Dr. Said M. El-Kurdi1 Chapter 4 An introduction to molecular symmetry

Dr. Said M. El-Kurdi2 Understanding of symmetry is essential in discussions of molecular spectroscopy and calculations of molecular properties. 4.1 Introduction consider the structures of BF 3, and BF 2 H, both of which are planar BF bond distances are all identical (131 pm) trigonal planar the BH bond is shorter (119 pm) than the BF bonds (131 pm). pseudo-trigonal planar

Dr. Said M. El-Kurdi3 the molecular symmetry properties are not the same In this chapter, we introduce the fundamental language of group theory (symmetry operator, symmetry element, point group and character table). Group theory is the mathematical treatment of symmetry.

Dr. Said M. El-Kurdi4 4.2 Symmetry operations and symmetry elements A symmetry operation is an operation performed on an object which leaves it in a configuration that is indistinguishable from, and superimposable on, the original configuration. The rotations are performed about an axis perpendicular to the plane of the paper and passing through the boron atom; the axis is an example of a symmetry element.

Dr. Said M. El-Kurdi5 A symmetry operation is carried out with respect to points, lines or planes, the latter being the symmetry elements. Rotation about an n-fold axis of symmetry The symmetry operation of rotation about an n-fold axis (the symmetry element) is denoted by the symbol C n, in which the angle of rotation is: n is an integer, e.g. 2, 3 or 4.

Dr. Said M. El-Kurdi6 Applying this notation to the BF 3 molecule BF 3 molecule contains a C 3 rotation axis

Dr. Said M. El-Kurdi7 If a molecule possesses more than one type of n-axis, the axis of highest value of n is called the principal axis; it is the axis of highest molecular symmetry. For example, in BF 3, the C 3 axis is the principal axis.

Dr. Said M. El-Kurdi8 Reflection through a plane of symmetry (mirror plane) If reflection of all parts of a molecule through a plane produces an indistinguishable configuration, the plane is a plane of symmetry

Dr. Said M. El-Kurdi9 the symmetry operation is one of reflection and the symmetry element is the mirror plane (denoted by  )  If the plane lies perpendicular to the vertical principal axis, it is denoted by the symbol  h.  If the plane contains the principal axis, it is labeled  v.

Dr. Said M. El-Kurdi10 Consider the H 2 O molecule A special type of  plane which contains the principal rotation axis, but which bisects the angle between two adjacent 2-fold axes, is labeled  d

Dr. Said M. El-Kurdi11 A square planar molecule: XeF 4 One C 2 axis coincides with the principal (C 4 ) axis; the molecule lies in a  h plane which contains two C 2 ’ and two C 2 ’’ axes.

Dr. Said M. El-Kurdi12 Each of the two  v planes contains the C 4 axis and one C 2 ’ axis.

Dr. Said M. El-Kurdi13 Each of the two  d planes contains the C 4 axis and one C 2 ’’ axis. In the notation for planes of symmetry, , the subscripts h, v and d stand for horizontal, vertical and dihedral respectively.

Dr. Said M. El-Kurdi14 Reflection through a centre of symmetry (inversion centre) If reflection of all parts of a molecule through the centre of the molecule produces an indistinguishable configuration, the centre is a centre of symmetry, also called a centre of inversion; it is designated by the symbol i. CO 2 SF 6 benzene

Dr. Said M. El-Kurdi15 H2SH2S cis-N 2 F 2 SiH 4 Rotation about an axis, followed by reflection through a plane perpendicular to this axis If rotation through about an axis, followed by reflection through a plane perpendicular to that axis, yields an indistinguishable configuration, the axis is an n-fold rotation–reflection axis, also called an n-fold improper rotation axis. It is denoted by the symbol S n. No i

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Dr. Said M. El-Kurdi17 Identity operator All objects can be operated upon by the identity operator E. The operator E leaves the molecule unchanged.

Dr. Said M. El-Kurdi Successive operations For NH 3

Dr. Said M. El-Kurdi19 For example, in planar BCl 3, the S 3 improper axis of rotation corresponds to rotation about the C 3 axis followed by reflection through the  h plane.

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Dr. Said M. El-Kurdi21 Summary Table of Symmetry Elements and Operations

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Dr. Said M. El-Kurdi Point groups The number and nature of the symmetry elements of a given molecule are conveniently denoted by its point group C 1 point group C 1 = E

Dr. Said M. El-Kurdi25 C  v point group  -fold axis of rotation; that possessed by a linear molecule It must also possess an infinite number of  v planes but no  h plane or inversion centre. These criteria are met by  Asymmetrical diatomics such as HF, CO and [CN  ], and  linear polyatomics that do not possess a centre of symmetry, e.g. OCS and HCN.

Dr. Said M. El-Kurdi26 D  h point group  Symmetrical diatomics (e.g. H 2, [O 2 ] 2  ) and  linear polyatomics that contain a centre of symmetry (e.g. [N 3 ],CO 2, HCCH) possess a  h plane in addition to a C  axis and an infinite number of  v planes

Dr. Said M. El-Kurdi27 T d, O h or I h point groups Molecular species that belong to the T d, O h or I h point groups possess many symmetry elements

Dr. Said M. El-Kurdi28 Determining the point group of a molecule or molecular ion

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Dr. Said M. El-Kurdi30 Determine the point group of trans-N 2 F 2. Apply the strategy shown in Figure 4.10:

Dr. Said M. El-Kurdi31 Determine the point group of PF 5.

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