Jan BLASIUS1, Tom FRÖMBGEN1, Leonard DICK1, Wenbo DONG1, Barbara KIRCHNER1
1Mulliken Center for Theoretical Chemistry, Clausius Institute for Physical and Theoretical Chemistry, University of Bonn, Bonn, Germany
The structure of ionic liquids (ILs) is affected by a variety of different interactions, ranging from Coulomb forces and dispersion interactions up to directional atom contacts such as hydrogen bonds. Vibrational spectroscopy is one of the most valuable tools to study these interactions in greater detail, as the vibrational frequencies in a molecular system are affected by intra- and intermolecular interactions. The calculation of vibrational spectra is therefore an ongoing subject of computational chemistry. In this contribution, we will highlight our developments with respect to the calculation of infrared (IR) and vibrational circular dichroism (VCD) spectra from ab initio molecular dynamics (AIMD) simulations and point out critical aspects regarding the system size and sampling issues.
On the basis of molecular dynamics simulations, IR and VCD spectra can be obtained by auto- and crosscorrelation functions of the molecular electric and magnetic dipole moments. In AIMD simulations, the electric dipole moments can be obtained from our radical Voronoi tessellation and the magnetic dipole moments from the Thomas—Kirchner approach. Both provide excellent agreement between theoretical and experimental IR and VCD spectra. The disadvantage of AIMD simulations is, however, that they are computationally rather expensive. Therefore it would be beneficial to keep the system size as small as possible, but also as large as necessary in order to obtain reliable vibrational spectra. Two critical aspects of a small system size are the possible neglection of non-local effects and an insufficient sampling of the conformational space. We therefore show that the electric dipole moment is a local property in ILs which allows for the use of small system sizes in order to obtain converged IR spectra. Additionally, the locality of the electric dipole moment supports the hypothesis of rattling ions in long-living cages. Yet, a small system size can reduce the sampling of the conformational space. This can cause severe convergence issues for computed VCD spectra, as they are very sensitive to the conformational distribution. We present possible problems of such an insufficient sampling at the example of an enantiomeric recognition process of butan-2-ol in the chiral IL [C2C1Im][L-ala]. As conclusion, we provide guidelines for the robust calculation of IR and VCD spectra from AIMD simulations.