Ab initio study of M(CH3CN)n clusters (M = Li+, Na+, Mg2+) in the gas phase

Calculations for dusters consisting of an Li+, Na+ or Mg2+ ion and up to six acetonitrile molecules were carried out using the HF, DFT/B3LYP and MP2 methods with the 6-31+G* basis set. The three methods led to the same minima, which correspond to highly symmetric structures, where the dipole moment of acetonitrile points at the cation in order to facilitate a charge-dipole interaction. Intermolecular distances follow the sequence Na+ > Mg2+ > Li+ with the three methods used; on the other hand, interaction energies decrease in the sequence Mg2+ > Li+ > Na+. Intermolecular distances increase gradually with increasing cluster size. By contrast, the energy change resulting from incorporation of an additional molecule into a given cluster decreases with increasing number of molecules. The coordination sphere of Li+ saturates with five acetonitrile molecules, which reflects in increased distances and in a markedly decreased interaction energy for the cluster containing five acetonitrile molecules. The other two ions can easily accommodate up to six acetonitrile molecules. The interaction is essentially electrostatic, but ligand polarization contributions are significant. (C) 2000 Elsevier Science B.V. All rights reserved.