Ionic liquids from theoretical investigations B. Kirchner Wilhelm-Ostwald Institut für Physikalische und Theoretische Chemie, Universität Leipzig, Linnéstr. 2, 04103 Leipzig,
[email protected], www.uni-leipzig.de/~quant/ Why does hydrogen bond elimination lead to an increase in viscosity?
Unexpected hydrogen bond dynamics in ionic liquids
What keeps ionic liquids in flow?
• Born-Oppenheimer molecular dynamics (BOMD) simulations of 32 ion pairs of [Emim][SCN] for 18.2 ps with cp2k
• Intermolecular forces of ILs were investigated by the symmetry adapted perturbation theory
Classical MD-Simulations with GROMACS 3.3.1, Box: 4520 pm (T=[C2C1mim][Cl])/ 4350 pm (D=[Emim][Cl]) 343 NVT, 10 ns, tstep=1 fs and T=460 K, scaled by factor 0.85 • • •
• Dispersion and induction forces have a significant impact upon interaction energy and equilibrium structure in ILs while in typical salts dispersion forces play no role
Cl is for T not at C2 C2-C2: Two T-ion pairs look at each other: Compare Seddon et al. Struct. Chem. 1990, 1, 391 D: C2-C2/C4/C5 similar, T: C2-C4 100 pm shifted. • Structural picture: H2 shows larger peaks than H4 and H5
D C2 C4
Fig. 1:
C5
T2 a-d: Interaction energies versus distance for MmimCl (a), MPyCl (b), N2111Cl (c) and NaCl (d) obtained by the SAPT-approach.
Spatial distribution function
• Structural picture: H2 shows larger peaks than H4 and H5
ILs have a shallower interaction potential than typical salts.
No coordination at C2 for T
• Dynamics: continuous hydrogen bond dynamics at H2 faster than at H4 or H5.
Ions act in the repulsive if only electrostatics are S. Zahn, F. Uhlig, J. Thar, C. Spickermann and B. Kirchner, “Intermolecular forces in an ionic liquid considered
Enhanced coordination at C4 and C5
• Side chain rotation might induce fast dynamics:
D [10-5 cm2/s]
Diffusion
Cation Anion Autocorrelation function of the continuous hydrogen bond lifetime
460/500 K [Emim]Cl
0.46
0.43
[C2C1mim]Cl
0.29
0.27
[Emim]Cl
0.80
0.73
[C2C1mim]Cl
0.60
0.64
([Mmim][Cl]) versus Those in a typical salt (NaCl)“, Angew. Chem. Int. Ed., 2008, 47, 3639-3641
S. Zahn, G. Bruns, J. Thar and B. Kirchner, “What keeps ionic liquids in flow?”, Phys. Chem. Chem. Phys., 2008, 10, 6921-6924
Why are ionic liquid ions mainly associated in water? • IL ion pairs in water consist longer than typical salts • Car-Parrinello molecular dynamics (CPMD) simulations of one [Emim][Cl] dissolved in 60 water molecules for 22 ps • Maxima of the Cl-H (~220 pm) and Cl-O (~320 pm) RDFs indicate an optimal incorporation of chloride into water H-bond network
S. Zahn, G. Bruns, J. Thar and B. Kirchner, “What keeps ionic liquids in flow?”, Phys. Chem. Chem. Phys., 2008, 10, 6921-6924
Cooperativity in ionic liquids • Large [Mmim][Cl] clusters by static quantum chemistry calculations (BP86(RI)/TZVP) • Charge of chloride anion converges from -0.735 a.u. towards 0.823 a.u. → Dowscaling of charges in classical molecular dynamics simulations S. Koßmann, J. Thar, B. Kirchner, P. A. Hunt and T. Welton, ”Cooperativity in ionic liquids”, J. Chem. Phys., 2006, 174506
J.Thar, M. Brehm, A. P. Seitsonen and B. Kirchner “Unexpected hydrogen bond dynamics in imidazolium-based ionc liquids”, submitted
Ion dynamics and ion pairing of a protic ionic liquid • Car-Parrinello molecular dynamics (CPMD) simulations of 48 [CH3NH3][NO3] ion pairs for 19.5 ps • Each ion has seven counter ions in the first solvation sphere
RDF between ion pair atoms and water atoms. “etring” indicates the carbon next to the imidazolium ring of the ethyl group and “et-term” indicates the terminal carbon atom of the ethyl group.
Schematic illustration of the orientation of the water molecules around the [Emim][Cl] contact ion pair. black: hydrophobic part; orange: cationic part; cyan: anionic part
• Water molecules hydrating the cation show typical arrangement for hydrophobic hydration (tangential orientation)
• One hydrogen bond donor and acceptor per ion pair remains free. • Hydrogen bond network is found.
Distribution of angles between water O-H vectors and chloride respectively the methyl hydrogens.
a: O; b: H2; c: C; d: H1; e: N1 Interface of the cationic and anionic solvation shells in case of spherical ionic compounds shows rather strong structural differences to the hydrogen bond network in pure water thus favoring dissociation.
Charge on chloride atom with the number of IL-pairs. Red: charge at the first anion of the ion chain; blue: average charge of all anions.
Validation of KS-DFT for ionic liquid systems • Common functionals show deviations in interaction energies and equilibrium structures compared to ab-initio calculations • Dispersion corrected approaches reduce the deviations to the reference values to negligible amounts. S. Zahn and B. Kirchner, “Validation of dispersion corrected DFT-approaches for ionic liquid systems”, J. Phys. Chem. A, 2008, 112, 8430-8435
• Hydrogen bonds sub-shells have a competing impact upon orientation of like ions. In total, the impact of hydrogen bonds upon molecular orientation nearly vanishes compared to non hydrogen bonded ions.
Water around the hydrophobic parts of the IL cation fits the orientation around the anion → favoring contribution towards the ion pair association of the IL ≡ solvent enhanced ion pair! C. Spickermann, J. Thar, S. B. C. Lehmann, S. Zahn, J. Hunger, R. Buchner, P. A. Hunt, T. Welton and B. Kirchner, “Why are ionic liquid ions mainly associated in water? A Car-Parrinello study of 1-ethyl-3methyl-imidazolium chloride water mixture”, J. Chem. Phys., 2008, 129, 104505
Further Publications B. Kirchner and A. P. Seitsonen, Inorg. Chem., 2007, 47, 2751-2754 P. A. Hunt, I. R. Gould and B. Kirchner, Aust. J. Chem., 2007, 60, 9-14 P. Nockemann et al.., J. Phys. Chem. B, 2007, 111, 5254-5263 P. Nockemann et al., J. Phys. Chem. B, 2006, 110, 20978-20992 P. A. Hunt, B. Kirchner and T. Welton, Chem. Eur. J., 2006, 12, 6762-6775
Acknowledgment Comparison of equilibrium structures of EmimDCA with cyclopentadiene (DAI) or methylacrylate (DAII) employing the HF (red), MP2 (blue), B3LYP (magenta), BLYP-D (yellow) and BP86DCACP (green) approach
Financial: DFG, SPP-1191, ERA-Chemistry, CPU: RZ-Leipzig, NIC-Jülich, HLRSStuttgart, ZIH-Dresden Angular distribution of two cations (a) and two anions (b)
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