No abstract

The main static and dynamic properties of some ionic heteroclusters, involving K+, C6H6, and Ar, have been investigated. A new representation of the intermolecular potential energy, which takes into account both electrostatic and non-electrostatic contributions to the overall noncovalent interaction, was used. Dynamical calculations were performed for a microcanonical ensemble. Particular attention was paid to the opening of the isomerization and dissociation processes for K+-C6H6-Ar(n) and to the formation of some of its fragments at increasing temperatures of the cluster considered.

The achievement of extensive and meaningful molecular dynamics simulations requires both the detailed knowledge of the basic features of the intermolecular interaction and the representation of the involved potential energy surface in a simple, natural and analytical form. This double request stimulated us to extend to ion-molecule systems a semiempirical method previously introduced for the description of weakly interacting atom-molecule aggregates and formulated in terms of atomic species-molecular bond interaction additivity. The method is here applied to the investigation of the prototypical M(+)-C6H6 systems (M = Li, Na, K, Rb and Cs) and some of its predictions are tested against accurate ab initio calculations. Such calculations have been performed by employing the MP2 method and large basis sets, privileging the description of the metal atoms. The agreement between potential energy scans semiempirically obtained and ab initio results is good for all the investigated geometries, thus showing that the adopted representation is in general able to reproduce all the main features of the potential energy surface for these systems. The role of the various noncovalent interaction components, as a function of the geometry and of the intermolecular distance in the M(+)-C6H6 complexes, is also investigated for a more detailed assessment of the results of the semiempirical method.

The supramolecular organization of liquid water is discussed in connection with both the spectral profile of the OH stretching Raman signal measured in pure water and the distribution of tetrahedral order computed by molecular dynamics simulations. Both curves show common features and a similar temperature dependence never pointed out before. Energetic information extracted from the Raman profiles using a recently proposed integration method is also discussed. Overall results confirm that thermally induced structural variations of water in the interval of temperature ranging from 10 to 75 degrees C involve a change of the local tetrahedral order associated with a redistribution energy of 1.6-1.7 kcal/mol.