An insight into microscopic properties of aprotic ionic liquids: A DFT study Ali Heidar Pakiari a, * , Samira Siahrostami a , Tom Ziegler b a Chemistry Department, College of Sciences, Shiraz University, Shiraz, Iran b Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alta., Canada article info Article history: Received 25 April 2010 Received in revised form 26 May 2010 Accepted 26 May 2010 Available online 10 June 2010 Keywords: Aprotic ionic liquid (AIL) Inter-ionic interaction ETS–NOCV Electrostatic interaction Pauli repulsion Orbital interaction abstract The relationship between the structure of counter-ions and inter-ionic interaction in ion-pairs is system- atically studied for 15 aprotic ionic liquids (AILs) using density functional theory. Five different substi- tuted imidazolium cations and three different polyatomic anions (PA) are taken into account. Theoretical calculations show that the inter-ionic interaction decreases as the vdW volume of the coun- ter-ions increases. By means of a new decomposition scheme called ETS–NOCV [33], which decomposes the total interaction energy into different parts, it can be shown that there is a clear linear relationship between the electrostatic part of the interaction energy and the total interaction energy. Further, for a given PA the electrostatic interaction decreases with increase of the vdW volume of the cation as one would expect. A similar decrease is seen in the electrostatic interaction between a given imidazolium cat- ion on the PAs as the PA vdW volume increases. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Ionic liquids (IL) represent a new class of solvents with un- ique characteristics. Due to the presence of a strong anion–cat- ion interaction, they have much higher viscosities, polarities and much lower vapor pressures than those of normal solvents. Their unique solvent properties make them key targets for the development of a large number of emerging technologies, span- ning a wide range of applications [1]. For instance, ILs can be exploited as useful solvents for new homogeneous catalytic reactions and other chemical processes with relation to ‘‘green chemistry” [2–9]. A wide variety of ionic combinations makes it possible to design ILs with specific properties. They can be classified as aprotic or protic ILs depending on their cation structures. Aprotic ILs (AILs) containing organic cations have low melting points due to the inef- ficient packing of a large irregular organic cation with a mono atomic or polyatomic inorganic anion [4]. AILs are one of the most important solvent classes in organic chemistry for experimental determination of chemical and physical properties such as melting point, density, viscosity and conductivity [10–14]. The second class is protic ionic liquids (PILs). They are related to the AILs but differ in that the cation has been formed by transferring a proton from a Brønsted acid to a Brønsted base. This class has shown great prom- ise as electrolytes for fuel cells in recent years, because its mem- bers can have aqueous solution-like conductivities [15,16]. The study of the relationship between the structures of AILs and their physical properties has been an important theoretical re- search area in recent years [15,17–20]. There are three extensively used theoretical procedures by which these relationships have been established. (i) The quantitative structure–property relation- ship (QSPR) can be employed to correlate the melting point with structure [17,18]. (ii) Molecular dynamic simulation is used to compare calculated versus experimental values for some physical properties [19–23]. (iii) Quantum mechanical methods are em- ployed in order to determine interaction energies [24,25], thermo- dynamic properties [26–29] and also to find a correlation between melting points and interaction energies of ion-pairs [24]. Some AILs have been selected by Dong et al. to inspect the nature of hydrogen bonds between cations and anions [25]. Recently, Fumi- no et al. have employed density functional theory (DFT) calcula- tions to investigate the role of hydrogen bonding in aprotic and protic ILs [30]. DFT calculations have also been used to investigate intermolecular interactions [31] and to predict vibrational fre- quencies [32] of some AILs. In this work, we have performed a systematic theoretical study on five classes of imidazolium-based AILs to determine their inter- ionic interactions by means of DFT. We have made some compar- isons among the five classes according to (i) the nature of the poly- atomic anion (PA), and (ii) the nature of the cation substitution. Finally, we have used a new scheme based on the Extended Tran- sition State and Natural Orbitals for Chemical Valency (ETS–NOCV 0166-1280/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2010.05.029 * Corresponding author. E-mail address: pakiari@shirazu.ac.ir (A.H. Pakiari). Journal of Molecular Structure: THEOCHEM 955 (2010) 47–52 Contents lists available at ScienceDirect Journal of Molecular Structure: THEOCHEM journal homepage: www.elsevier.com/locate/theochem