Ionic liquids theoretical calculations

In addition to the obvious structural information, vibrational spectra can also be obtained from both semi-empirical and ab initio calculations. Computer-generated IR and Raman spectra from ab initio calculations have already proved useful in the analysis of chloroaluminate ionic liquids [19]. Other useful information derived from quantum mechanical calculations include and chemical shifts, quadru-pole coupling constants, thermochemical properties, electron densities, bond energies, ionization potentials and electron affinities. As semiempirical and ab initio methods are improved over time, it is likely that investigators will come to consider theoretical calculations to be a routine procedure. 

It is interesting to compare this transition state in the solid with the one calculated from the HF-SbF5 system. In the liquid superacid, the ionic character is very strong and it is easier to connect the reactivity with the unusual activity of the proton even when solvated by the HF solvent. In contrast, on the solid the theoretical calculated transition state is further away from the carbonium ion type and in line with the much higher temperatures needed to activate the alkane with weaker acids. 

Abstract Theoretical investigations of ionic liquids are reviewed. Three main categories are discussed, i.e., static quantum chemical calculations (electronic structure methods), traditional molecular dynamics simulations and first-principles molecular dynamics simulations. Simple models are reviewed in brief. 

Theoretical density functional calculations on the possibility of addition of imidazolium salts to electron-rich palladium centers predicted an exothermic enthalpy for such a process [36]. These results suggested that, under appropriate reaction conditions and with the use of a proper carbene precursor, this reaction should present a feasible synthetic path to carbene/palladium complexes. Only recently, the addition of the C(2)-H bond of an imidazolium salt, in the form of an ionic liquid, to a Pd(0)/NHC complex with the formation of a stable Pd-H bond has been reported [41]. These complexes bear three carbenes per metal center, the fourth coordination position being occupied by hydrogen. The isolation of these complexes has proven that the beneficial role of ionic liquids as solvent can lead to the formation of catalytically active palladium-carbene complexes (see Scheme 7). 

The electronic structure of the imidazolium cations is of interest because it impacts on the hydrogen bond acceptor and donor properties of ionic liquids. This in turn relates to the penchant of the solvent to coordinate to, or react with the solvated species. The imidazolium cation is isoelectronic with the carbene -like imidazole-2-ylidene. Theoretical calculations on deprotonation of the unsubstituted imidazolium cations determine pfCaS of24.90 and 32.97 for the proton at the and C positions... 

Recent studies combine the results for the reorientational dynamics with viscosity data to compare experimental correlation times with correlation times calculated from hydrodynamic models [15]. By combining experimental and theoretical methods, it is possible to make statements about the intermolecular structure and interactions in ionic liquids and about the molecular basis for the specific properties of ionic liquids [15-20]. 

Task-specific ionic liquids are of special interest in process intensification. The great variety of existing ionic liquids leads to an unavoidable theoretical screening at the beginning of a project. To date, the only fast and predictive virtual screening method is COSMO-RS, which provides activity coefficients for the relevant species in a set of ionic liquids. From these, selectivites and capacities are then calculated. Figure 2.4 depicts data from the open literature [13] to illustrate the performance of COSMO-RS. 

The theoretical simulation of ionic liquids requires a combined approach that involves the use of more than one techniques. It is customary to perform a series of preliminary ab-initio calculations before moving to molecular dynamics simulations. The ab-initio simulations are dictated by the necessity of obtaining the "true" charge distribution in order to check whether it matches the partial atomic charges of the force field and, if necessary, to provide new partial atomic charges. 

In this Chapter we have reviewed the t)rpical theoretical approaches that are commonly used to describe ionic liquids using in silico simulations. The study of these materials requires a combined approach made by ab-initio calculations and MD simulations. These techniques require a special attention in the application to ionic liquids because they of the intrinsic difficulties due to the extremely slow dynamics in the real systems. We have concluded our chapter by reporting few highlights from a recent work that we are carrying out on a special class of these compoxmds and we have pointed out a few interesting features we have discovered in order to provide an example of how powerful theoretical predictions can be for molecular systems which are still relatively xmexplored experimentally. 

---