Background Steric or Shape Influence

In chemical phenomena, two classes can be described as being influenced by steric effects. First, groups of atoms affect the reactivity of a nearby part of a molecule. This steric effect frequently manifests itself as a repulsive influence toward a reagent attacking a nearby reaction center. These effects are analyzed and encoded into substituent indexes from a standard reaction, based on their relative inhibitory effect. 

In the second class of phenomena are intermolecular interactions, where a certain degree of complementarity must be achieved for a reaction to occur. There must be a fit or recognition between molecules or between molecule and receptor, governed by the shape of each. The degree of fit or complementarity (similarity) has led to indexes encoding shape. 

Studies of these two facets of steric influence have spawned different approaches to the quantitation of this structural feature. A brief review of some methods illustrates the progress and problems in this area. 

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