Free-Electron Network Model for Conjugated Systems. III. A Demonstration Model Showing Bond Order and ``Free Valence'' in Conjugated Hydrocarbons

A mechanical model of a conjugated hydrocarbon has been constructed, based on the free-electron density theorems of Ruedenberg and Scherr. Stiff steel music wires strung on dowels above a baseboard follow the carbon skeleton. When the height of the wire above the board at each atomic position is fixed to constant theoretical values, and when simple boundary conditions are met at branch points and terminal atoms, then the height in the centers of bonds is a direct measure of electron density and bond order and ``free valence.'' The measured heights may be used to predict bond lengths and positions of substitution, even for large or unsymmetrical molecules which are hard to handle theoretically. The model should be especially useful for demonstrations and for giving students a feeling for electronic behavior in conjugated systems.