Bonding considerations geometry

At present it seems that before such theoretical considerations may be used in a truly predictive manner, further development and refinement of the methods are necessary. This development may be derived from consideration of (i) the reactivity of other types of states, for example, IL and CT excited state compounds (34,48) (ii) theoretical models for radiationless transitions of reactive excited state compounds (60-66) (iii) the symmetry requirements of excited state reactions (67) and (iv) correlations of photoreactivity with detailed calculations of excited state bonding properties, geometry, and so on (56, 58,68,69). The accumulation of data regarding primary photoprocesses should provide a solid basis for this maturation and also a means of evaluating more highly developed models. 

The origin of a torsional barrier can be studied best in simple cases like ethane. Here, rotation about the central carbon-carbon bond results in three staggered and three eclipsed stationary points on the potential energy surface, at least when symmetry considerations are not taken into account. Quantum mechanically, the barrier of rotation is explained by anti-bonding interactions between the hydrogens attached to different carbon atoms. These interactions are small when the conformation of ethane is staggered, and reach a maximum value when the molecule approaches an eclipsed geometry. 

The Universal Force Field, UFF, is one of the so-called whole periodic table force fields. It was developed by A. Rappe, W Goddard III, and others. It is a set of simple functional forms and parameters used to model the structure, movement, and interaction of molecules containing any combination of elements in the periodic table. The parameters are defined empirically or by combining atomic parameters based on certain rules. Force constants and geometry parameters depend on hybridization considerations rather than individual values for every combination of atoms in a bond, angle, or dihedral. The equilibrium bond lengths were derived from a combination of atomic radii. The parameters [22, 23], including metal ions [24], were published in several papers. 

The detailed coordination geometry about As, Sb or Bi in these clusters varies substantially, and is of considerable signilicancc in describing the nature of the bonding in these species. 

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