Structure and Stereochemistry
The terms “structure” and “molecular structure” are used frequently in chemical literature. However, their meaning in most instances is quite diffuse. Generally, structure is defined as an arrangement or relationship of elements in a substance, body, or system. Regarding chemistry, the term structure was used in the beginning of this science to point out a connectivity relationship between the atoms of a molecule. Later on more and more relationships were included until, especially in X-ray crystallographic studies, the complete geometrical picture with all its various details was meant. To emphasize this meaning and to distinguish it from the structures resulting from packing many molecules together as is the case in crystals, the term molecular structure was introduced. Therefore it seems to be convenient to avoid the term structure in favour of several “structural aspects”, whose definitions and scopes will be detailed in the following . Further it will be important to indicate the state of matter when structural aspects of a molecule are considered, i.e., wether the material is amorphous, crystalline, liquid, solute, or gaseous state. Of course, for linear oligopyrroles only the solid phase and the state of solution will be of interest.