Interactions and Transitions in Imidazolium Cation Based Ionic Liquids

Ionic liquids (ILs) raise considerable research interest not only as promising new solvents for the replacement of conventional solvents in synthesis, but also as new liquid materials.[1-2] It appears from the recent breakthroughs that the novel properties of various ILs are of more interest than mere application as ‘green solvent’ in traditional organic chemistry. ILs are now rediscovered to be whole new materials with many wonderful properties, much of it are yet to be discovered. The defining characteristic of ILs is of their constitutions molecular ions as their building blocks as opposed to molecules in the traditional solvents. In other words, ILs or molten salts in general are defined as liquids composed of ions only, either at room temperature or at elevated temperatures (below 100C). ILs are rather unique in the sense that in addition to ionic and covalent interactions, there are relatively weaker interactions such as H-bondings, and π-stacking, which are not commonly found in conventional solvents.[3-4] The nature of the forces in different ILs may however differ from one another and mainly control their physical properties. As the properties of any material depends on the structure of molecules in different phases, it is very important to understand the structural features of ILs in depth. Researchers have paid considerable attention towards pyridinium, imidazolium and pyrrolidinium based ILs in addition to ammonium and phosphonium ILs. Pyridinium based ILs are heterocyclic aromatic compounds proven to have great potential in organic synthesis and biocatalyst. Compared with the imidazolium based ILs, few studies have examined the biodegradability of pyridinium based ones.[5] Due to biodegradable property of these pyridinium and pyrrolidinium ILs, it has been extensively studied.[6-12] On the other hand, pyrrolidinium ILs are mainly involved in dye sensitized solar cell and batteries.[ 11] Since structure plays the vital role for any application, recently detailed x-ray scattering studies have been reported on the pyrrolidinium cations based ILs while varying the length of the alkyl chain attached with the ring.[12] Interestingly, diffraction pattern shows signature of intermediate range ordering similar to that of in imidazolium based ILs. Among all ionic liquids, imidazolium cation based ILs are the most extensively studied ILs, and therefore our discussion will mainly be confined with imidazolium cation based ILs. As of true ‘designer solvent’, it has been observed that a small variation in imidazolium cation (such as increase or decrease in alkyl chain length) alters their physical properties


Introduction
Ionic liquids (ILs) raise considerable research interest not only as promising new solvents for the replacement of conventional solvents in synthesis, but also as new liquid materials.[1][2] It appears from the recent breakthroughs that the novel properties of various ILs are of more interest than mere application as 'green solvent' in traditional organic chemistry.ILs are now rediscovered to be whole new materials with many wonderful properties, much of it are yet to be discovered.The defining characteristic of ILs is of their constitutions -molecular ions as their building blocks as opposed to molecules in the traditional solvents.In other words, ILs or molten salts in general are defined as liquids composed of ions only, either at room temperature or at elevated temperatures (below 100C).ILs are rather unique in the sense that in addition to ionic and covalent interactions, there are relatively weaker interactions such as H-bondings, and π-stacking, which are not commonly found in conventional solvents.[3][4] The nature of the forces in different ILs may however differ from one another and mainly control their physical properties.As the properties of any material depends on the structure of molecules in different phases, it is very important to understand the structural features of ILs in depth.Researchers have paid considerable attention towards pyridinium, imidazolium and pyrrolidinium based ILs in addition to ammonium and phosphonium ILs.Pyridinium based ILs are heterocyclic aromatic compounds proven to have great potential in organic synthesis and biocatalyst.Compared with the imidazolium based ILs, few studies have examined the biodegradability of pyridinium based ones.[5] Due to biodegradable property of these pyridinium and pyrrolidinium ILs, it has been extensively studied.[6][7][8][9][10][11][12] On the other hand, pyrrolidinium ILs are mainly involved in dye sensitized solar cell and batteries.[ 11] Since structure plays the vital role for any application, recently detailed x-ray scattering studies have been reported on the pyrrolidinium cations based ILs while varying the length of the alkyl chain attached with the ring.[12] Interestingly, diffraction pattern shows signature of intermediate range ordering similar to that of in imidazolium based ILs.Among all ionic liquids, imidazolium cation based ILs are the most extensively studied ILs, and therefore our discussion will mainly be confined with imidazolium cation based ILs.As of true 'designer solvent', it has been observed that a small variation in imidazolium cation (such as increase or decrease in alkyl chain length) alters their physical properties coloured.[23,26] The colour issue is important since during the synthesis of these derivatives (e.g., halogen and PF 6 , BF 4 anion containing alkylimidazolium ILs), often impurity imparts colour making it difficult to understand whether the colour is inherent to ILs concerned or is due to the colour impurity.[28][29] Especially, from spectroscopic application point of view it is very important to have optically transparent windows of the ILs.[28][29][30] TD-DFT theoretical calculation found to play a crucial role to explain, whether colour of given IL is intrinsic or due to impurity present in it.

Experimental
2.1 Synthesis of 1-butyl-3-methylimidazolium halide (bmimX, where X -= Cl, Br, I, NTf 2 , PF 6 and BF 4 ) Synthesis of all bmimX have been done following the reported method [4,23,31] with some modifications to get the product as much pure as possible.The major modification is mainly of the temperature at which reactions were done.Since we have used considerable less temperature than generally reported, the time required for the reactions are also much higher.Nevertheless, this low temperature reaction has been found to provide much pure ILs.

Cl - +
N-methylimidazole was dissolved in required amount of dry acetonitrile followed by slow addition of butyl halide at 0 0 C with constant stirring under N 2 atmosphere in dark.The solution was left at 0 0 C for 2h and then stirred for 3 days while keeping the temperature below 40 0 C. The progress of the reaction was monitored by checking the TLC (removal of methylimidazole).Acetonitrile (ACN) was then evaporated at reduced pressure at the same temperature.Pale yellow colour viscous liquid was obtained (yield: 70%) which was then washed with dry ethyl acetate and then followed by dry diethyl ether.During washing bmimCl was solidified.bmimCl was further purified with activated charcoal to remove coloured impurities.Finally, it was kept under reduced pressure (10 -3 bar) for 5 h at 40 0 C. Precautions were taken to eliminate the presence of water or organic solvents in the purified IL.The product was confirmed by IR: 754 cm -1 , 1168 cm -1 , 1463 cm -1 , 1571 cm -1 , 1628 cm -1 , 2870 cm -1 , 2960 cm -1 , 3098 cm -1 and 3152 cm -1 and 1 H NMR, (in CDCl 3 , ppm); 0.93(t), 1.36(sextet), 1.84(q), 4.11(s), 4.31(t), 7.50(s), 7.65(s), 10.38(s).Following the above mentioned procedure, bmimBr was synthesized.Here stirring was carried out for 2 days.The pale yellow coloured solid was obtained which was further treated with activated charcoal to remove coloured impurities.Thus obtained bmimBr was kept under reduced pressure (10 -3 bar) for 5 h at 40 0 C. Precautions were taken to eliminate the presence of water or organic solvents in the purified IL.The product was confirmed by IR: 794 cm -1 , 1168 cm -1 , 1463 cm -1 , 1570 cm -1 , 1629 cm -1 , 2871 cm -1 , 2960 cm -1 , 3086 cm -1 and 3155 cm -1 and

Computational method
The Gaussian 03 program [32] was used for the Density Functional Theory (DFT) calculation of different ILs.The basis sets already implemented in the program were used for the different types of calculations.The geometries of isolated ion pairs of ILs were optimized at the Becke's three parameter hybrid method with LYP correlation ( i.e.B3LYP).[33] While 6-31G++(d,p) basis set was used for C, H, N, O, S, F and P, DGDZVP basis set was used for I atom.[34] Due to large atomic number of iodine atom, 6-31 G++(d,p) basis set is found to be incapable of predicting accurate structure and vibrational spectrum of bmimI.Optimisation, frequency calculation and Time-Dependent Density functional Theory (TD-DFT) were also performed at the B3LYP level of calculation.The absence of imaginary vibrational frequencies in vibrational spectrum ensures the presence of a true minimum.

Studies of interactions by single crystal x-ray diffraction and Raman spectroscopic techniques
Among all imidazolium cation based ILs mentioned above, bmimCl and bmimBr are the two prototype ILs that have been studied extensively.[4,21,31] The interesting aspects which make these two monoatomic anion based ILs well studied is their existence both in www.intechopen.comsolid and super cooled liquid phases.Therefore these two ILs give the opportunity to study the structures and interactions present in both solid and liquid state with a possibility to make a correlation.Crystal structures of bmimCl and bmimBr at room temperature have been well studied.[4,21,31] It has been discovered simultaneously and independently by,  [31] that bmimCl crystal shows two crystal polymorphism.Schematic diagram of bmim cation has been presented in Figure 1.In crystal 1 (Figure 2a), the n-butyl group takes trans-trans (TT) conformation with respect to C 7 -C 8 and C 8 -C 9 bonds, while in crystal 2 (Figure 2b), n-butyl group takes gauche-trans (GT) conformation with respect to C 7 -C 8 and C 8 -C 9 bonds similar to that of bmimBr conformation.[4,25] Powdered x-ray diffraction patterns of the two polymorphs of bmimCl and bmimBr was recorded and found that the pattern for crystal 2 and bmimBr somewhat resembles each other, while that of bmimCl crystal 1 is distinct from the other two.[4,21] Fig. 2. The structure of bmim cation in bmimCl.(a) n-butyl chain has trans-trans conformation with respect to C7-C8 and C8-C9 bonds and therefore described as Crystal 1 or TT in text.(b) The crystal structure of bmim cation in bmimBr.In this case, n-butyl chain has gauche-trans conformation with respect to C7-C8 and C8-C9 bond and therefore termed as GT or Crystal 2 in the text.
Presence of polymorphs is found to be due to the different interactions in the crystal systems.In crystal 1, a couple of bmim cations form a pair through a hydrophobic interaction among stretched n-butyl group.Those pairs stack together and form a column in which all imidazolium ring planes are parallel with one another.Two types of cation columns with different orientations exist.Hamaguchi and co-workers have been studied these ILs extensively by Raman spectroscopic techniques.[21] It was found that Raman spectra of two polymorphs are markedly different from each other.In the wavenumber region 600-700 cm -1 , where ring deformation bands are expected, two bands appear at 730 cm -1 and 625 cm -1 in bmimCl crystal 1 (i.e., TT conformation), while another couple of bands appear at 701 and 603 cm -1 in bmimCl crystal 2 (GT) and bmimBr but not in bmimCl crystal 1.In order to clarify the origin of these bands, Ozawa et.al. has done extensive DFT calculation on these Ils.[22] It shows that the 625 cm -1 band of bmimCl crystal 1 and the 603 cm -1 band of bmimBr originates from the same ring but have different magnitude of couplings with the CH 2 rocking vibration of the C 7 carbon.The coupling occurs more effectively for the gauche conformation around the C 7 -C 8 bond, resulting in a lower frequency in the GT form (603 cm -1 ) than in the TT form (625 cm -1 ).It was noted that the coupling with the CH 2 rocking mode having a higher frequency pushes down the frequency of the ring deformation vibration.The same coupling scheme holds for another ring deformation mode and the GT form has a lower frequency (701 cm -1 ) than the TT form (730 cm -1 ).It is therefore elucidated from the DFT calculation that the 625 and 730 cm -1 bands are the characteristics of the trans conformation around the C 7 -C 8 bond, while the 603 and 701 cm -1 bands are the characteristics of the gauche conformation.In other words, one can use these bands as key bands to probe the conformation around the C 7 -C 8 bond of the bmim cation.The 500 cm -1 band of the bmimBr is ascribed to the C 7 -C 8 -C 9 deformation vibration of the gauche conformation around the C 7 -C 8 bond.The Raman spectra of all liquid bmimX (where X= Cl, Br, I, BF 4 , PF 6 ) found to be similar, suggesting that bmim cation is similar for all these liquids.Both the two sets of marker bands, 625 and 730 cm -1 for the trans conformation and the 603 and 701 cm -1 band for the gauche conformation appears in all the liquid spectra.Therefore, at least two rotational isomers, one having a trans conformation and other having gauche conformation around the C 7 -C 8 bond co-exist in liquid bmimX.The physical properties of these two polymorphs are found to be different, e.g., the melting point of crystal 1 is 41 0 C while that of crystal 2 is 66 0 C. Different melting points are due to different types of interactions present in the system which is further due to the different kind of molecular conformations (e.g., TT, GT etc.).

Studies of interaction through
1 HNMR NMR data clearly explain the strength of interaction between cation and anion.Figure 3 shows 1 HNMR spectra of bmimX.C 2 -H proton shiftings were recorded to be 10.38ppm,Fig. 3. 1 HNMR shift of 1-butyl-3-methylimidazolium (bmim) cation with various halide anions.10.35ppm, 9.95ppm in bmimCl, bmimBr and bmimI respectively, explaining that H-X interaction is strongest in bmimCl and then in bmimBr and least in bmimI.When these values compared with the bmimNT 2 , bmimBF 4 and bmimPF 6 , this shift was found to be 8.73 ppm, 8.82 ppm and 8.33 ppm respectively, explaining that interaction between C 2 -H and anion in these ILs is weak when compared with bmimCl, bmimBr and bmimI.Variation of IR spectra which is discussed later in this chapter have been found to be nicely correlated with the NMR studies of these ILs.  1 that dihedral angles (~180 0 ) of the butyl group explain the trans conformations in all 1-butyl-3-methyl-imidazolium halide ILs studied.Similar observation was observed for bmimPF 6 , bmimBF 4 and bmimNTf 2 ILs.While the C 2 -H-----X distance increases gradually from Cl¯ to I¯ following the trend of electronegativity (Cl¯=3.0,Br¯=2.8,I¯=2.5), the bond angle also decreases gradually from bmimCl to bmimI.The C 2 -H---Cl interaction is thus found to be the strongest among all and the order was found to be C 2 -H----Cl > C 2 -H----Br > C 2 -H---I.Further the position of anion was found to be crucial in determining whether an IL will be liquid at ambient temperature or not.As mentioned earlier that bmimI exist in liquid state (m.p.-72C) whereas its corresponding chloride, (bmimCl) and bromide (bmimBr) ILs are solids at 20C.As mentioned earlier, melting point of bmimCl are 41C and 66C depending on the type of crystal polymorph, while that of bmimBr is 79C.[21] From our calculations it is observed that while Cl¯and Br¯ ions are present in the plane of the imidazolium ring, position of iodide anion is found to be below or above the plane.[23,34] The position of Cl¯ and Br¯ are satisfactorily matching with what was found in single crystal x-ray diffraction data.[21] This led us to conclude that because of the small size of the Cl¯and Br¯, there is compact packing of ions in the crystal lattice leading to higher melting points.Due to the large size of iodide anion, it does not fit well in the lattice.This makes it difficult to have close packing leading to liquid state at ambient temperature.It is important to mention here that in addition to ion size, interionic interactions also play major role in determining the state of an IL.Further, various dihedral angles of type <N 1 -C 2 -H-X (see Table 1) also indicates that the Cl¯ and Br¯ anions are in the plane of the imidazolium ring whereas I¯ ion is out of plane.[ Further in addition to the C2-H hydrogen bonding, C4-H and C5-H also participate in Hbonding interaction with halide anion but stabilization due to H-bonding interaction between cation and anion is found to be more in case of interaction involving C2-H compared to that of involving C4 and C5-H.[35] It is because, while C2 is positive (due to the electron deficient  bond formation of C=N bond), the C4 and C5 are essentially neutral (due to  bond formation between two carbon sharing equally the available electrons).[36]

Calculated vibrational spectra, analysis and its correlation with experiment
Vibrational spectra of all the ILs has been calculated at same level of calculation and using the same basis set in gas phase.The calculated vibrational spectra for bmimCl, bmimBr and bmimI have been shown in Figure 5 and the spectral bands with the corresponding assignments are presented in Table 2. Vibrational frequencies corresponding to different vibrational motions such as stretching, in plane bending (ip) out of plane bending (op), wagging, rocking etc were assigned based on the predominant motion of the atoms and magnitude of the displacement vector associated with the atoms involved in concerned vibration.Most of the assignments were found to be matching with the literature values of similar compounds.[27] It is interesting to note that wavenumber for most of the motions for bmimCl and bmimBr are nearly same, but differs considerably for the same motion in bmimI.Among all, the most prominent and interesting observation is the shift of C 2 -H stretching band in different ILs studied.While the band appears at 2652 cm -1 for bmimCl, 126 cm -1 blue shift is observed for the same in bmimBr.A further blue shift of 212 cm -1 is observed in bmimI (i.e., appears at 2990 cm -1 ).This can be explained considering the fact that this hydrogen participates in hydrogen bond formation with the halide anion.Since the strength of hydrogen bond depends on the type of anion (mainly on its electronegativity), shift in frequencies were observed.Where sym-symmetric, asym-asymmetric, ip-inplane, op-out of plane Table 2. Selected DFT calculated harmonic vibrational frequencies for bmimX ILs in gas phase and their band assignment.

Bands
In other word, this shift in C 2 -H stretching frequency is an indicator of the strength of the hydrogen bond which is formed between C 2 -H and halide anion.These data suggests that the hydrogen bonding is strongest in case of bmimCl and weakest in bmimI.[23] This observation is exactly matching with the finding of 1 H NMR results asdiscussed earlier in this chapter.Further the single crystal X-ray crystallographic data that are available for bmimCl and bmimBr also corroborate with our results discussed here.[4,17,25] In addition, as expected, the position of the band arising due to 'out of plane' bending motion of C 2 -H is also sensitive to the type of anion.Interestingly the I¯ is found to be present out of plane of imidazolium ring which restricts the out of plane bending of C 2 -H bond, making it absent in the IR spectrum whereas band at 1019 cm -1 in bmimCl and 1017 cm -1 in bmimBr are due to out of plane bending of C 2 -H bond.Hence in bmimCl and bmimBr, Cl¯ and Br¯ found to exist in plane of imidazole ring and hence provide strong hydrogen bonding with C 2 -H and therefore found to exist in solid state.Whereas in case of bmimI, I¯ is present out of plane thus providing less interaction with C 2 -H and therefore found to exist in liquid state.[23] Infrared (IR) spectrum of synthesised bmimI is compared with the calculated one and is presented in vibrational stretching frequency which is anharmonic in nature and the DFT calculated vibrational frequency which is harmonic in nature.Frequency of low wavenumber is found to match excellently while that of higher wavenumber deviated from theoretical values which are due to the increase in anharmonicity at higher wavenumber.Hence a scaling factor of 0.9889 was used for all calculated values above 3000 cm -1 .The experimental results can be interpreted and rationalized in the light of theoretical investigations.Table 2 presents the assignments of different bands based on calculation.DFT calculated IR spectra for bmimNTf 2, bmimPF 6 and bmimBF 4 at B3LYP/6-31G++(d,p) level of calculation are shown in Figure 7.In bmimNTf 2 spectra S=O, N-S, C-F and C-S bond found to be at 1281 cm -1 , 1195 cm -1 , 1122 cm -1 and 1046cm -1 .[37] In bmimPF 6 , P-F symmetric and asymmetric stretching found at 827 cm -1 and 841 cm -1 respectively.In bmimBF 4 , B-F stretching (vertical) found at 1174 cm -1 and B-F stretching (in plane) found at 1009 cm -1 and 975 cm -1 .In higher wavenumber region intense peak found at 3373 cm -1 , 3233 cm -1 and 3278 cm -1 in bmimNTf 2 , bmimPF 6 and bmimBF 4 respectively which is due to C 2 -H stretching in imidazolium cation.Hence with variation of anion, C 2 -H stretching frequency shift is observed.Experimental IR spectra of the bmimNTf 2 , bmimPF 6 and bmimBF 4 ILs which we have synthesized have been correlated with the DFT calculated data and are shown in Figure 8(a), 8(b) and 8(c) respectively.To correlate well a scaling factor of 0.943, 0.975 and 0.965 has been applied in higher wavenumber region (above 3000cm -1 ) for bmimNTf 2 , bmimPF 6 and bmimBF 4 respectively.Hence the DFT calculated results found to be matching well with the experimental data.As can be seen from the above figures, DFT can predict very well the vibrational bands of various ILs as discussed above.Therefore, assignment of experimental bands becomes a easy task, thereby helping us to understand completely the structure and interactions present in the ILs.

Transitions in bmimX: UV-Vis spectra and TD-DFT calculations
The UV-Visible spectrum of neat bmimCl and bmimBr could not recorded in absorbance mode, as they are solid at ambient temperature.The UV-Visible spectrum of pale yellow coloured bmimI was recorded and is presented in Figure 9.
Fig. 9. UV-Vis spectrum (smooth line) of neat bmimI RTIL recorded using quartz cuvette having 5mm path length (Vertical lines correspond to calculated transitions using TD-DFT calculation)) While the peak of the spectrum appears at about 380 nm, long absorption tail is extended to about 525 nm.The interesting aspect is that the bmimI has pale yellow colour while the analogous derivatives of pure chloride and bromide are colourless.It is well known fact that during the process of synthesising these ILs, often colour impurity makes the resultant ILs coloured.In most of the cases improperly purified or unpurified ILs shows pale colour whereas on persistent and proper purifications, the final IL comes out to be colourless.[26,38] So, to understand whether the bmimI is inherently pale yellow coloured or the colour is due to presence of impurity, time dependent DFT (B3LYP) calculations were performed on bmimCl, bmimBr and bmimI.For bmimCl and bmimBr, transitions found to occur below 356 nm (only in UV region), whereas in case of bmimI, transition found to occur in visible region also.The wavelengths of calculated transitions for bmimI, along with their oscillator strengths are presented in Figure 10.Even though the calculated oscillator strengths of various possible transitions do not exactly corresponds with that of experimentally observed spectrum (vide Figure 10), it does strongly indicate couple of transitions (though weak, at 371 and 380nm) in visible range.
The important point here to note is that a couple of transitions also occur at much longer wavelengths 434 and 475 nm having reasonably good oscillator strengths.Thus it is confirmed that the pale yellow colour of the bmimI is inherently due to the transition in bmimI itself.Being confirmed that the observed colour of bmimI is inherent to this IL, we have investigated the involvement of orbitals through Molecular Orbital (MO) analysis.Figure 10 depicts the orbitals responsible for various low energy transitions.From this frontier molecular orbital pictures it is clear that HOMO is the non-bonding orbital localized on iodide ion (due to presence of lone pair electron) while LUMO is mainly localized on imidazolium ring and is antibonding in nature (π*).Thus the transition which is responsible for pale yellow colour of bmimI is due to charge transfer transition occurring from HOMO on iodide anion to LUMO on imidazolium cation.The strongest transition in the visible range (434nm) was found to be from MO 63 to MO 66 (shown in Figure 10) having oscillator strength of 0.0912.
Hence the pale yellow colour of bmimI is found to be intrinsic of this room temperature IL and is due to the charge transfer from iodide anion to imidazolium ring (n  π*).UV-visible spectra of bmimNTf2, bmimPF6 and bmimBF4 were also recorded and these ILs were found to be colourless liquids.Similar TD-DFT calculations for these RTILs are also done and supports the experimental observation.In all of these RTILs, as shown in Figure 11 (a-c) transitions found to occur in UV region only, that is why, they are transparent in visible region.Because of their clear optical window, these RTILs have extensively been used as a medium for photophysical studies of various probe molecules and solvation dynamics in recent years.[30,[39][40][41]

Conclusion
Modified synthesis of different imidazolium based ILs having simple mono atomic to complex anions have been described here.In addition to usual characterizations, 1 HNMR spectra are used to extract information on existence and strength of H-bondings present in the ILs having the mono-atomic anion.IR spectroscopy has extensively been used to analyse the interaction present in the ILs.A through DFT calculation at B3LYP level have been performed on the all the ILs to get fully optimized geometry and obtained complete vibrational spectra.These calculations help us to understand the structure and existence of various interactions including H-bonding interactions.Monoatomic anion based ILs (bmimCl, bmimBr and bmimI) found to have only one predominant H-bonding between cation and anion through C 2 -H while multiple H-bonding interactions were found to exist in bmimNTf 2 , bmimPF 6 and bmimBF 4 ILs.TD-DFT calculations were also presented to understand and interpret the experimental UV-Visible spectra of various ILs.That iodide ILs have pale yellow color while corresponding chloride and bromide ILs are colorless can well be explained with the TD-DFT calculations.

Acknowledgement
Financial assistance from CSIR, India (01(2210)/08/EMR-II) and DST, India (SR/FTP/CS-70/2006) are gratefully acknowledged.MLS and NS thanks UGC and CSIR respectively for fellowships.SS thanks Banaras Hindu University for providing the infrastructure and facilities.

Fig. 1 .
Fig. 1.Schematic representation of bmim catiom [bmim: 1-butyl-3-methylimidazolium]Saha et.al.[4] and Holbrey et.al.[31] that bmimCl crystal shows two crystal polymorphism.Schematic diagram of bmim cation has been presented in Figure1.In crystal 1 (Figure2a), the n-butyl group takes trans-trans (TT) conformation with respect to C 7 -C 8 and C 8 -C 9 bonds, while in crystal 2 (Figure2b), n-butyl group takes gauche-trans (GT) conformation with respect to C 7 -C 8 and C 8 -C 9 bonds similar to that of bmimBr conformation.[4,25]Powdered x-ray diffraction patterns of the two polymorphs of bmimCl and bmimBr was recorded and found that the pattern for crystal 2 and bmimBr somewhat resembles each other, while that of bmimCl crystal 1 is distinct from the other two.[4,21]

4. 3 Fig. 4 .
Fig. 4. DFT optimized structure of different imidazolium based Ils in gas Phase The optimized structures reveal that the imidazolium ring is a planar pentagon as expected.The average bond length of 1.34 Å for the N1-C2 and C2-N3 in all the three ILs indicates the nature of conjugated double bond character.Further N1-C5 and N3-C4 bond lengths are (1.38 Å) shorter than a pure C-N single bond (1.47 Å).Bond length of C4-C5 is in the range of normal C=C bond (1.33 Å).It can be seen from the Table1that dihedral angles (~180 0 ) of the butyl group explain the trans conformations in all 1-butyl-3-methyl-imidazolium halide ILs studied.Similar observation was observed for bmimPF 6 , bmimBF 4 and bmimNTf 2 ILs.While the C 2 -H-----X distance increases gradually from Cl¯ to I¯ following the trend of electronegativity (Cl¯=3.0,Br¯=2.8,I¯=2.5), the bond angle also decreases gradually from bmimCl to bmimI.The C 2 -H---Cl interaction is thus found to be the strongest among all and

Fig. 5 .
Fig. 5. DFT calculated IR spectra of bmimX where X=Cl¯, Br¯, I¯indicating the blue shift of the C 2 -H stretching band (g) with different anions.

Fig. 10 .
Fig. 10.Some important Frontier Molecular Orbitals of bmimI ion pair are shown which is obtained by calculation at B3LYP/DGDZVP level.

Table 1 .
Selected bond lengths, bond angles and dihedral angles as obtained from DFT calculation.