Molecular symmetry

Molecular symmetry is a basic idea in chemistry. It describes the shapes of molecules in terms of the permutations of similar atoms in those molecules when they are transformed by the symmetry operations described below. It puts molecules into groups according to their symmetry. It can predict or explain many of a molecule's chemical properties. Chemists study symmetry to explain how crystals are made up and how chemicals react. The molecular symmetry of the reactants help predict how the product of the reaction is made up and the energy needed for the reaction.

Molecular symmetry can be studied several different ways. Group theory is the most popular idea. Group theory is also useful in studying the symmetry of molecular orbitals. This is used in the Hückel method, ligand field theory, and the Woodward–Hoffmann rules. Another idea on a larger scale is the use of crystal systems to describe crystallographic symmetry in bulk materials.

Scientists find molecular symmetry by using X-ray crystallography and other forms of spectroscopy. Spectroscopic notation is based on facts taken from molecular symmetry.

Historical background

Physicist Hans Bethe used characters of point group operations in his study of ligand field theory in 1929. Eugene Wigner used group theory to explain the selection rules of atomic spectroscopy. The first character tables were compiled by László Tisza (1933), in connection to vibrational spectra. Robert Mulliken was the first to publish character tables in English (1933). E. Bright Wilson used them in 1934 to predict the symmetry of vibrational normal modes. The complete set of 32 crystallographic point groups was published in 1936 by Rosenthal and Murphy.

Symmetry concepts

Mathematical group theory has been adapted to study of symmetry in molecules.

Elements

The symmetry of a molecule can be described by 5 types of symmetry elements.

Water molecule is symmetrical

• Symmetry axis: an axis around which a rotation by ${\tfrac {360^{\circ }}{n}}$ results in a molecule that appears identical to the molecule before rotation. This is also called an n-fold rotational axis and is shortened to C_n. Examples are the C₂ in water and the C₃ in ammonia. A molecule can have more than one symmetry axis; the one with the highest n is called the principal axis, and by convention is given the z-axis in a Cartesian coordinate system.
• Plane of symmetry: a plane of reflection through which an identical copy of the original molecule is given. This is also called a mirror plane and abbreviated σ. Water has two of them: one in the plane of the molecule itself and one perpendicular (at right angles) to it. A symmetry plane parallel with the principal axis is dubbed vertical (σ_v) and one perpendicular to it horizontal (σ_h). A third type of symmetry plane exists: if a vertical symmetry plane additionally bisects the angle between two 2-fold rotation axes perpendicular to the principal axis, the plane is dubbed dihedral (σ_d). A symmetry plane can also be identified by its Cartesian orientation, e.g., (xz) or (yz).

Benzene

• Center of symmetry or inversion center, shortened to i. A molecule has a center of symmetry when, for any atom in the molecule, an identical atom exists diametrically opposite this center an equal distance from it. There may or may not be an atom at the center. Examples are xenon tetrafluoride (XeF₄) where the inversion center is at the Xe atom, and benzene (C₆H₆) where the inversion center is at the center of the ring.
• Rotation-reflection axis: an axis around which a rotation by ${\tfrac {360^{\circ }}{n}}$, followed by a reflection in a plane perpendicular to it, leaves the molecule unchanged. Also called an n-fold improper rotation axis, it is shortened to S_n, with n necessarily even. Examples are present in tetrahedral silicon tetrafluoride, with three S₄ axes, and the staggered conformation of ethane with one S₆ axis.
• Identity (also E), from the German 'Einheit' meaning Unity. It is called "Identity" because it is like the number one (unity) in multiplication. (When a number is multiplied by one, the answer is the original number.) This symmetry element means no change. Every molecule has this element. The identity symmetry element helps chemists use mathematical group theory.

Operations

Each of the five symmetry elements has a symmetry operation. People use a caret symbol (^) to talk about the operation rather than the symmetry element. So, Ĉ_n is the rotation of a molecule around an axis and Ê is the identity operation. A symmetry element can have more than one symmetry operation associated with it. Since C₁ is equivalent to E, S₁ to σ and S₂ to i, all symmetry operations can be classified as either proper or improper rotations.

Point groups

A point group is a set of symmetry operations forming a mathematical group, for which at least one point remains fixed under all operations of the group. A crystallographic point group is a point group which will work with translational symmetry in three dimensions. There are a total of 32 crystallographic point groups, 30 of which are relevant to chemistry. Scientists use Schoenflies notation to classify point groups.

Group theory

Mathematics define a group. A set of symmetry operations form a group when:

• the result of consecutive application (composition) of any two operations is also a member of the group (closure).
• the application of the operations is associative: A(BC) = (AB)C
• the group contains the identity operation, denoted E, such that AE = EA = A for any operation A in the group.
• For every operation A in the group, there is an inverse element A⁻¹ in the group, for which AA⁻¹ = A⁻¹A = E

The order of a group is the number of symmetry operations for that group.

For example, the point group for the water molecule is C_2v, with symmetry operations E, C₂, σ_v and σ_v'. Its order is thus 4. Each operation is its own inverse. As an example of closure, a C₂ rotation followed by a σ_v reflection is seen to be a σ_v' symmetry operation: σ_v*C₂ = σ_v'. (Note that "Operation A followed by B to form C" is written BA = C).

Another example is the ammonia molecule, which is pyramidal and contains a three-fold rotation axis as well as three mirror planes at an angle of 120° to each other. Each mirror plane contains an N-H bond and bisects the H-N-H bond angle opposite to that bond. Thus ammonia molecule belongs to the C_3v point group which has order 6: an identity element E, two rotation operations C₃ and C₃², and three mirror reflections σ_v, σ_v' and σ_v".

Common point groups

The following table contains a list of point groups with representative molecules. The description of structure includes common shapes of molecules based on VSEPR theory.

Point group	Symmetry elements	Simple description, chiral if applicable	Illustrative species
C1	E	no symmetry, chiral	CFClBrH, lysergic acid
Cs	E σh	planar, no other symmetry	thionyl chloride, hypochlorous acid
Ci	E i	Inversion center	anti-1,2-dichloro-1,2-dibromoethane
C∞v	E 2C∞ σv	linear	hydrogen chloride, dicarbon monoxide
D∞h	E 2C∞ ∞σi i 2S∞ ∞C2	linear with inversion center	dihydrogen, azide anion, carbon dioxide
C2	E C2	"open book geometry," chiral	hydrogen peroxide
C3	E C3	propeller, chiral	triphenylphosphine
C2h	E C2 i σh	planar with inversion center	trans-1,2-dichloroethylene
C3h	E C3 C32 σh S3 S35	propeller	Boric acid
C2v	E C2 σv(xz) σv'(yz)	angular (H2O) or see-saw (SF4)	water, sulfur tetrafluoride, sulfuryl fluoride
C3v	E 2C3 3σv	trigonal pyramidal	ammonia, phosphorus oxychloride
C4v	E 2C4 C2 2σv 2σd	square pyramidal	xenon oxytetrafluoride
D2	E C2(x) C2(y) C2(z)	twist, chiral	cyclohexane twist conformation
D3	E C3(z) 3C2	triple helix, chiral	Tris(ethylenediamine)cobalt(III) cation
D2h	E C2(z) C2(y) C2(x) i σ(xy) σ(xz) σ(yz)	planar with inversion center	ethylene, dinitrogen tetroxide, diborane
D3h	E 2C3 3C2 σh 2S3 3σv	trigonal planar or trigonal bipyramidal	boron trifluoride, phosphorus pentachloride
D4h	E 2C4 C2 2C2' 2C2 i 2S4 σh 2σv 2σd	square planar	xenon tetrafluoride
D5h	E 2C5 2C52 5C2 σh 2S5 2S53 5σv	pentagonal	ruthenocene, eclipsed ferrocene, C70 fullerene
D6h	E 2C6 2C3 C2 3C2' 3C2 i 3S3 2S63 σh 3σd 3σv	hexagonal	benzene, bis(benzene)chromium
D2d	E 2S4 C2 2C2' 2σd	90° twist	allene, tetrasulfur tetranitride
D3d	E C3 3C2 i 2S6 3σd	60° twist	ethane (staggered rotamer), cyclohexane chair conformation
D4d	E 2S8 2C4 2S83 C2 4C2' 4σd	45° twist	dimanganese decacarbonyl (staggered rotamer)
D5d	E 2C5 2C52 5C2 i 3S103 2S10 5σd	36° twist	ferrocene (staggered rotamer)
Td	E 8C3 3C2 6S4 6σd	tetrahedral	methane, phosphorus pentoxide, adamantane
Oh	E 8C3 6C2 6C4 3C2 i 6S4 8S6 3σh 6σd	octahedral or cubic	cubane, sulfur hexafluoride
Ih	E 12C5 12C52 20C3 15C2 i 12S10 12S103 20S6 15σ	icosahedral	C60, B12H122-

Representations

Symmetry operations can be written in many ways. A good way to write them is by using matrices. For any vector representing a point in Cartesian coordinates, left-multiplying it gives the new place of the point transformed by the symmetry operation. Composition of operations is done by matrix multiplication. In the C_2v example this is:

$\underbrace {\begin{bmatrix}-1&0&0\\0&-1&0\\0&0&1\\\end{bmatrix}} _{C_{2}}\times \underbrace {\begin{bmatrix}1&0&0\\0&-1&0\\0&0&1\\\end{bmatrix}} _{\sigma _{v}}=\underbrace {\begin{bmatrix}-1&0&0\\0&1&0\\0&0&1\\\end{bmatrix}} _{\sigma '_{v}}$

Although an infinite number of such ways of showing things exist, the irreducible representations (or "irreps") of the group are commonly used, as all other representations of the group can be described as a linear combination of the irreducible representations. (The irreps span the vector space of the symmetry operations.) Chemists use the irreps to sort the symmetry groups and to talk about their properties.

Character tables

For each point group, a character table summarizes information on its symmetry operations and on its irreducible representations. The tables are square because there are always equal numbers of irreducible representations and groups of symmetry operations.

The table itself is made of characters which show how a particular irreducible representation changes when a particular symmetry operation is put to it. Any symmetry operation in a molecule's point group acting on the molecule itself will leave it unchanged. But for acting on a general entity (thing), such as a vector or an orbital, this does not have to be what happens. The vector could change sign or direction, and the orbital could change type. For simple point groups, the values are either 1 or -1: 1 means that the sign or phase (of the vector or orbital) is unchanged by the symmetry operation (symmetric) and -1 denotes a sign change (asymmetric).

The representations are labeled according to a set of conventions:

• A, when rotation around the principal axis is symmetrical
• B, when rotation around the principal axis is asymmetrical
• E and T are doubly and triply degenerate representations, respectively
• when the point group has an inversion center, the subscript g (German: gerade or even) signals no change in sign, and the subscript u (ungerade or uneven) a change in sign, with respect to inversion.
• with point groups C_∞v and D_∞h the symbols are borrowed from angular momentum description: Σ, Π, Δ.

The tables also tell the Cartesian basis vectors, rotations about them, and quadratic functions of them transformed by the symmetry operations of the group. The table also shows which irreducible representation transforms in the same way (on the right hand side of the tables). Chemists use this because chemically important orbitals (in particular p and d orbitals) have the same symmetries as these entities.

The character table for the C_2v symmetry point group is given below:

C2v	E	C2	σv(xz)	σv'(yz)
A1	1	1	1	1	z	x2, y2, z2
A2	1	1	-1	-1	Rz	xy
B1	1	-1	1	-1	x, Ry	xz
B2	1	-1	-1	1	y, Rx	yz

For example, water (H₂O) which has the C_2v symmetry described above. The 2p_x orbital of oxygen is oriented perpendicular to the plane of the molecule and switches sign with a C₂ and a σ_v'(yz) operation, but remains unchanged with the other two operations (obviously, the character for the identity operation is always +1). This orbital's character set is thus {1, -1, 1, -1}, corresponding to the B₁ irreducible representation. Similarly, the 2p_z orbital is seen to have the symmetry of the A₁ irreducible representation, 2p_y B₂, and the 3d_xy orbital A₂. These assignments and others are in the rightmost two columns of the table.

Other websites

• Molecular symmetry at the University of Exeter Archived 2008-06-19 at the Wayback Machine
• Molecular symmetry at Imperial College London
• Molecular Point Group Symmetry Tables
• Molecular symmetry at Otterbein University
• Pictorial overview of the 32 groups