Molecular Shape as a Critical Factor in Structure-Property-Activity Studies

The dominant role of molecular mass in determining many physical properties of substances often masks significant variations of these same properties with molecular shape. Here attention is drawn to the important influence of molecular shape on molecular properties. The interdependences of the heats of vaporization, boiling points, molar volumes, molar refractions, critical temperatures, critical pressures and surface tensions of alkanes in general, and of the octanes in particular, are used to illustrate the different roles of molecular mass and shape in influencing bulk properties. The advantages of using a hierarchical separation of influences by means of orthogonal descriptors in structure-property studies are emphasized. The high interrelatedness of many physical properties in alkanes does not extend to isomeric variations, but rather reflects the dominant influence of mass on these properties. When the role of mass is removed, the influence of other structural factors is more readily discerned.