Molecular States and Molecular Orbitals

Molecules change their properties on acquisition or loss of energy. The state of a molecule can be characterized by the difference between its energy and that of the preceding initial state or that of the subsequent final state, as well as by the respective charge distribution. Molecules in their various energy states each display a particular structure and characteristic properties, and it is these entities which the chemist uses as synthetic building blocks.—The best model hitherto available for describing molecular states is the molecular orbital representation. Molecular orbitals are especially suitable for comparing mutually corresponding molecular states of similar compounds, and thus provide a manifold and stimulating overview of large areas of chemistry.—An attempt will now be made to point out important characteristic parameters of molecular states and to explain the rules governing their description by molecular orbitals, while circumventing mathematical barriers. A principal objective is the comparison of states of “chemically related” molecules with regard to the connection, the arrangement, and the potential of their atoms. The simple models employed for this purpose, whose inherent limitations and potential expansions are discussed, have meanwhile become valuable tools in the hand of the chemist.