George C. Brumlik, Edward 3. Barrett, and Reuben 1. Baumgalten Hunter College N e w York, N e w York
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Framework Moletular Orbital Models
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Recent developments in chemistry have created a demand for models that would depict not only the relative positions of atoms in molecules but would in addition portray the symmetries, coutours, and relative orientation of atomic and molecular orbitals. This demand was met in the area of spacefilling molecular models,' hut the field of the framework (skeletal) molecular models has lagged behind this development. We have designed and constructed a new set of framework models: The Framework Molecular Orbital Models (abbreviated the FMO Set, as they shall hereafter be referred to) which outline the orientation of the symmetry axes and the symmetry planes of atomic and molecular orbitals in three dimensions and show on relative scale how far these orbitals reach out into molecular space. The Design of the FMO Set
The FMO Set was designed to meet the requirements of accurate representation of the largest variety of molecules in all possible detail and yet to achieve this with only a few components. The goal was attained by means of two degrees of freedom provided in the construction of the FMO Set: facility to change bond angles and ability to adjust individual linear parameters (iternuclear distances, covalent radii, and van der Waals radii).
Atoms and their volence angler Figure 1. Components of the F M O Set. ore represented by the valence clvrterr lmiddle row). Lefl to right: tetroInternuclear distances as well or hedron, trigonal bipyramid, octahedron. covalent and von der Wools mdii ore represented by connectors cut from plastic tubing (in color for otom-coding) or from metal tubing (tap ond bottom re3pectively) in ogreement with the cho9en scale (range 5-100 mm/i). h i n g only there three valence clusters and sections of tubing, molecvler containing m y of the atoms of the periodic table con be built on relative %ale.
Atomic units (from now on called the "valence clusters") and metal and plastic tubing are the sole components of the FMO Set (Fig. 1). The valence clusters are made from sections of wire brazed end-on a t a common point-the center of the atom. The wire elements point along the symmetry
axes of the atomic valence orbitals, and the angles a t which the wires are attached determine bond angles in assembled molecular models. Three types of valence clusters are provided: (Fig. 1, middle row, left to right) tetrahedron, trigonal bipyramid, and octahedron. All atoms of the periodic table having up to twelve electrons in their valence shell can be represented on relative scale with these three units and sections of tubmg. The valence clusters hold their shape even when they are used for the construction of strained models, yet their angles can he adjusted by bending the wire elements a t the point of their attachment by means of long pieces of tubing used as lever arms. Plastic (elastomer) or metal tubing (Fig. 1, top and bottom, respectively) serves as raw material for connectors which are cut to size in agreement with the selected scale (range 5-100 mm/A). The inside diameter of the tubing is matched to the diameter of the wire elements of the clusters so as to provide a snug friction-fit. Colored plastic or elastomer tubing is used for atom coding (C-black, H-white, O-red, I\'-blue, S-yellow, Br-orange, etc.). Metal tuhing is of service for demonstration of special effects, such as simulation of restricted rotation around single bonds. Rigid connectors cut from the metal tubing prevent distortion of the molecular framework that is under stress.
Figure 2. Relotive sealer. A large model of cycloheime (far simpliciW is controrted with o m o l l model placed in shown without hydrogen its center. The shadow cost b y the large choir tokes the form of the familiar hexagon. One of the some sire of valence clusters is used for both models.
Features
Building Models on a Variety of Scales. Several relative scales are currently employed in using molecular models. These scales are chosen so as to meet inditdual requirements; namely, large models (5-10 cm/A) are used for lecture demonstration, smaller models (1-5 cm/A) for individual observation, and the smallest working scale (
