Absolute structure

Definition

The spatial arrangement of the atoms of a physically identified non-centrosymmetric crystal and its description by way of unit-cell dimensions, space group, and representative coordinates of all atoms.

Background

Absolute structure is a specification of the orientation of a crystal under the operation of inversion through a point otherwise known as parity (P). Under P a centrosymmetric object is invariant so there is clearly no absolute structure to specify. However the orientation of The need for the notion of absolute structure arises due

Absolute structure is to a crystal what absolute configuration is to a molecule. Both describe certain aspects of the spatial arrangement of the atoms in their respective object. However the object, its symmetry and the method of its specification are different. The table compares the two cases.

                Absolute Structure        Absolute Configuration
Property       spatial  arrangement        spatial  arrangement
Content               atoms                       atoms
Object               crystal                     molecule
Symmetry        non-centrosymmetric               chiral
Specification     crystallographic            stereochemical
                    space group                    CIP
                  cell dimensions
                 atomic coordinates

Whereas absolute configuration distinguishes between and specifies enantiomers (chiral molecules of opposite hand), absolute structure does the same for inversion-related models of the crystal structure.

Related chemical terms

The IUPAC Basic Terminology of Stereochemistry contains definitions of related chemical terms of use to the crystallographer, viz: absolute configuration, chiral, chirality, chirality sense, enantiomer, enantiomerically pure, enantiomorph, racemate, racemic, racemic conglomerate and relative configuration.

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