Inter-cluster interactions

In further computational work concerned with isopropylzinc-derived species, the enthalpies and structures of the four possible closed tetramers were derived (Fig. 22). Dramatic differences from the methylzinc analogues were observed, simply because the bulk of the larger alkyl group dictates the level of inter-cluster interactions. Cubic structures directly derived from homoehiral isopropyl-dimers possessed severe and inescapable non-bonding... 

Theoretical and computational methodologies are treated in detail elsewhere in this book. Experimental techniques for studying isolated molecules rely on their observation in the gas phase, where molecules can be studied free of interactions. This is different from single molecule studies in which molecules can interact with their environment, but are studied one by one [2], In the gas phase one may study a large ensemble of molecules or clusters, but each one of those is isolated and does not interact with its environment. Clusters represent a transition area between gas phase and bulk by allowing infra-cluster interactions, while being isolated from inter-cluster interactions. 

Specific cluster-counterion and/or inter-cluster interactions are evident in many solid structures containing formally naked clusters. With the exception of the mercury clusters, these interactions are generally weak enough to validate the notion of the cluster as naked . However, they are nevertheless interesting since in some cases they influence the cluster structures. In addition, the cation-Zintl ion and inter-cluster interactions are of interest since they represent bridges to the structures found in Zintl phases and other intermetallics. ... 

Transition metal clusters, however, need still to be tested in the engineering of crystalline materials. Crystal engineering has been defined as the capacity to make crystals with a purpose. In transition metal cluster chemistry this purpose is that of utilizing the distinct characteristics mentioned above to construct crystals that can function as the result of the inter-cluster interactions. To do this the experimentalist needs to conceive ways of directing the crystal-building process towards given architectures, i. e. needs to learn how to make non-covalent crystal synthesis. Clearly, the growth and success of a solid-state chemistry of transition metal clusters depends crucially on a close interaction between synthesis, theory, solid state characterization, and evaluation of properties. 

Donor ligands can influence not only the inter-cluster interactions but also the nuclearity of the clusters. An illustrative example of such an effect is observed for phenyl lithium which, depending on the donor ligands, shows nuclearities of one in [PhLi (pmdta)] (pmdta = pentamethyldiethylentriamine), two in [PhLi-(tmeda)]2 and four in the diethyl ether derivative [PhLi(OEt2)]4. 

We have found that dendrimers can be used to encapsulate active moieties, thereby preventing them from interacting. This passivation effect limits inter-molecular interactions such as self-aggregation and molecular clustering. We also found that dendrimers can be made dipolar. This asymmetry in molecular orientation enables dendrimers to be used in NLO. In this chapter we describe our application of dendrimers to lasers and NLO. 

Their comparatively simple crystal structures may be described as a primitive packing of quasi-molecular units, and, in a way, they represent the border line between molecular and infinitely extended units in a solid (Cheetham and Day 1992). The Mo6 type core is completely surrounded by X atoms and inter-cluster bonding essentially occurs through the Mo-X interactions. The Mo—Mo bonding between clusters is very weak. 

When small clusters are placed adjacent to one another, but at random with respect to inter-cluster distances and orientations, as is the case in the amorphous arrangement of the clusters of Aujj, the interactions between the individual clusters will lead to small shifts of the intra-cluster energy levels which are slightly different for each cluster. Since both macroscopic as well as microscopic... 

The term metalloid cluster is used to describe a multinuclear molecular species in which the metal atoms exhibit closest packing (and hence delocalized inter-metallic interactions) like that in bulk metal, and the metal-metal contacts outnumber the peripheral metal-ligand contacts. Most examples are found in the field of precious-metal cluster chemistry. In recent years, an increasing number of cluster species of group 13 elements have been synthesized with cores... 

Besides the mobility on the cluster surface in solution the ball-like character of the huge molecules obviously prevents crystallization. If we assume a closest packed structure for the M55 cluster sphere in a crystal, then there would exist octahedral lattice vacancies with a diameter of 8-9 Al There is some hope to find ligands causing inter-molecular interactions and thus facilitating crystallization. 

A straightforward consequence of such strong interactions (in the absence of inter-chain interactions, such as in very dilute solutions) would be aggregation (or clustering) of these segments, a situation that can be termed self-association. Such a... 

Recently we have proposed a decomposition scheme of inter-molecular interaction energy in polyhedral water clusters. Central to the scheme is the concept of effective pair interaction energy (effective H-bond energy). In additive approximation, the interaction energy of any molecular system is equal to the sum of all pair interactions. The effective energy of pair interaction (effective energy of H-bond) is... 

Using the concept of effective pair interactions we have developed a new discrete model of inter-molecular interactions in gas hydrate frameworks. Contrary to H-bond types on a surface of polyhedral clusters, in this case it is beyond reason to divide additionally the trans- and c/ -configurations of distinct H-bonded dimer. It is necessary to take into consideration an interactions of all surrounding molecules. We have found that there are 1098 symmetry-distinct types of H-bond surroundings (extended H-bonds). Among them 369 types (33.6%) correspond to strong H-bonds. The rest of them contain weak H-bonded dimer. Taking into account antisymmetry, the number of different extended H-bonds is 567. 

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