Molecular clusters intermolecular forces

Our interest is in the connection between the intermolecular forces that cause condensation and/or gas phase molecular clustering and thermodynamics. To set the stage consider the following simple model ... 

Quantum-chemical ab initio calculations have become an alternative to experiments for determining accurately structures, vibrational frequencies and electronic properties as well as intermolecular forces and molecular reactivity.28-31 Two specific approximations were developed to solve the problems of surface chemistry periodic approximation, where quantum-chemical method employs a periodic structure of the calculated system and cluster approximation, where a model of solid phase of finite size is created as a cutoff from the system of solid phase (it produces unsaturated dangling bonds at the border of the cluster). Cluster approximation has been widely used for studying interactions of molecules with all types of solids and their surfaces.32 This approach is powerful in calculating the systems with deviations from the ideal periodic structure like doping and defects. 

In order to obtain a better understanding of the forces acting in molecular clusters we consider an appropriate partitioning of intermolecular energies. For the formation of an aggregate of these subsystems A, B and C... 

In this section we consider clusters of some polyatomic molecules. The structure of such clusters is expected to depend greatly on both the shape of the molecules and the intermolecular forces. These characteristic properties will also govern the possibility that a molecular cluster will undergo structural transitions according to its size. It is worthwhile to note that diffraction patterns contain, in addition to structural information, various features related to dynamic effects such as translational and librational molecular motions, this making it more difficult to elucidate the cluster structure. On the other hand, size effects may be detected in both structural and dynamic properties. 

Because of their importance to nucleation kinetics, there have been a number of attempts to calculate free energies of formation of clusters theoretically. The most important approaches for the current discussion are harmonic models, " Monte Carlo studies, and molecular dynamics calcula-tions. In the harmonic model the cluster is assumed to be composed of constituent atoms with harmonic intermolecular forces. The most recent calculations, which use the harmonic model, have taken the geometries of the clusters to be those determined by the minimum in the two-body additive Lennard-Jones potential surface. The oscillator frequencies have been obtained by diagonalizing the Lennard-Jones force constant matrix. In the harmonic model the translational and rotational modes of the clusters are treated classically, and the vibrational modes are treated quantum mechanically. The harmonic models work best at low temjjeratures where anharmonic-ity effects are least important and the system is dominated by a single structure. 

Material particles consisting of a few to a few thousand atoms are called clusters. Cluster properties can have dramatic size and shape dependence. Clusters can serve as building blocks for new materials and electronic devices. Hence clusters of metals, semiconductors, ionic solids, rare gases, and small molecules have been studied using both theoretical and experimental methods. Atomic and molecular clusters [1] are held together by hydrogen bonds [2] or by relatively weak intermolecular forces [3, 4]. 

The intermolecular forces encountered in. solvation are generally weak in comparison with the interactions that give rise to covalent chemical bonds, thus the molecular integrity of both solute and solvent is preserved. An obvious exception is the large interaction between an ionic solute and polar solvent molecules. In such cases one may wish to redefine the solute, e.g., as a small cluster consisting of an ion and the solvent molecules in the first solvation shell. ... 

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