Clusters intermolecular forces

Nesbitt D J 1994 Fligh-resolution, direct infrared-laser absorption-spectroscopy in slit supersonic ]ets—intermolecular forces and unimolecular vibrational dynamics in clusters Ann. Rev. Phys. Chem. 45 367-99... 

Supramolecular aggregations are commonly referred to by a variety of terms, including adduct, complex, and van der Waals molecule. In this chapter we shall primarily employ the more neutral term cluster, which may, if desired, be qualified with the type of intermolecular interaction leading to clustering (e.g., H-bonded cluster ). General and specific types of intermolecular forces are discussed in the following sections. 

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 ... 

The problem of influence of the electric field intensity on the permittivity of solvents has been discussed in many papers. The high permittivity of water results from the intermolecular forces and is a cumulative property. The electric field intensity is the lowest at the potential of zero charge (pzc), thus allowing water molecules to adsorb in clusters. When the electrode is polarized, the associated molecules, linked with hydrogen bonds, can dissociate due to a change in the energy of their interaction with the electrode. Moreover, the orientation of water molecules may also change when the potential is switched from one side of the pzc to the otha. 

The overall relative orientation of the secondary structures of an enzyme determines its three-dimensional shape, or tertiary structure. Some enzymes require multiple copies of the same enzyme to function. The individual enzymes cluster into groups of two or more (called dimers, trimers, etc.) and are held together by intermolecular forces. The relative positioning of the separate enzymes in the cluster determines the overall structure, or quaternary structure, of the supramolecular complex. While all enzymes have tertiary structure, only clusters of multiple enzyme subunits have quaternary structure. The overall folded conformation of a protein in its active, catalytic form is called the active or native conformation. 

The relation between the supermolecule coupled cluster approach and the perturbation theory of intermolecular forces in even less obvious than the case of the Mpller-Plesset theory, and no formal analysis has been reported in the literature thus far. Rode et al.68 analyzed the long-range behavior of the CCSD(T) method65, and showed that this method, although very popular and in principle accurate, may lead to wrong results for systems with the electrostatic term strongly depending on the electronic correlation, e.g. the CO dimer. 

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 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. 

An atomistic approach, which has relevance to the current work, is the previously discussed normal-mode method. In the normal-mode method the constituent monomer units in the cluster are assumed to interact with a reasonable model potential in a fixed structure. From the assumed structure and model potential a normal-mode analysis is jjerformed to determine a vibrational partition function. Rotational and translational partition functions are then included classically. The normal-mode method treats the cluster as a polyatomic molecule and is most appropriate at very low temperatures where anharmonic contributions to the intermolecular forces can be ignored. As we shall show by numerical example, as the temperature is increased, the... 

Pure solvents Both UV-vis and fluorescent probes, which exhibit solvatochromic shifts, have been used to study clustering. Solvatochromic shifts are caused by the same types of solute-solvent intermolecular forces (i.e. dispersion, induction, and dipole-dipole forces) that influence solubilities, interacting over the same range. Consequently, the values of clustering determined spectroscopically are appreciate for considering the effect of clustering on solubilities. 

---