gaumont, A.-C. Tetrahedron: Asymmetry 2005, 16, 3921–3945. (33) Plaquevent, J.; Levillain, J.; Guillen, F.; Malhiac, C.; Gaumont, A. Chem. ReV. 2008, 108, 5035–5060. (34) Ohno, H.; Fukumoto, K. Acc. Chem. Res. 2007, 40, 1122–1129. (35) Liu, W. S.; Tao, G. H.; He, L.; Kou, Y. Chin. J. Org. Chem. 2006, 26, 1031–1038. (36) Bao, W. L.; Wang, Z.; Li, Y. J. Org. Chem. 2003, 68, 591–593. (37) Wasserscheid, P.; Bo¨smann, A.; Bolm, C. Chem. Commun. 2002, 200–201. (38) Clavier, H.; Boulanger, L.; Audic, N.; Toupet, L.; Mauduit, M.; Guillemin, J. C. Chem. Commun. 2004, 1224–1225. (39) Fukumoto, K.; Yoshizawa, M.; Ohno, H. J. Am. Chem. Soc. 2005, 127, 2398–2399. (40) Tao, G.-H.; He, L.; Sun, N.; Kou, Y. Chem. Commun. 2005, 3562– 3564. (41) Tao, G.-H.; He, L.; Liu, W. S.; Xu, L.; Xiong, W.; Wang, T.; Kou, Y. Green Chem. 2006, 8, 639–646. (42) Dean, J. A. Lange’s Handbook of Chemistry, 15th ed.; McGrawHill: New York, 1999; p 4.39. (43) Angell, C. A. Chem. ReV. 2002, 102, 2627–2650. (44) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (45) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785–789. (46) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989, 157, 200–206. (47) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; reven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;

J. Phys. Chem. B, Vol. 113, No. 46, 2009 15169 Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision D.01; Gaussian, Inc.: Wallingford, CT, 2004. (48) Glendening, E. D.; Reed, A. E.; Carpenter J. E.; Weinhold, F. NBO Version 3.1, as Implemented in Gaussian 03, reVision, D.01; Gaussian, Inc.: Wallingford, CT, 2004. (49) Kobrak, M. N.; Sandalow, N. An Electrostatic Interpretation of Structure-Property Relationships in Ionic Liquids. In Molten Salts XIV; Mantz, R. A., Ed.; The Electrochemical Society: Pennington, NJ, 2004; pp 417-425. (50) Hunt, P. A. J. Phys. Chem. B 2007, 111, 4844–4853. (51) Fumino, K.; Wulf, A.; Ludwig, R. Angew. Chem., Int. Ed. 2008, 47, 8731–8734. (52) APEX2 V2.1-0. Bruker AXS Inc.: Madison, Wisconsin, 2006. (53) SAINT V7.34A. Bruker AXS Inc.: Madison, Wisconsin, 2005. (54) XPREP V2005/2. Bruker AXS Inc.: Madison, Wisconsin, 2004. (55) SADABS V2004/1, Bruker AXS Inc.: Madison, Wisconsin, 2004. (56) SHELXTL V6.12 (b). Bruker AXS Inc.: Madison, Wisconsin, 2000.

JP905079E