Ab Initio Structures of Ionic Liquids

The basis set is 6-31G(d,p), and electron correlation at the MP2 level is included. A similar structure is obtained with the AMI and PM3 semi-empirical methods. Density functional theory at the B3LYP/6-31G(dp,p) level also produced the same structure for this ion-pair. The only observed differences between the semi-empiri-cal and the ab initio structures were slightly shorter hydrogen bonds (PM3 and AMI) between FI, F2, and F5 and the G2-F1 (H18) on the imidazolium ring. 

Note that DFT structures are as reliable as or more reliable than HF structures obtained with similar or less complex basis sets. 

Robert Carper, Zhizhortg Mertg, Andreas Dblle 

---

T. I. Morrow, E. J. Maginn, Molecular Structure of Various Ionic Liquids from Gas Phase Ab Initio Calculations, in Ionic Liquids as Green Solvents. ACS Symposium Series 856, K. R. Seddon, R. D. Rogers (Eds.), American Chemical Society, Washington, DC, 2003. 

Storhaug, V. J., and Carper, W. R., Ab initio molecular structure and vibrational spectra of ionic liquids. Trends in Phys. Chem., 9, 173-177,2003. 

A group of theoretical methods exists where the electronic wavefuntion is computed, and the atomic nuclei are propagated (using classical equations of motion). The Car-Parrinello MD method is one of this type [22-24]. These methods he between the extremes of the classical and ab initio methods, as they include some (quantum) electronic information and some (classical) dynamics information. These methods are called ah initio or first principles MD if you come from the classical community and semi-classical MD if you come firom the quantum community [9], Ah initio MD methods are far more expensive and cannot simulate as many molecules for as long as the classical simulations, but they are more flexible in that structures are not predetermined and information on the electronic structure is retained. Semi-classical MD can be carried out under periodic boundary conditions and thus the local liquid environment, and any extended bonding network, vyill be present. These methods hold a great deal of promise for the future study of ionic liquid systems, the first such calciilations on ionic liquids were reported in 2005 [21,25]. 

The last section was devoted to a range of real-world applications treated with ab initio molecular dynamics simulations. Results of gas to liquid phase transition simulations, structural and dynamical properties of liquids such as common solvents as well as the emerging neoteric media of ionic liquids were presented. After a short discussion of chemical reactions concerning homogeneous catalysis, we presented an overview of electrochemical reactions and related processes. 

The theoretical framework in which it is possible to provide high quality studies of the microscopic structure of the ionic liquid is mainly represented by classical molecular mechanics and, only very recently, by ab-initio molecular dynamics. While the employed theoretical techniques are not very different from those used for conventional fluids, many difficulties arise because of the microscopic nature of ionic liquids. In particular these substances are extremely viscous and simulation times become quickly prohibitive if one wants to describe dynamical properties, even as simple as diffusion coefficients. Recent technological advances such as the introduction of GPU clusters might allow unprecedented possibilities in the simulation of these material opening the route to the simulation of rare events and long time scale phenomena. 

Computer simulations of ionic liquids have been reported in increasing numbers during the past few years. Some of the modeling work has been focused on the development of force field parameters, specific to an ionic liquid or an ionic liquid family [4-10]. Structural, dynamic, electric, and thermodynamic properties of several pure ionic liquids have been simulated [10-12] using these force field tools, and the solvation of small solutes in ionic liquids has also been investigated [13-19]. More recently, an ab initio molecular dynamics study has also been published [20], pushing the capabilities of computational chemistry to its current limits. 

Figure 4.2-1 shows the calculated ab initio molecular structure of the ionic liquid [BMIM][PFg] (l-butyl-3-methylimidazolium hexafluorophosphate). 

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. 

Bhargava, B.L., and Balasubramanian, S., Insights into the structure and dynamics of a room-temperature ionic liquid Ab initio molecular dynamics simulation studies of l-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PFj]) and the [bmim][PFj]-C02 mixture, /. Phys. Chem. B, 111, 4477-4487, 2007. 

Meng, Z., Dolle, A., and Carper, W. R, Gas phase model of an ionic liquid Semi-empirical and ab initio bonding and molecular structure, J. Molec. Struct. (THEOCHEM), 585, 119-128, 2002. 

Ab initio molecular dynamics were used to investigate the structure of liquid SbF5. The results confirm the high tendency of SbF5 to oligomerize and impart a highly ionic character to the Sb-F bond. It confirms also the cw-bridged chain polymer as the most stable structure.39... 

Structure and Dynamics of a Room Temperature Ionic Liquid Ab Initio Molecular Dynamics Simulation Studies of l- -Butyl-3-methylimidazolium Hexafluorophosphate ([bmim][PF ]) and the [bmim][PF ]-C02 Mixture. 

Especially for imidazolium based ionic liquids, several force fields can be found in the literature. The first force field of Hanke, Price, and Lynden-Bell was developed to reproduce experimental crystal structures. Unfortunately, a comparison of the reported liquid structure to later published ab initio molecular dynamics simulations show large deviations. 

---