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Quantum-Chemical Calculations of Ionic Liquids
Rolf Öttking, Wichard J.D. Beenken
Organic Ionic Lequids are advanced Materials with astonishing properties. Electically they behave like fused salts, but are liquid already at room temperature. Their vapor pressure Ris etremly low. Therefore they do not evaporate even in ultra-high vacuum. This qualified them e.g. as lubricant in UHV apparatus.
Experimental studies of the electronic and vibrational surface-properties of the ionic liquid [EMIM][Tf2N] by XPS, UPS, MIES, and HREELS were done in the Department Technical Physics I. For comparision and better interpretation of the data we performed quantum-chemical calculations on the MaPaCC.
The computationally demanding optimization of structure revealed that the cation [EMIM]+ and the anion [Tf2N]- most probably form dimers (see Fig. 2). The energetic positions of the calculated electronic states (DFT-DOS) agree very well with the peaks in the experimental XPS spectrum (Fig. 1).
- Fig. 1: Experimental XPS spectrum (black line) compared to the density of states (red line) as obtained by convolution of the quantum-chemically calculated stick-spectrum (red peaks) with an Gaussian.
Furthermore, we found that the electronic state with the lowest binding energy (HOMO) is located on the [Tf2N]- anion (Fig. 3), while the first state located on the [EMIM]+ cation (Fig. 4) has a binding energy of ~8.8eV. this result could not have been found by overlaying of the single ion XPS-spektra but needs the visualization of the calculated molecular orbitals.
References
- S. Krischok, R. Öttking, W.J.D. Beenken, M. Himmerlich, P. Lorenz, O. Höfft, S. Bahr, V. Kempter, J.A. Schaefer, Z. Phys. Chem. 220 (2006) 1407
- S. Krischok, M. Eremtchenko, M. Himmerlich, P. Lorenz, J. Uhlig, A. Neumann, R. Öttking, W. J. D. Beenken, O. Höfft, S. Bahr, V. Kempter, J. A. Schaefer, J. Phys. Chem. B 111 (2007) 4801