Symmetry and group theory

Transcription

1 Symmetry and group theory or How to Describe the Shape of a Molecule with two or three letters Natural symmetry in plants Symmetry in animals 1

2 Symmetry in the human body The platonic solids Symmetry in modern art M. C. Escher 2

3 Symmetry in arab architecture La Alhambra, Granada (Spain) Symmetry in baroque art Gianlorenzo Bernini Saint Peter s Church Rome Symmetry in words, music 3

4 Re 2 (CO) 10 C24 C60 Symmetry in chemistry Molecular structures Wave functions Description of orbitals and bonds Reaction pathways Optical activity Spectral interpretation (electronic, IR, NMR)... 4

5 Molecular structures A molecule is said to have symmetry if some parts of it may be interchanged by others without altering the identity or the orientation of the molecule Symmetry Operation: Movement of an object into an equivalent or indistinguishable orientation Symmetry Elements: A point, line or plane about which a symmetry operation is carried out 5 types of symmetry operations/elements Identity: this operation does nothing, symbol: E Element is entire object 5

6 Proper Rotation: Rotation about an axis by an angle of 2π/n C 2 C3 H 2 O NH 3 How about: NO2? The Operation: Proper rotation C n is the movement (2π/n) The Element: Proper rotation axis C n is the line 180 (2π/2) Applying C 2 twice Returns molecule to original oreintation C 22 = E C 2 Proper rotation axes C 2 180º C 3, 120º H 2 O NH 3 How about: NO2? 6

7 Rotation angle 60º 120º 180º 240º 300º 360º Symmetry operation C 6 C 3 (= C 62 ) C 2 (= C 63 ) C 32 (= C 64 ) C 6 5 E (= C 66 ) Proper Rotation: Rotation about an axis by an angle of 2π/n PtCl 4 C 2, C 4 m C n Rotation 2πm/n C 2 C C n n n+1 n = E = C n C 2 2π/2 = C 2 2π/4 = C 4 C nn = E The highest order rotation axis is the principal axis and it is chosen as the z axis 7

8 Reflection and reflection planes (mirrors) σ σ σ (reflection through a mirror plane) σ NH 3 Only one σ? H 2 O σ 8

9 H 2 O σ B If the plane contains the principal axis it is called σ v If the plane is perpendicular to the principal axis it is called σ h B σ n = E (n = even) σ n = σ (n = odd) Inversion: i Center of inversion or center of symmetry (x,y,z) (-x,-y,-z) i n = E (n is even) i n = i (n is odd) 9

10 Inversion not the same as C 2 rotation!! igures with center of inversion igures without center of inversion 10

11 Improper rotation (and improper rotation axis): S n rotation about an axis by an angle 2π/n followed by reflexion through perpendicular plane S 42 = C 2 Also, S 44 = E; S 2 = i; S 1 = σ Symmetry operations and elements Operation proper rotation improper rotation reflection inversion Identity Element axis (C n ) axis (S n ) plane (s) center (i) Molecule (E) 11

12 Symmetry point groups The set of all possible symmetry operations on a molecule is called the point group (there are 28 point groups) The mathematical treatment of the properties of groups is Group Theory In chemistry, group theory allows the assignment of structures, the definition of orbitals, analysis of vibrations,... See: Chemical applications of group theory by. A. Cotton Point Groups Special Shapes Linear D h O C C 2 O C C v H-Cl C Icosahedral Group, I h rare The platonic solid with 20 triangular faces e.g. B 12 H common symmetry for viruses (human rhinovirus, polio virus, T-3 viruses) Two of the most common C 4 C 3 Octahedral Group, O h common e.g. most MX 6 species Lots of symmetry Tetrahedral, T d Most non-transitional AX 4 species. 12

13 But for Intermediate Symmetries (low Chart) To determine the point group of a molecule Low Symmetry (no C n ) C 1 : only E P Cl Br C s : E and σ only CHICl P O Cl H D Cl H 3 N NH 3 Rh H 3 N O O O O 13

14 The C 4 vs. C 3 paradox? [Cr(NH 3 ) 6 ] 3+ H Cr N H H vs. Cr N [e(oh 2 ) 6 ] 2+, Si(CH 3 ) 4, Pb(C 2 H 5 ) 4 H H C C H H B Cl Cl Pt Cl Cl H H H B B H H H P C 3h NH 3 Cl Cl Pt Cl Cl H 3 N D 4h Ru Cr D 5h D 6h D 5d OC OC Cu Cu OC CO CO CO e Cr D 6d D 3h H2O: C2v H2O2: C2 B(OH)3: C3h NH3: C3v S5Cl: C4v HCN: C v 14

15 Here s two examples Chirality Dipole Moments Raman Spectroscopy Chirality No Mirror Planes or Mirror Centers 15

16 Symmetry and Dipole Moments Permanent Dipoles: Never have an inversion center (i) or a S n Molecules which belong to: Ci, Sn, Dn, Cnh, Dnh, Dnd, Td, Oh, Ih, Dinfh NEVER Have permanet dipoles. Only those molecules which belong to C1, Cn, Cs, Cn, Cnv can have a permanent dipole moment. Character Table Point group Symmetry operations Mülliken symbols Characters +1 symmetric behavior -1 antisymmetric Each row is an irreducible representation Naming of Irreducible representations One dimensional (non degenerate) representations are designated A or B. Two-dimensional (doubly degenerate) are designated E. Three-dimensional (triply degenerate) are designated T. Any 1-D representation symmetric with respect to C n is designated A; antisymmétric ones are designated B Subscripts 1 or 2 (applied to A or B refer) to symmetric and antisymmetric representations with respect to C 2 C n or (if no C 2 ) to σ v respectively Superscripts and indicate symmetric and antisymmetric operations with respect to σ h, respectively In groups having a center of inversion, subscripts g (gerade) and u (ungerade) indicate symmetric and antisymmetric representations with respect to i 16

17 Character Tables Irreducible representations are the generalized analogues of σ or π symmetry in diatomic molecules. Characters in rows designated A, B,..., and in columns other than E indicate the behavior of an orbital or group of orbitals under the corresponding operations (+1 = orbital does not change; -1 = orbital changes sign; anything else = more complex change) Characters in the column of operation E indicate the degeneracy of orbitals Symmetry classes are represented by CAPITAL LETTERS (A, B, E, T,...) whereas orbitals are represented in lowercase (a, b, e, t,...) The identity of orbitals which a row represents is found at the extreme right of the row Pairs in brackets refer to groups of degenerate orbitals and, in those cases, the characters refer to the properties of the set Let s use character tables! Symmetry and molecular vibrations # of atoms N (linear) degrees of freedom 3 x 2 Translational modes 3 Rotational modes 2 Vibrational modes 3N-5 = 1 Example 3 (HCN) N (nonlinear) 3N 3 3 3N-6 Example 3 (H 2 O) Symmetry and molecular vibrations A molecular vibration is IR active only if it results in a change in the dipole moment of the molecule A molecular vibration is Raman active only if it results in a change in the polarizability of the molecule In group theory terms: A vibrational motion is IR active if it corresponds to an irreducible representation withthesamesymmetryas anx, y, z coordinate (or function) and it is Raman active if the symmetry is the same as x 2, y 2, z 2, or one of the rotational functions R x, R y, R z 17

18 How many vibrational modes belong to each irreducible representation? You need the molecular geometry (point group) and the character table Symmetry of molecular movements of water Vibrational modes If the symmetry label of a normal mode corresponds to x, y, or z, then the fundamental transition for this normal mode will be IR active. If the symmetry label of a normal mode corresponds to products of x, y, or z (such as x 2 or yz) then the fundamental transition for this normal mode will be Raman active. Water has three normal modes: two of A 1 symmetry and one of B 2 symmetry. All of these are IR and Raman active. We would expect water to have three peaks corresponding to fundamental vibrations in the IR spectrum. There also would be three peaks in its Raman spectrum at the same frequencies as in the IR. 18

19 Which of these vibrations having A 1 and B 1 symmetry are IR or Raman active? Raman active IR active What about the trans isomer? O L C M 2 1 C L O D 2h A g E 1 C 2 (z) C 2 (y) C 2 (x) i σ v (xy) σ v (xz) σ v' (yz) B 3u x Γ Only one IR active band and no Raman active bands Remember cis isomer had two IR active bands and one Raman active 19