Asymmetric unit

Unité asymétrique (Fr). Asymmetrische Einheit (Ge). Unità asimmetrica (It). Unidad asimétrica (Sp). Мотивная единица (Ru).

An asymmetric unit of a space group is the simply connected smallest group of atoms – closed part of space from which, by application of all symmetry operations of the space group, the whole of space is filled. This implies that:

· mirror planes must form boundary planes of the asymmetric unit;

· rotation axes must form boundary edges of the asymmetric unit;

· inversion centres must either form vertices of the asymmetric unit or be located at the midpoints of boundary planes or boundary edges.[1]

In structures composed of molecules of a single type which have no symmetry themselves, the asymmetric unit is a molecule, or occasionally two or more molecules which differ from one another in orientation or conformation. When the molecule has symmetry, which can conform to crystallographic symmetry, it may occupy a special position, and the asymmetric unit will then be a half molecule or even some smaller fraction.[2] The asymmetric unit may contain multiple molecules of two different types.

References:

1 T. Hahn, “International tables for crystallography”, Volume A, 5th Ed., Springer, Dordrecht 2005, 25.

2 W. Massa, “Crystal structure determination”, 3rd Ed., Books on Demand, Norderstedt 2016, 79-80.