Molecular clusters

Molecular clusters

Atomic and molecular clusters are aggregates of 5-105 atomic or molecular units. They classify according to the forces holding them together:

  • Van der Waals clusters - attraction between induced electric dipoles and repulsion between electron cores of closed electronic configurations
  • Metallic clustes - long range valence electron sharing (over many successive adjacent atoms) and partially directional
  • Ionic clusters - valence electrons are almost entirely transferred among closest neighbors to yield 2 net, equal but opposite, electric charge distributions that mutually attract.

Quantum many-body mechanisms are also important.

The role of cluster formation in the precipitation of liquid mixtures and in the condensation, adsorption to surface or solidification phase transitions has been long investigated from a theoretical standpoint.

Cluster system properties — stem both from their size and composition (which contributes to the binding force types) that determine:

  • the number of dimensions of their phase space
  • the ranges of accessible positions and velocities of their atomic components

A gradual transition occurs between the properties of the molecular species and those of the corresponding bulk mix. And yet the clusters exhibit physical and chemical properties specific only to their configuration space (in turn strongly atom-count-dependent) and not specific to their bulk counterparts.

Cluster systems are metastable with respect to at least one of the following evolution classes:

  • atom elimination or adsorption at cluster surface as a cause for their disassociation or growth
  • configuration switches among a set of stable structures (a.k.a. an “isomer class”) accessible to all clusters of a same atom count and a same relative component composition.

A large fraction of their component atoms is found at their surface defines many of their properties. With increasing size, the relative number of atoms at the cluster surface will scale approximately as N-1/3. One has to reach beyond a variable threshold of 9-27 component molecules (depending on the strength of the inter-molecular forces) to find global minimum configurations that hold at least one interior molecule. At the other end of the scale a cluster of about 105 atoms will expose only about 10% of the atoms at its surface, a still significant percentage in comparison to the bulk.


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