Molecular symmetry

Symmetry elements of formaldehyde. C₂ is a two-fold rotation axis. σ_v and σ_v' are two non-equivalent reflection planes.

Molecular symmetry in chemistry describes the symmetry present in molecules and the classification of molecules according to their symmetry. Molecular symmetry is a fundamental concept in chemistry, as it can predict or explain many of a molecule's chemical properties, such as its dipole moment and its allowed spectroscopic transitions (based on selection rules such as the Laporte rule). Many university level textbooks on physical chemistry, quantum chemistry, and inorganic chemistry devote a chapter to symmetry.^[1]^[2]^[3]^[4]^[5]

While various frameworks for the study of molecular symmetry exist, group theory is the predominant one. This framework is also useful in studying the symmetry of molecular orbitals, with applications such as the Hückel method, ligand field theory, and the Woodward-Hoffmann rules. Another framework on a larger scale is the use of crystal systems to describe crystallographic symmetry in bulk materials.

Many techniques for the practical assessment of molecular symmetry exist, including X-ray crystallography and various forms of spectroscopy, for example infrared spectroscopy of metal carbonyls. Spectroscopic notation is based on symmetry considerations.

Symmetry concepts

The study of symmetry in molecules is an adaptation of mathematical group theory.

Examples of the relationship between chirality and symmetry
Rotational axis (C_n)	Improper rotational elements (S_n)
	Chiral no S_n	Achiral mirror plane S₁ = σ	Achiral inversion centre S₂ = i
C₁
C₂

Elements

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

Symmetry axis: an axis around which a rotation by ${\tfrac {360^{\circ }}{n}}$ results in a molecule indistinguishable from the original. This is also called an n-fold rotational axis and abbreviated C_n. Examples are the C₂ axis in water and the C₃ axis 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 aligned with 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 generated. 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 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).
Center of symmetry or inversion center, abbreviated 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. In other words, a molecule has a center of symmetry when the points (x,y,z) and (-x,-y,-z) correspond to identical objects. For example, if there is an oxygen atom in some point (x,y,z), then there is an oxygen atom in the point (-x,-y,-z). There may or may not be an atom at the inversion center itself. Examples are xenon tetrafluoride 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 abbreviated S_n. Examples are present in tetrahedral silicon tetrafluoride, with three S₄ axes, and the staggered conformation of ethane with one S₆ axis.
Identity, abbreviated to E, from the German 'Einheit' meaning unity.^[6] This symmetry element simply consists of no change: every molecule has this element. While this element seems physically trivial, it must be included in the list of symmetry elements so that they form a mathematical group, whose definition requires inclusion of the identity element. It is so called because it is analogous to multiplying by one (unity). In other words, E is a property that any object needs to have regardless of its symmetry properties.^[7]

Operations

XeF₄, with square planar geometry, has 1 C₄ axis and 4 C₂ axes orthogonal to C₄. These five axes plus the mirror plane perpendicular to the C₄ axis define the D_4h symmetry group of the molecule.

The five symmetry elements have associated with them five types of symmetry operations, which leave the molecule in a state indistinguishable from the starting state. They are sometimes distinguished from symmetry elements by a caret or circumflex. Thus, 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. For example, the C₄ axis of the square xenon tetrafluoride (XeF₄) molecule is associated with two Ĉ₄ rotations (90°) in opposite directions and a Ĉ₂ rotation (180°). Since Ĉ₁ is equivalent to Ê, Ŝ₁ to σ and Ŝ₂ to î, all symmetry operations can be classified as either proper or improper rotations.

Molecular symmetry groups

Groups

The symmetry operations of a molecule (or other object) form a group, which is a mathematical structure usually denoted in the form (G,*) consisting of a set G and a binary combination operation say '*' satisfying certain properties listed below.

In a molecular symmetry group, the group elements are the symmetry operations (not the symmetry elements), and the binary combination consists of applying first one symmetry operation and then the other. An example is the sequence of a C₄ rotation about the z-axis and a reflection in the xy-plane, denoted σ(xy)C₄. By convention the order of operations is from right to left.

A molecular symmetry group obeys the defining properties of any group.

(1) closure property:
For every pair of elements x and y in G, the product x*y is also in G.
( in symbols, for every two elements x, y∈G, x*y is also in G ).
This means that the group is closed so that combining two elements produces no new elements. Symmetry operations have this property because a sequence of two operations will produce a third state indistinguishable from the second and therefore from the first, so that the net effect on the molecule is still a symmetry operation.
(2) associative property:
For every x and y and z in G, both (x*y)*z and x*(y*z) result with the same element in G.
( in symbols, (x*y)*z = x*(y*z ) for every x, y, and z ∈ G)
(3) existence of identity property:
There must be an element ( say e ) in G such that product any element of G with e make no change to the element.
( in symbols, x*e=e*x= x for every x∈ G )
(4) existence of inverse property:
For each element ( x ) in G, there must be an element y in G such that product of x and y is the identity element e.
( in symbols, for each x∈G there is a y ∈ G such that x*y=y*x= e for every x∈G )

The order of a group is the number of elements in the group. For groups of small orders, the group properties can be easily verified by considering its composition table, a table whose rows and columns correspond to elements of the group and whose entries correspond to their products.

Point group

Point Group Chart

The successive application (or composition) of one or more symmetry operations of a molecule has an effect equivalent to that of some single symmetry operation of the molecule. For example, 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).^[7] Moreover, the set of all symmetry operations (including this composition operation) obeys all the properties of a group, given above. So (S,*) is a group, where S is the set of all symmetry operations of some molecule, and * denotes the composition (repeated application) of symmetry operations.

This group is called the point group of that molecule, because the set of symmetry operations leave at least one point fixed (though for some symmetries an entire axis or an entire plane remains fixed). In other words, a point group is a group that summarizes all symmetry operations that all molecules in that category have.^[7] The symmetry of a crystal, by contrast, is described by a space group of symmetry operations, which includes translations in space.

Examples

Assigning each molecule a point group classifies molecules into categories with similar symmetry properties. For example, PCl₃, POF₃, XeO₃, and NH₃ all share identical symmetry operations.^[8] They all can undergo the identity operation E, two different C₃ rotation operations, and three different σ_v plane reflections without altering their identities, so they are placed in one point group, C_3v, with order 6.^[9] Similarly, water (H₂O) and hydrogen sulfide (H₂S) also share identical symmetry operations. They both undergo the identity operation E, one C₂ rotation, and two σ_v reflections without altering their identities, so they are both placed in one point group, C_2v, with order 4.^[10] This classification system helps scientists to study molecules more efficiently, since chemically related molecules in the same point group tend to exhibit similar bonding schemes, molecular bonding diagrams, and spectroscopic properties.^[7]

Common point groups

The following table contains a list of point groups labelled using the Schoenflies notation, which is common in chemistry and molecular spectroscopy. The description of structure includes common shapes of molecules, which can be explained by the VSEPR model.

Point group	Symmetry operations	Simple description of typical geometry	Example 1	Example 2	Example 3
C₁	E	no symmetry, chiral	bromochlorofluoromethane (both enantiomers shown)	lysergic acid	L-leucine and most other α-amino acids except glycine
C_s	E σ_h	mirror plane, no other symmetry	thionyl chloride	hypochlorous acid	chloroiodomethane
C_i	E i	inversion center	meso-tartaric acid	mucic acid (meso-galactaric acid)	(S,R) 1,2-dibromo-1,2-dichloroethane (anti conformer)
C_∞v	E 2C_∞ ∞σ_v	linear	hydrogen fluoride (and all other heteronuclear diatomic molecules)	nitrous oxide (dinitrogen monoxide)	hydrocyanic acid (hydrogen cyanide)
D_∞h	E 2C_∞ ∞σ_i i 2S_∞ ∞C₂	linear with inversion center	oxygen (and all other homonuclear diatomic molecules)	carbon dioxide	acetylene (ethyne)
C₂	E C₂	"open book geometry," chiral	hydrogen peroxide	hydrazine	tetrahydrofuran (twist conformation)
C₃	E C₃	propeller, chiral	triphenylphosphine	triethylamine	phosphoric acid
C_2h	E C₂ i σ_h	planar with inversion center	trans-1,2-dichloroethylene	trans-dinitrogen difluoride	trans-azobenzene
C_3h	E C₃ C₃² σ_h S₃ S₃⁵	propeller	boric acid	phloroglucinol (1,3,5-trihydroxybenzene)
C_2v	E C₂ σ_v(xz) σ_v'(yz)	angular (H₂O) or see-saw (SF₄)	water	sulfur tetrafluoride	pyrrole
C_3v	E 2C₃ 3σ_v	trigonal pyramidal	ammonia	phosphorus oxychloride	cobalt tetracarbonyl hydride, HCo(CO)₄
C_4v	E 2C₄ C₂ 2σ_v 2σ_d	square pyramidal	xenon oxytetrafluoride	pentaborane(9), B₅H₉	nitroprusside anion [Fe(CN)₅(NO)]²⁻
C_5v	E 2C₅ 2C₅² 5σ_v	'milking stool' complex	Ni(C₅H₅)(NO)	corannulene
D₂	E C₂(x) C₂(y) C₂(z)	twist, chiral	biphenyl (skew conformation)	twistane (C₁₀H₁₆)	cyclohexane twist conformation
D₃	E C₃(z) 3C₂	triple helix, chiral	Tris(ethylenediamine)cobalt(III) cation	tris(oxalato)iron(III) anion
D_2h	E C₂(z) C₂(y) C₂(x) i σ(xy) σ(xz) σ(yz)	planar with inversion center	ethylene	pyrazine	diborane
D_3h	E 2C₃ 3C₂ σ_h 2S₃ 3σ_v	trigonal planar or trigonal bipyramidal	boron trifluoride	phosphorus pentachloride	cyclopropane
D_4h	E 2C₄ C₂ 2C₂' 2C₂ i 2S₄ σ_h 2σ_v 2σ_d	square planar	xenon tetrafluoride	octachlorodimolybdate(II) anion	Trans-[Co^III(NH₃)₄Cl₂]⁺ (excluding H atoms)
D_5h	E 2C₅ 2C₅² 5C₂ σ_h 2S₅ 2S₅³ 5σ_v	pentagonal	cyclopentadienyl anion	ruthenocene	C₇₀
D_6h	E 2C₆ 2C₃ C₂ 3C₂' 3C₂‘’ i 2S₃ 2S₆ σ_h 3σ_d 3σ_v	hexagonal	benzene	bis(benzene)chromium	coronene (C₂₄H₁₂)
D_7h	E C₇ S₇ 7C₂ σ_h 7σ_v	heptagonal	tropylium (C₇H₇⁺) cation
D_8h	E C₈ C₄ C₂ S₈ i 8C₂ σ_h 4σ_v 4σ_d	octagonal	cyclooctatetraenide (C₈H₈²⁻) anion	uranocene
D_2d	E 2S₄ C₂ 2C₂' 2σ_d	90° twist	allene	tetrasulfur tetranitride	diborane(4) (excited state)
D_3d	E 2C₃ 3C₂ i 2S₆ 3σ_d	60° twist	ethane (staggered rotamer)	dicobalt octacarbonyl (non-bridged isomer)	cyclohexane chair conformation
D_4d	E 2S₈ 2C₄ 2S₈³ C₂ 4C₂' 4σ_d	45° twist	sulfur (crown conformation of S₈)	dimanganese decacarbonyl (staggered rotamer)	octafluoroxenate ion (idealised geometry)
D_5d	E 2C₅ 2C₅² 5C₂ i 3S₁₀³ 2S₁₀ 5σ_d	36° twist	ferrocene (staggered rotamer)
S₄	E 2S₄ C₂		tetraphenylborate anion
T_d	E 8C₃ 3C₂ 6S₄ 6σ_d	tetrahedral	methane	phosphorus pentoxide	adamantane
O_h	E 8C₃ 6C₂ 6C₄ 3C₂ i 6S₄ 8S₆ 3σ_h 6σ_d	octahedral or cubic	sulfur hexafluoride	molybdenum hexacarbonyl	cubane
I_h	E 12C₅ 12C₅² 20C₃ 15C₂ i 12S₁₀ 12S₁₀³ 20S₆ 15σ	icosahedral or dodecahedral	Buckminsterfullerene	dodecaborate anion	dodecahedrane

Representations

The symmetry operations can be represented in many ways. A convenient representation is by matrices. For any vector representing a point in Cartesian coordinates, left-multiplying it gives the new location of the point transformed by the symmetry operation. Composition of operations corresponds to matrix multiplication. Within a point group, a multiplication of the matrices of two symmetry operations leads to a matrix of another symmetry operation in the same point group.^[7] For example, 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 representations 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.

Character tables

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

The table itself consists of characters that represent how a particular irreducible representation transforms when a particular symmetry operation is applied. 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, such as a vector or an orbital, this need not be the case. 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 capture information about how the Cartesian basis vectors, rotations about them, and quadratic functions of them transform by the symmetry operations of the group, by noting which irreducible representation transforms in the same way. These indications are conventionally on the righthand side of the tables. This information is useful 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:

C_2v	E	C₂	σ_v(xz)	σ_v'(yz)
A₁	1	1	1	1	z	x², y², z²
A₂	1	1	−1	−1	R_z	xy
B₁	1	−1	1	−1	x, R_y	xz
B₂	1	−1	−1	1	y, R_x	yz

Consider the example of water (H₂O), which has the C_2v symmetry described above. The 2p_x orbital of oxygen has B₁ symmetry as in the fourth row of the character table above, with x in the sixth column). It 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. Likewise, the 2p_z orbital is seen to have the symmetry of the A₁ irreducible representation (i.e.: none of the symmetry operations change it), 2p_y B₂, and the 3d_xy orbital A₂. These assignments and others are noted in the rightmost two columns of the table.

Historical background

Hans Bethe used characters of point group operations in his study of ligand field theory in 1929, and Eugene Wigner used group theory to explain the selection rules of atomic spectroscopy.^[11] 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), and E. Bright Wilson used them in 1934 to predict the symmetry of vibrational normal modes.^[12] The complete set of 32 crystallographic point groups was published in 1936 by Rosenthal and Murphy.^[13]

Nonrigid molecules

The symmetry groups described above are useful for describing rigid molecules which undergo only small oscillations about a single equilibrium geometry, so that the symmetry operations all correspond to simple geometrical operations. However Longuet-Higgins has proposed a more general type of symmetry groups suitable for non-rigid molecules with multiple equivalent geometries.^[14]^[15] These groups are known as permutation-inversion groups, because a symmetry operation may be an energetically feasible permutation of equivalent nuclei, or an inversion with respect to the centre of mass, or a combination of the two.

For example, ethane (C₂H₆) has three equivalent staggered conformations. Conversion of one conformation to another occurs at ordinary temperature by internal rotation of one methyl group relative to the other. This is not a rotation of the entire molecule about the C₃ axis, but can be described as a permutation of the three identical hydrogen atoms of one methyl group. Although each conformation has D_3d symmetry as in the table above, description of the internal rotation and associated quantum states and energy levels requires the more complete permutation-inversion group.

Similarly, ammonia (NH₃) has two equivalent pyramidal (C_3v) conformations which are interconverted by the process known as nitrogen inversion. This is not an inversion in the sense used for symmetry operations of rigid molecules, since NH₃ has no inversion center. Rather it is a reflection of all atoms about the centre of mass (close to the nitrogen atom), which happens to be energetically feasible for this molecule. Again the permutation-inversion group is used to describe the interaction of the two geometries.

A second and similar approach to the symmetry of nonrigid molecules is due to Altmann.^[16]^[17] In this approach the symmetry groups are known as Schrödinger supergroups and consist of two types of operations (and their combinations): (1) the geometric symmetry operations (rotations, reflections, inversions) of rigid molecules, and (2) isodynamic operations, which take a nonrigid molecule into an energetically equivalent form by a physically reasonable process such as rotation about a single bond (as in ethane) or a molecular inversion (as in ammonia).^[17]

References

↑ Quantum Chemistry, Third Edition John P. Lowe, Kirk Peterson ISBN 0-12-457551-X
↑ Physical Chemistry: A Molecular Approach by Donald A. McQuarrie, John D. Simon ISBN 0-935702-99-7
↑ The chemical bond 2nd Ed. J.N. Murrell, S.F.A. Kettle, J.M. Tedder ISBN 0-471-90760-X
↑ Physical Chemistry P.W. Atkins and J. de Paula (8th ed., W.H. Freeman 2006) ISBN 0-7167-8759-8, chap.12
↑ G. L. Miessler and D. A. Tarr Inorganic Chemistry (2nd ed., Pearson/Prentice Hall 1998) ISBN 0-13-841891-8, chap.4.
↑ LEO Ergebnisse für "einheit"
1 2 3 4 5 Pfenning, Brian (2015). Principles of Inorganic Chemistry. John Wiley & Sons. ISBN 9781118859025.
↑ Pfennig, Brian. Principles of Inorganic Chemistry. Wiley. p. 191. ISBN 978-1-118-85910-0.
↑ pfennig, Brian. Principles of Inorganic Chemistry. Wiley. ISBN 978-1-118-85910-0.
↑ Miessler, Gary. Inorganic Chemistry. Pearson. ISBN 9780321811059.
↑ Group Theory and its application to the quantum mechanics of atomic spectra, E. P. Wigner, Academic Press Inc. (1959)
↑ Correcting Two Long-Standing Errors in Point Group Symmetry Character Tables Randall B. Shirts J. Chem. Educ. 2007, 84, 1882. Abstract
↑ Rosenthal, Jenny E.; Murphy, G. M. (1936). "Group Theory and the Vibrations of Polyatomic Molecules". Rev. Mod. Phys. 8: 317–346. Bibcode:1936RvMP....8..317R. doi:10.1103/RevModPhys.8.317.
↑ Longuet-Higgins, H.C. (1963). "The symmetry groups of non-rigid molecules". Molecular Physics. 6 (5): 445–460. doi:10.1080/00268976300100501.
↑ Fundamentals of Molecular Symmetry by Philip R. Bunker and Per Jensen (Institute of Physics Publishing 2005) ISBN 0-7503-0941-5
↑ Altmann S.L. (1977) Induced Representations in Crystals and Molecules, Academic Press
1 2 Flurry, R.L. (1980) Symmetry Groups, Prentice-Hall, ISBN 0-13-880013-8, pp.115-127

External links

Point group symmetry @ Newcastle University Link
Molecular symmetry @ Imperial College London Link
Molecular Point Group Symmetry Tables
Symmetry @ Otterbein
An internet lecture course on molecular symmetry @ Bergische Universitaet
Character tables for point groups for chemistry Link

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