Instructor: Dr. Upali Siriwardane - PowerPoint Presentation

Instructor: Dr. Upali Siriwardanee-mail: upali@latech.eduOffice: CTH 311 Phone 257-4941Office Hours: M,W 8:00-9:00 & 11:00-12:00 am; Tu,Th, F 9:30 - 11:30 a.m. April 7 , 2016: Test 1 (Chapters 1, 2, 3, 4) April 28, 2016: Test 2 (Chapters  (6 & 7) May 17, 2016: Test 3 (Chapters. 19 & 20) May 18, Make Up: Comprehensive covering all Chapters Chemistry 481(01) Spring 2016

Chapter 6. Molecular symmetry An introduction to symmetry analysis 6.1 Symmetry operations, elements and point groups 1796.2 Character tables 183Applications of symmetry 6.3 Polar molecules 1866.4 Chiral molecules 1876.5 Molecular vibrations 188The symmetries of molecular orbitals 6.6 Symmetry-adapted linear combinations 1916.7 The construction of molecular orbitals 1926.8 The vibrational analogy 194Representations 6.9 The reduction of a representation 1946.10 Projection operators 196

SymmetryM.C. Escher

Symmetry Butterflies

Fish and Boats Symmetry

Symmetry elements and operations A symmetry operation is the process of doing something to a shape or an object so that the result is indistinguishable from the initial stateIdentity (E)Proper rotation axis of order n (Cn)Plane of symmetry (s)Improper axis (rotation + reflection) of order n (Sn), an inversion center is S2

2) What is a symmetry operation?

E - the identity elementThe symmetry operation corresponds to doing nothing to the molecule. The E element is possessed by all molecules, regardless of their shape.C1 is the most common element leading to E, but other combination of symmetry operation are also possible for E.

Cn - a proper rotation axis of order nThe symmetry operation Cn corresponds to rotation about an axis by (360/n)o.H2O possesses a C2 since rotation by 360/2o = 180o about an axis bisecting the two bonds sends the molecule into anindistinguishable form:

s - a plane of reflectionThe symmetry operation corresponds to reflection in a plane. H2O possesses two reflection planes. Labels: sh, sd and sv.

i - an inversion centerThe symmetry operation corresponds to inversion through the center. The coordinates (x,y,z) of every atom are changed into (-x,-y,-z):

Sn - an improper axis of order nThe symmetry operation is rotation by (360/n)o and then a reflection in a plane perpendicular to the rotation axis. operation is the same as an inversion is S2 = ia reflection so S1 = s.

2) What are four basic symmetry elements and operations?

3) Draw and identify the symmetry elements in:a) NH3:b) H2O:c) CO2:d) CH4:e) BF3:

Point GroupAssignmentThere is a systematic way of naming most point groups C, S or D for the principal symmetry axisA number for the order of the principal axis (subscript) n.A subscript h, d, or v for symmetry planes

4) Draw, identify symmetry elements and assign the point group of following molecules:a) NH2Cl:b) SF4:c) PCl5:d) SF6:e) Chloroformf) 1,3,5-trichlorobenzene

Special Point GroupsLinear molecules have a C∞ axis - there are an infinite number of rotations that will leave a linear molecule unchangedIf there is also a plane of symmetry perpendicular to the C∞ axis, the point group is D∞h If there is no plane of symmetry, the point group is C∞vTetrahedral molecules have a point group TdOctahedral molecules have a point group OhIcosahedral molecules have a point group Ih

Point groupsIt is convenient to classify molecules with the same set of symmetry elements by a label. This label summarizes the symmetry properties. Molecules with the same label belong to the same point group. For example, all square molecules belong to the D4h point group irrespective of their chemical formula.

5) Determine the point group to which each of the following belongs:a) CCl4b) Benzenec) Pyridined) Fe(CO)5e) Staggered and eclipsed ferrocene, (η5-C5H5)2Fef) Octahedral W(CO)6g) fac- and mer-Ru(H2O)3Cl3

Character tables Summarize a considerable amount of information and contain almost all the data that is needed to begin chemical applications of molecule. C2v   E   C2 sv   sv' 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

Character Table Td

Information on Character TableThe order of the group is the total number of symmetry elements and is given the symbol h. For C2v h = 4. First Column has labels for the irreducible representations. A1 A2 B1 B2The rows of numbers are the characters (1,-1)of the irreducible representations.px, py and pz orbitals are given by the symbols x, y and z respectivelydz2, dx2-y2, dxy, dxz and dyz orbitals are given by the symbols z2, x2-y2, xy, xz and yz respectively.

H2O molecule belongs to C2v point group

Symmetry ClassesThe symmetry classes for each point group and are labeled in the character tableLabel Symmetry Class A Singly-degenerate class, symmetric with respect to the principal axis B Singly-degenerate class, asymmetric with respect to the principal axis E Doubly-degenerate class T Triply-degenerate class

Molecular Polarity and Chirality Polarity:Only molecules belonging to the point groups Cn, Cnv and Cs are polar. The dipole moment lies along the symmetry axis for molecules belonging to the point groups Cn and Cnv. Any of D groups, T, O and I groups will not be polar

ChiralityOnly molecules lacking a Sn axis can be chiral.This includes mirror planesand a center of inversion as S2=s , S1=i and Dn groups.Not Chiral: Dnh, Dnd,Td and Oh.

Meso-Tartaric Acid

Optical Activity

Symmetry allowed combinationsFind symmetry species spanned by a set of orbitalsNext find combinations of the atomic orbitals on central atom which have these symmetries. Combining these are known as symmetry adapted linear combinations (or SALCs). The characters show their behavior of the combination under each of the symmetry operations. several methods for finding the combinations.

Example: Valence MOs of WaterH2O has C2v symmetry.The symmetry operators of the C2v group all commute with each other (each is in its own class).Molecualr orbitals should have symmetry operators E, C2, v1, and v2.

Building a MO diagram for H2Oxz y

a1 orbital of H2O

b1 orbital of H2O

b1 orbital of H2O, cont.

b2 orbital of H2O

b2 orbital of H2O, cont.

[Fe(CN)6]4-

Reducing the RepresentationUse reduction formula

MO forML6 diagram Molecules

Group Theory and Vibrational SpectroscopyWhen a molecule vibrates, the symmetry of the molecule is either preserved (symmetric vibrations) or broken (asymmetric vibrations).The manner in which the vibrations preserve or break symmetry can be matched to one of the symmetry classes of the point group of the molecule.Linear molecules: 3N - 5 vibrationsNon-linear molecules: 3N - 6 vibrations (N is the number of atoms)

Reducible Representations(3N) for H2O: Normal Coordinate MethodIf we carry out the symmetry operations of C2v on this set, we will obtain a transformation matrix for each operation.E.g. C2 effects the following transformations:x1 -> -x2, y1 -> -y2, z1 -> z2 , x2 -> -x1, y2 -> -y1, z2 -> z1, x3 -> -x3 , y3 -> -y3, z3 -> z3.

Summary of Operations by a set of four 9 x 9 transformation matrices.

Use Reduction Formula

Example H2O, C2v

Use Reduction Formula:to show that here we have:G3N = 3A1 + A2 + 2B1 + 3B2 This was obtained using 3N cartesian coordinate vectors . Using 3N (translation + rotation + vibration) vectors would have given the same answer. But we are only interested in the 3N-6 vibrations . The irreducible representations for the rotation and translation vectors are listed in the character tables, e.g. for C 2 v,

GT = A1 + B1 + B2GR = A2 + B1 + B2i.e. GT+R = A1 + A2 + 2B1 + 2B2But Gvib = G 3N - G T+R Therefore G vib = 2A 1 + B 2 i.e. of the 3 (= 3N-6) vibrations for a molecule like H 2 O, two have A 1 and one has B 2 symmetry

INTERNAL COORDINATE METHODWe used one example of this earlier - when we used the "bond vectors" to obtain a representation corresponding to bond stretches.We will give more examples of these, and also the other main type of vibration - bending modes.For stretches we use as internal coordinates changes in bond length, for bends we use changes in bond angle.

Deduce G3N for our triatomic molecule, H2O in three lines: E C2 sxz syzunshifted atoms 3 1 1 3c/unshifted atom s 3 -1 1 3 \ G 3N 9 -1 1 3 For more complicated molecules this shortened method is essential!! Having obtained G 3N , we now must reduce it to find which irreducible representations are present.

Example H2O, C2v

Use Reduction Formula:to show that here we have:G3N = 3A1 + A2 + 2B1 + 3B2 This was obtained using 3N cartesian coordinate vectors . Using 3N (translation + rotation + vibration) vectors would have given the same answer. But we are only interested in the 3N-6 vibrations . The irreducible representations for the rotation and translation vectors are listed in the character tables, e.g. for C 2 v,

GT = A1 + B1 + B2GR = A2 + B1 + B2i.e. GT+R = A1 + A2 + 2B1 + 2B2But Gvib = G 3N - G T+R Therefore G vib = 2A 1 + B 2 i.e. of the 3 (= 3N-6) vibrations for a molecule like H 2 O, two have A 1 and one has B 2 symmetry

Further examples of the determination of Gvib, via G3N:NH3 (C3v) C3v E 2C3 3sv 12 0 2 \ G 3N 12 0 2 Reduction formula ® G 3N = 3A 1 + A 2 + 4E G T+R (from character table) = A 1 + A 2 + 2E, \ G vib = 2A 1 + 2E (each E "mode" is in fact two vibrations (doubly degenerate)

CH4 (Td)Td E 8C3 3C2 6S4 6sd 15 0 -1 -1 3 \ G 3N 15 0 -1 -1 3 Reduction formula ® G 3N = A 1 + E + T 1 + 3T 2 G T+R (from character table) = T 1 + T 2 , \ G vib = A 1 + E + 2T 2 (each E "mode" is in fact two vibrations (doubly degenerate), and each T 2 three vibrations (triply degenerate)

XeF4 (D4h)D4h E 2C4 C2 2C2' 2C2" i 2S4 sh 2sv 2s d 15 1 -1 -3 -1 -1 -1 5 3 1 \ G 3N 15 1 -1 -3 -1 -1 -1 5 3 1 Reduction formula ® G 3N = A 1g + A 2g + B 1g + B 2g + E g + 2A 2u + B 2u + 3E u G T+R (from character table) = A 2g + E g + A 2u + E u , \ G vib = A 1g + B 1g + B 2g + A 2u + B 2u + 2E u For any molecule, we can always deduce the overall symmetry of all the vibrational modes, from the G 3N representation. To be more specific we need now to use the INTERNAL COORDINATE method .

INTERNAL COORDINATE METHODWe used one example of this earlier - when we used the "bond vectors" to obtain a representation corresponding to bond stretches.We will give more examples of these, and also the other main type of vibration - bending modes.For stretches we use as internal coordinates changes in bond length, for bends we use changes in bond angle.

Let us return to the C2v molecule:Use as bases for stretches:Dr1, Dr2.Use as basis for bend:Dq C 2v E C 2 s xz s yz G stretch 2 0 0 2 G bend 1 1 1 1 N.B. Transformation matrices for G stretch : E, s yz : C 2 , s xz : i.e. only count UNSHIFTED VECTORS ( each of these ® +1 to c ) .

Gbend is clearly irreducible, i.e. A1.Gstretch reduces to A1 + B2We can therefore see that the three vibrational modes of H2O divide into two stretches (A1 + B2) and one bend (A1). We will see later how this information helps in the vibrational assignment.

Other examples:NH3Bases for stretches: Dr1, Dr2, Dr3. Bases for bends: Dq 1 , Dq 2 , Dq 3 . C 3v E 2C 3 3 s G stretch 3 0 1 G bend 3 0 1 Reduction formula ® G stretch = A 1 + E G bend = A 1 + E (Remember G vib (above) = 2A 1 + 2E)

CH46 angles q1,.....q6, where q1 lies between r1 and r2 etc.Bases for stretches: D r 1 , D r 2 , D r 3 , D r 4 . Bases for bends: Dq 1 , Dq 2 , Dq 3 , Dq 4 , Dq 5 , Dq 6 . T d E 8C 3 3C 2 6S 4 6 s d G stretch 4 1 0 0 2 G bend 6 0 2 0 2 Reduction formula ® G stretch = A 1 + T 2 G bend = A 1 + E + T 2 But G 3N (above) = A 1 + E + 2T 2

IR AbsorptionsInfra-red absorption spectra arise when a molecular vibration causes a change in the dipole moment of the molecule. If the molecule has no permanent dipole moment, the vibrational motion must create one; if there is a permanent dipole moment, the vibrational motion must change it.Raman AbsorptionsDeals with polarizability

Raman SpectroscopyNamed after discoverer, Indian physicist C.V.Raman (1927). It is a light scattering process. Irradiate sample with visible light - nearly all is transmitted; of the rest, most scattered at unchanged energy (frequency) (Rayleigh scattering), but a little is scattered at changed frequency (Raman scattering). The light has induced vibrational transitions in molecules (ground ® excited state) - hence some energy taken from light, scattered at lower energy, i.e. at lower wavenumber. Raman scattering is weak - therefore need very powerful light source - always use lasers (monochromatic, plane polarised, very intense).Each Raman band has wavenumber:where n = Raman scattered wavenumber n0 = wavenumber of incident radiation nvib = a vibrational wavenumber of the molecule (in general several of these)

Molecular VibrationsAt room temperature almost all molecules are in their lowest vibrational energy levels with quantum number n = 0. For each normal mode, the most probable vibrational transition is from this level to the next highest level (n = 0 -> 1). The strong IR or Raman bands resulting from these transitiions are called fundamental bands. Other transitions to higher excited states (n = 0 -> 2, for instance) result in overtone bands. Overtone bands are much weaker than fundamental bands.

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 x2 or yz) then the fundamental transition for this normal mode will be Raman active.