J. Phys. Chem. B 2006, 110, 11475-11480
11475
Ionic Liquids from Car-Parrinello Simulations, Part I: Liquid AlCl3 Barbara Kirchner,*,† Ari P. Seitsonen,*,‡,§ and Ju1 rg Hutter*,‡ Lehrstuhl fu¨r Theoretische Chemie, Institut fu¨r Physikalische und Theoretische Chemie, UniVersita¨t Bonn, Wegelerstr. 12, D-53115 Bonn, Germany, Physikalisch-Chemisches Institut, UniVersita¨t Zu¨rich, Winterthurerstr. 190, CH-8057 Zu¨rich, Switzerland, and Institut de Mine´ ralogie et de Physique des Milieux Condense´ s (IMPMC), CNRS + UniVersite´ Pierre et Marie Curie, 4 place Jussieu, case 115, F-75252 Paris, France ReceiVed: March 5, 2006; In Final Form: April 25, 2006
The properties of isolated AlCl3 clusters and the bulk system are investigated by means of static and dynamic electronic structure methods. We find important structural motifs with the edge connectivity dominant in a dimer and the corner connectivity dominant in a trimer. Furthermore, the trimer cluster exhibits an interesting ring structure with large cooperative effects relative to the dimer. Comparing the found structural motifs in isolated molecule calculations with the structure of the liquid allows us to determine the dominace of edge connectivity in the liquid. The size of the clusters present in the liquid indicates indeed that the dimer is the most abundant species, but there are also trimers, tetramers, and pentamers present. From the local dipole analysis both for the isolated clusters as well as for the liquid, further proof for the edge connectivity is given. However, all results point to the fact that there is also some small percentage of corner connectivity present that might be attributed to the most stable corner-connected cluster, namely the trimer. Importantly, we find that energetic considerations of isolated (static) clusters only do not represent the findings in liquid phase. Instead, a quantum cluster equilibrium approach or simulations are needed.
1. Introduction Recently, new solvents and solutions have received increasing attention in chemical research because many of the materials commonly used nowadays in laboratories and in the chemical industry are considered to be unsafe for reasons of environmental protection.1 The new solvents that are called ionic liquids are salt mixtures with melting points below the ambient temperature. In contrast to classical molten salts, which are usually melting at high temperatures, viscous and very corrosive substances, ionic liquids are fluid at low temperatures (
