Intermolecular interactions and cooperative effects in acetonitrile clusters. An ab initio molecular orbital study

Abstract

Ab initio calculations were performed on clusters of acetonitrile molecules employing the HF, DFT/B3LYP and MP2 methods with the 6-31+G* basis set. Several minima were observed and their interaction energies were calculated using the counterpoise method to avoid basis set superposition error. Single point calculations were performed with the 6-311++G(2d,2p) basis set as well as with the MP4 method. All methods provide similar results, but HF and DFT/B3LYP give smaller interaction energies and larger intermolecular distances than MP2. The dimer presents a minimum corresponding to an antiparallel arrangement with an interaction energy of −4.9kcal/mol at the MP2/6-31+G* level (−5.8kcal/mol with the 6-311++G(2d,2p) basis set). Two minima were observed for the trimer. The global minimum is a cyclic structure with an interaction energy of −9.9kcal/mol, about 1kcal/mol more stable than the other structure. A significant shortening of intermolecular distances is appreciated for the global minimum compared to the values obtained for the dimer; also, cooperative effects amount to 10% of the total interaction energy for this structure. For the other minimum neither shortening nor significant contribution of cooperative effects was observed. Acetonitrile tetramer shows similar features as those found for the trimer. Two minima were observed with interaction energies of about −12kcal/mol in which molecules are arranged by pairs in an antiparallel manner giving zero dipole moment.