Predicting frequency dependent cross-relaxation rates from Molecular Dynamics simulations to provide a basis for a reliable interpretation of experimental NOE data

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There is still a large degree of uncertainty about the detailed structure and dynamics of ionic liquids (ILs). In particular, how those liquids are organized at the molecular level in terms of intermolecular atom-atom contacts. It has been argued that specific intermolecular interactions matter, and thus might be employed for tuning properties of ILs. To resolve those issues, it is essential to have a profound understanding of the detailed intermolecular structure and dynamics of ILs. Exploiting the Nuclear Overhauser Effect to the fullest could be instrumental in providing such a structural and dynamical picture for those molecular details. However, the situation for a neat ionic liquids is substantially different than for example for the case of large globular proteins. In ILs the conformational diversity is large, the lifetimes of encounter-configurations are typically short, and nonidealities in translatoric diffusion and reorientational dynamics have to be taken into account. Moreover, the specific nanoscale structure of ILs, imposed by the alternating cation/anion charge pattern has to be considered. Diffusion based formulae, developed for simple liquids, cannot be assumed to be adequate. We propose to use molecular dynamics simulations of realistic all atom models of imidazolium based ionic liquids with different counter ions, to fully characterize the frequency dependent cross-relaxation phenomena for particular meaningful spin-combinations. Temperature variation will be used to better characterize and dissect the influence of the structural and dynamical aspects of the intra- and inter-molecular relaxation processes. Moreover, the addition of molecules, such as chloroform, will be used to study the possible preferential interactions of those additives with anions or cations. Our aim is to lay a reliable foundation for the interpretation of experimentally determined NOEs carried out by our collaborator Ralf Giernoth within the SPP. In addition, the simulations will also help to suggest and design future NMR experiments of e.g.varying isotopic composition to verify and test the simulation results.