Schoenflies notation

The Schoenflies (or Schönflies) notation, named after the German mathematician Arthur Moritz Schoenflies, is one of two conventions commonly used to describe point groups. This notation is used in spectroscopy. The other convention is the Hermann–Mauguin notation, also known as the International notation. A point group in the Schoenflies convention is completely adequate to describe the symmetry of a molecule; this is sufficient for spectroscopy. The Hermann–Mauguin notation is able to describe the space group of a crystal lattice, while the Schoenflies notation isn't. Thus the Hermann–Mauguin notation is used in crystallography.

Symmetry elements

Symmetry elements are denoted by i for centers of inversion, C for proper rotation axes, σ for mirror planes, and S for improper rotation axes (rotation-reflection axes). C and S are usually followed by a subscript number (abstractly denoted n) denoting the order of rotation possible.

By convention, the axis of proper rotation of greatest order is defined as the principal axis. All other symmetry elements are described in relation to it. A vertical mirror plane (containing the principal axis) is denoted σv; a horizontal mirror plane (perpendicular to the principal axis) is denoted σh.

Point groups

In three dimensions, there are an infinite number of point groups, but all of them can be classified by several families.

  • Cnh is Cn with the addition of a mirror (reflection) plane perpendicular to the axis of rotation (horizontal plane).
  • Cnv is Cn with the addition of n mirror planes containing the axis of rotation (vertical planes).
  • Dnh has, in addition, a horizontal mirror plane and, as a consequence, also n vertical mirror planes each containing the n-fold axis and one of the twofold axes.
  • Dnd has, in addition to the elements of Dn, n vertical mirror planes which pass between twofold axes (diagonal planes).
  • Td includes diagonal mirror planes (each diagonal plane contains only one twofold axis and passes between two other twofold axes, as in D2d). This addition of diagonal planes results in three improper rotation operations S4.
  • Th includes three horizontal mirror planes. Each plane contains two twofold axes and is perpendicular to the third twofold axis, which results in inversion center i.
  • Oh includes horizontal mirror planes and, as a consequence, vertical mirror planes. It contains also inversion center and improper rotation operations.
  • Ih includes horizontal mirror planes and contains also inversion center and improper rotation operations.

All groups that do not contain several higher-order axes (order 3 or more) can be arranged in a table:

n 12345678...
Cn C1 C2 C3 C4 C5 C6 C7 C8
Cnv C1v = C1h C2v C3v C4v C5v C6v C7v C8v
Cnh C1h = Cs C2h C3h C4h C5h C6h C7h C8h
Sn S1 = Cs S2 = Ci S3 = C3h S4 S5 = C5h S6 S7 = C7h S8
S = C∞h
Cni C1i = Ci C2i = Cs C3i = S6 C4i = S4 C5i = S10 C6i = C3h C7i = S14 C8i = S8
C∞i = C∞h
Dn D1 = C2 D2 D3 D4 D5 D6 D7 D8
Dnh D1h = C2v D2h D3h D4h D5h D6h D7h D8h
Dnd D1d = C2h D2d D3d D4d D5d D6d D7d D8d
D∞d = D∞h

The symbols that should not be used are marked with red color.

In crystallography, due to the crystallographic restriction theorem, n is restricted to the values of 1, 2, 3, 4, or 6. The noncrystallographic groups are shown with grayed backgrounds. D4d and D6d are also forbidden because they contain improper rotations with n = 8 and 12 respectively. The 27 point groups in the table plus T, Td, Th, O and Oh constitute 32 crystallographic point groups.

Groups with n = ∞ are called limit groups or Curie groups. There are two more limit groups, not listed in the table: K (for Kugel, German for ball, sphere), the group of all rotations in 3-dimensional space; and Kh, the group of all rotations and reflections. In mathematics and theoretical physics they are known respectively as the special orthogonal group and the orthogonal group in three-dimensional space, with the symbols SO(3) and O(3).

Space groups

The space groups with given point group are numbered by 1, 2, 3, ... (in the same order as their international number) and this number is added as a superscript to the Schönflies symbol for the corresponding point group. For example, groups numbers 3 to 5 whose point group is C2 have Schönflies symbols C1
, C2
, C3

While in case of point groups, Schönflies symbol defines the symmetry elements of group unambiguously, the additional superscript for space group doesn't have any information about translational symmetry of space group (lattice centering, translational components of axes and planes), hence one needs to refer to special tables, containing information about correspondence between Schönflies and Hermann–Mauguin notation. Such table is given in List of space groups page.

See also

External links


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