Electronic Spectroscopy of Polyatomics

Transcription

1 Electronic Spectroscopy of Polyatomics We shall discuss the electronic spectroscopy of the following types of polyatomic molecules: 1. general AH 2 molecules, A = first-row element 2. formaldehyde 3. benzene & aromatic complexes (Hückel theory) 4. transition metal complexes In considering the electronic spectra of polyatomic molecules, symmetry arguments are very important, and each MO will belong to a symmetry species associated with the point group of the molecule. It is possible for polyatomic molecules that have linear ground states to change to bent conformations in the excited states, thereby changing the symmetry species associated with each of the MOs (e.g., H-C/C-H is linear in the 1 E g + ground state but trans bent in the first excited state) The total electron density and resulting surface of the total molecule is relatively easy to visualize, but identifying and rationalizing contributions from individual electrons to certain localized properties within a molecule is the real challenge: notably, promotion of electrons from one orbital to another (transitions) or removal of electrons (ionization) must both obey symmetry selection rules.

2 AH 2 Molecules: phah = 180 o The AH 2 molecules have two possible extreme geometries: linear (D 4h ) and bent at 90 o (C 2v ) phah = 180 o : Symmetry labels from the D 4h point group are assigned to all of the AOs (internuclear axis along z): + 2s AO on the A atom is spherically symmetric, F g + 2p z AO on the A atom is along z, F u 2p x, 2p y AOs on the A atom are degenerate, B u 1s AO on H cannot by itself be assigned a symmetry species, but rather is treated as a set with its partner by considering inphase and out-of-phase contributions: F g F u In-phase 1s+1s H AOs Out-of-phase 1s-1s H AOs Rules for MO formation are the same as for diatomics: (1) AOs must be of the same symmetry to form an MO (2) The MOs that make significant contributions to bonding and anti-bonding, have significantly different character than the AOs from which they are made, and are composed of AOs of comparable energies. The 1s+1s H AOs combine with the 2s on A only, since they both have F g + symmetry (no F g-, MOs so omit +). The MO is labelled 2F g, (convention is to number MOs of the same symmetry in order of increasing energy). If 2s AO on A is out-of-phase with the 1s+1s AO, the anti-bonding 3F g MO is formed (nodal plane between A and H).

3 AH 2 Molecules: phah = 90 o The 2s, 2p x, 2p y, 2p z on the A atom are assigned to the a 1, b 1, b 2 and a 1 symmetry species, respectively (C 2v point group, z- axis along the C 2 rotation axis). The 1s H AOs are again broken down into in-phase and outof-phase contributions: + + a b 2 In-phase 1s+1s H AOs Out-of-phase 1s-1s H AOs The 1s+1s a 1 AO combines with 2s or 2p z AOs on A to give the 2a 1, 3a 1 and 4a 1 MOs (see Walsh diagram). The 2p y AOs on A can combine only with the 1s-1s AOs to make the 1b 2 and 2b 2 MO, but 2p x cannot combine with AOs on the H atoms, and becomes the 1b 1 lone pair MO. Constructing the MOs: Using the aufbau principle, e- can be fed pairwise into the MOs to construct the ground or excited state configurations for the molecule (2 e-for F, 4 e-for B, 6 e- for *, etc.). The label X is for the ground state; A, B, C,... are used for the excited states with the same multiplicity as X; and a, b, c,... are used for excited state with different multiplicity. Sometimes the tilde (~) is used above the label to differentiate it from symmetry species labels.

4 Walsh Diagram The Walsh diagram shows the correlation of the MOs as the HAH angle changes from 90 o to 180 o (z axis in the linear molecule becomes the y axis in the bent molecule) phah 180 o : Note that the non-bonding 1F g MO is not shown, very much like the 1s AO of the A atom phah 90 o : Note that the non-bonding 1a 1 MO is not shown, very much like the 1s AO of the A atom + The 1s-1s AOs combines with the 2p z AO on A (both F u symmetry) to form the 1F u and 2F u MOs, which are bonding and anti-bonding, respectively. The 2p x and 2p y AOs on A cannot combine with either set of H 1s AOs for symmetry reasons - thus they remain as doubly degenerate AOs on A, labelled as the 1B u MOs. MOs are arranged in order of increasing energy, based on the principle that those with decreased s character or increased number of nodes will be higher in energy. (e.g., 2F g and 1F u MOs are bonding btw. A and H, but the nodal plane through A makes 1F u higher in energy).

5 Configurations and Geometries Some ground and excited state configurations are below: Molecule Configuration State phah LiH 2 (1σ g ) 2 (2σ g ) 2 (1σ u ) 1 X Σ % u 180 o (?) (1a 1 ) 2 (2a 1 ) 2 (3a 1 ) 1 A A 1 < 180 o (?) BeH 2 (1σ g ) 2 (2σ g ) 2 (1σ u ) 2 X Σ % g 180 o (?) (1a 1 ) 2 (2a 1 ) 2 (1b 2 ) 1 (3a 1 ) 1 ã 3 B 2, A 1 B 2 < 180 o (?), < 180 o (?) BH 2 (1a 1 ) 2 (2a 1 ) 2 (1b 2 ) 2 (3a 1 ) 1 X 2 A 1 (1σ g ) 2 (2σ g ) 2 (1σ u ) 2 (1π u ) 1 A 2 Π u 131 o 180 o CH 2 (1a 1 ) 2 (2a 1 ) 2 (1b 2 ) 2 (3a 1 ) 2 ã 1 A o 1 (1a 1 ) 2 (2a 1 ) 2 (1b 2 ) 2 (3a 1 ) 2 (1b 1 ) 1 X 3 B 1, b 1 B o, 140 o NH 2 (H 2 O + ) (1a 1 ) 2 (2a 1 ) 2 (1b 2 ) 2 (3a 1 ) 2 (1b 1 ) 1 X B o (110.5 o ) (1a 1 ) 2 (2a 1 ) 2 (1b 2 ) 2 (3a 1 ) 1 (1b 1 ) 2 A A o (180.0 o ) H 2 O (1a 1 ) 2 (2a 1 ) 2 (1b 2 ) 2 (3a 1 ) 2 (1b 1 ) 2 X 1 A o (all bond angles are determined from electronic spectra, except LiH 2 and BeH 2, which are unknown species) 1a 1 or 1F g AO is non-bonding, favouring neither the bent nor linear shapes. Occupation of 2F g or 1F u favours linearity, since energies are lowest for 180 o angle (Walsh). Thus, LiH 2 and BeH 2 should have linear ground states. Promotion of an electron to the next highest MO (i.e., the 3a 1-1B u ) has a drastic effect, since this MO favours the bent geometry energetically. Thus, from molecules like BH 2 and CH 2, which have been experimentally proven to have bent geometries, one e- in the 3a 1-1B u MO counterbalances the four e-in the 2a 1-2F g and 1b 2-1B u MOs, predicting that BeH 2 and LiH 2 should be bent in the excited state.

6 Configurations and Geometries, 2 BH 2 has a X 2 A 1 ground state, where the angle is known to be 131 o (due to the single e- in the 3a 1 MO). However, if this e- is promoted to the 1b 1-1B u MO produces a linear molecule (no particular geometry favoured). CH 2 has two electrons in the 3a 1 MO, favoured by a very small angle of o. Promotion of an e- from 3a 1 to 1b 1 results in both singlet and triplet states, where the molecule is still bent but with a larger angle. The triplet state of CH 2, X 3 B 1, lies lower in energy (by kj mol -1 ) than the singlet state, a 1 A 1. So the former is the ground state, and the latter a low-lying excited state. NH 2 has similar geometry changes to CH 2, the only difference in configuration being an extra electron in the 1b 1 MO, which does not seem to favour any particular geometry. The H 2 O + ion (isoelectronic with NH 2 ), is also equite similar. H 2 O has a ground configuration with 2 e- in the 3a 1 orbital, strongly favouring a bent molecule. Excited states of H 2 O have an e- promoted from the 1b 1 MO to a large (size of the molecule), high-energy MO called a Rydberg orbital - this orbital has little influence on the geometry of the molecule, so H 2 O in the Rydberg states has essentially the same geometry as the ground state. Summary: Thus, the Walsh MO diagram predicts (and agrees with theory & experiment) that AH 2 molecules with 4 or less valence e- are have linear ground states, while those with 5 or more will have bent ground states.

7 BAB Molecules The symmetric non-hydride molecules BAB are common molecular species which can also be rationalized with a Walsh diagram, and a Walsh diagram can be constructed using the same principles asfor AH 2. Valence s and p electrons complicate matters. Molecules should be linear with 16 or less valence electrons, and bent with 17 or more electrons. Linear examples: C 3 (12 VE); CO 2 (16 VE) Bent examples: NO 2 (17 or 18 VE); O 3 (18 VE) Exceptions: SiC 2 (isovalent with C3; T-shaped) - the reason for this is that Walsh s rules are for covalently bound molecules, not ionic molecules (i.e., Si + C2 - ).

8 Formaldehyde Formaldehyde has 16 e-. 12 e- are involved in the following MO s: 3 F bonding, 3 F* anti-bonding, 1 B bonding, 1 B* antibonding and 2 lone pair (oxygen). The MO diagram for formaldehyde can be constructed by assigning symmetry species to all of the AOs used to construct the MOs: C 2v E C 2 σ(xz) σ(yz) 2H (1s) a 1 + b 1 C (2s) a 1 C (2p x ) b 1 C (2p y ) b 2 C (2p z ) a 1 O (2s) a 1 O (2p x ) b 1 O (2p y ) b 2 O (2p z ) a 1 The O(2s) AO is mixed with the C-O and C-H F bonding orbitals, which all have a 1 symmetry, so these three MOs are a mixture of F-bonding and lone pair character (see diagram next page). The O(2p) lone pair (2b 2 ) falls between the 1b 1 and 2b 1 (B and B*) MOs, and the electrons in these orbitals are the main concern of electronic spectroscopy. The 2b 2 n MO is the highest occupied molecular orbital (HOMO), and the 2b 1 B* MO is the lowest unoccupied molecular orbital (LUMO). Spacings absorb in the UV/visible region!

9 Formaldehyde, 2 The complete MO diagram is shown below: The 4 valence electrons can occupy the higher energy MOs: H H C + _ O 1b 1 (B) bonding H H C + O _ 2b 2 (n) non-bonding H H + C _ O + 2b 1 (B*) anti-bonding y z Energy increases as B < n < B*. ground configuration:...(1b 1 ) 2 (2b 2 ) 2, state: X 1 A 1 Promotion of a electron from the non-bonding (n) 2b 2 MO to the anti-bonding (B*) 2b 1 MO gives an excited configuration:...(1b 1 ) 2 (2b 2 ) 1 (2b 1 ) 1 which has the states: a 3 A 2 and A 1 A 2.

10 Formaldehyde, 3 6 outer valence e- are involved in major electronic transitions: Ground: (5a 1 ) 2 (1b 1 ) 2 (2b 2 ) 2 (X 1 A 1 state) First excited state: (5a 1 ) 2 (1b 1 ) 2 (2b 2 ) 1 (2b 1 ) 1 (gives rise to a 3 A 2 and A 1 A 2 states) HOMO-LUMO transition involves transfer of in-plane, nonbonding O2p y (2b 2 ) e- to the anti-bonding C-O B* (2b 1 ) MO. This B*7n transition occurs for =C=O, =C=S, -N=O, -NO 2 and -O-N=O chromophores. The A 1 A 2 -X 1 A 1 transition is electric-dipole forbidden, but is one of the most famous electronic spectra ( Å). It shows up because of vibronic coupling with the L 4 out-ofplane bend (b 1 ) (since A 2 qb 1 = B 2, the A 1 A 2 -X 1 A 1 borrows some intensity from the B 1 B 2 -X 1 A 1 transition near 1750 Å.

11 Formaldehyde, 4 Not only is formaldehyde a prototype for electronic spectroscopy for heteronuclear chromophores, it was also the first electronic transition studied with fine rotational detail describing an asymmetric top. 1 The A 1 A 2 -X 1 A 1 transition can be observed by laser excitation, where a tunable laser excites the A state, and then total fluorescence is monitored. Much of the fine structure is unexplained by the simple electronic picture. Long progressions are attributed to the L 2 CO stretching mode, but also, non-planarity of the excited state. 1. Clouthier & Ramsay, Ann. Rev. Phys. Chem. 34, 31, (1983) 2. Miller & Lee, Chem. Phys. Lett. 33, 104, (1975)

12 Formaldehyde, 5 The a and A states of formaldehyde and other similar molecules are known as nb* states, and the a - X and A - X transitions such as that described above are known more commonly as B*-n or n-to-b* transitions. The B*-n transitions are easily distinguished from the more common B*-B transitions (which occur in many aromatic systems), since the former are blue shifted in a hydrogen bonding solvent. This is due to interaction of 1s AO of OH groups (e.g., ethanol) with the n orbital, which increases the energy of the B*-n transition. The e- in the 2b 1 B* MO energetically favour a pyramidal shape for the CH 2 O molecule, because the B* MO can overlap with 1s + 1s AO s on the H atoms and the 2s AO on the C atom, resulting in an increase in s character in this MO and a lowering of energy relative to the planar molecule (two equivalent schema shown below): _ + H H C O The bend of the C-H bond away from a the usual plane of the CH 2 O molecule is 38 o in the A 1 A 2 state and 43 o in the a 3 A 2 state. Note the MO s must be reclassified in terms of the C s point group for these excited states.

13 Formaldehyde, 6 A Walsh-type diagram can be constructed for formaldehyde, which correlates the MOs of the planar C 2v ground state and the C s first excited state:

14 Electronic Spectra of Aromatics In molecules with electrons in B-orbitals (notably conjugated organic aromatic systems), the ground and excited states of the molecule, can be described by an approximate LCAO method known as the Hückel method. The secular determinant used for diatomics (Lecture 9a) can be expanded for general use with polyatomics: /0 H 11 & E H 12 & ES H 1n & ES 1n /0 H 12 & ES 12 H 22 & E... H 2n & ES 2n!!! H 1n & ES 1n H 2n & ES 2n... H nn & E ' 0 where H nn are the Coulomb integrals, H mn (m n) are the resonance integrals, S mn (m n) are the overlap integrals, and E is the orbital energy. This may be abbreviated as: H mn & ES mn ' 0 Use of the Hückel method requires that a number of approximations be made, and that only the B electrons are considered.

15 Hückel Method 1. Only B electrons are considered, e- in F MOs are neglected. The B and F MOs in highly symmetric molecules do not have the proper symmetry for overlap, so in this case this is not even an approximation. In less symmetric molecules, the energies of the F MOs are much less than the B MOs (and F* MOs greater than the B* MOs), so that F MOs can still be neglected. 2. For m n: S mn ' 0 3. When m = n, H nn is the same for all atoms (set to "): H nn ' " 4. When m n, H mn is the same for any pair of directly bonded atoms (set to $): H mn ' $ 5. When m and n are not directly bonded H mn ' 0 The B-electron wavefunctions are given by R ' j c i P i i where P i correspond to only the 2p AOs on C, N and O atoms which are involved in the B MOs.

16 Benzene The Hückel treatment of benzenes 6 2p z AOs give a secular determinant: x x where: 0 1 x ' 0 " & E x 1 0 $ /0 /0 ' x x x Solving the determinant (or by methods of solving simultaneous equations), the solutions are obtained: x ' ±1, ±1 or ±2 E ' " ± $, " ± $, or" ±2$ The E = " ± $ solution appears twice, meaning it is a doubly degenerate MO energy level, and the resonance integral $ is negative, as usual. The 6 MOs are: E " - 2$ " - $ " " + $ The aufbau principle can be used to fill up the MOs in the usual way, the doubly degenerate MOs are the HOMO and LUMO in this case. " + 2$

17 Benzene, 2 The symmetry species of the D 6h point group can be assigned to the MOs (only parts above the rings are shown, parts below are identical, but of opposite parity) The energy of the MOs increases with the increasing number of nodal planes perpendicular to the ring. MOs are antisymmetric w.r.t. reflection through the ring plane, just like the p AOs from which they are constructed. The ground configuration is:...(1a 2u ) 2 (1e 1g ) 4, and is a totally symmetric singlet state: X 1 A 1g If an e- is promoted from e 1g MO to a e 2u MO, then the first excited configuration is...(1a 2u ) 2 (1e 1g ) 3 (1e 2u ) 1, Since a single vacancy in 1e 1g can be treated like an electron, this configuration has the same states as the configuration arising from...(1a 2u ) 2 (1e 1g ) 1 (1e 2u ) 1. Thus, the symmetry species of the orbital part of the electronic wavefunction is obtained from '(R o e ) ' e e ' B 1g 2u 1u % B 2u % E 1u In the partially occupied MOs, e- may be parallel (S = 1) or antiparallel (S = 0), meaning there are six possible states: 1,3 B 1u, 1,3 B 2u, 1,3 E 1u.

18 Transition Metal Complexes Transition metal complexes have partially filled 3d, 4d and 5d AOs (here we consider the first transition series, 3d) Transition metals readily form complexes of all sorts, e.g., [Fe(CN) 6 ] 4-, Ni(CO) 4 and [CuCl 4 ] 2-. Atoms or collections of atoms that bond to the transition metal centre are referred to as ligands. Ligands are normally arranged in a highly symmetrical manner: NC NC CN Fe CN CN CN 4- OC CO Ni CO CO Cl Cl Cu Cl Cl 2- octahedral tetrahedral square planar we will focus on the octahedral case. Two specialized forms of MO theory: Crystal Field Theory: In a transition metal complex, the higher energy occupied MOs can be regarded as the perturbed d AOs. If this perturbation is weak, the ligands can be treated as point charges located octahedrally on the axes of the Cartesian coordinate system (i.e., like Na + surrounded by its 6 nearest neighbour Cl - ions). Ligand Field Theory: When ligands interact more strongly with the d AOs, the MOs of the ligands must be taken into account.

19 Crystal Field Theory In the presence of six octahedrally arranged point charges on the Cartesian axes of the 5 d AOs, the d AOs are perturbed an assigned according to the O h point group. Symmetry species for a variety of point groups are below: d-orbitals Point Group d z 2 d x 2 & y 2 d xy d yz d xz O h e g t 2g T d e t 2 D 3h a ) 1 en eo D 4h a 1g b 1g b 2g e g D 4h + σ g δ g π g C 2v a 1 a 1 a 2 b 2 b 1 C 3v a 1 a 1 a 2 e C 4v a 1 b 1 b 2 e D 2d a 1 b 1 b 2 e D 4h a 1 e 2 e 3

20 Crystal Field Theory, 2 The set of 5 degenerate d AOs break into doubly degenerate e g and triply degeneate t 2g orbitals in a regular octahedral crystal field. The d(z 2 ) and d(x 2 -y 2 ) AOs have much of their electron density along the metal-ligand (M-L) bonds, and e- therein experience much more repulsion by the ligand e- than do the d(xz), d(yz) and d(xy) orbitals. Thus, the e g orbitals are pushed up in energy by (3/5)) 0 and the t 2g orbitals are pushed down in energy by (2/5)) 0, where ) 0 is the e g - t 2g splitting or crystal field splitting. e g (3/5)) 0 d E (2/5)) 0 t 2g The value of ) 0 normally corresponds to absorption in the visible (and sometimes UV) portion of the spectrum, accounting for the beautiful colourations of transition metal complexes () 0 is also known as 10Dq, or ) T for tetrahedral complexes, ) for misc. symmetry, etc.).

21 Crystal Field Theory, 3 For d 1, d 2, d 3, d 8, d 9 and d 10 configurations, e- prefer to have parallel spins for minimum energy, and pair as the levels fill according to the Pauli-exclusion principle. e.g., [Cr(H 2 O) 6 ] 3+, Cr 3+ has a d 3 configuration (lost one d and two s electrons from a 4s 2 3d 4 configuration). Each electron goes into a separate t 2g orbitals with parallel spin to minimize energy, giving a quartet ground state (X 4 A 2g ). For d 4, d 5, d 6 and d 7 configurations, the way the electrons are fed into the orbitals depends upon the size of the splitting ) 0 : (a) If small, e- prefer to go to the e g orbitals with parallel spins. (b) If large, e- prefer to go to the t 2g orbitals with antiparallel spins. (a) (b)

22 Ligand Field Theory When ligands interact more strongly with the d AOs, the MOs of the ligands must be taken into account. Ligand MOs are roughly classified as two types: F MOs, cylindrically symmetrical about the M-L bond B MOs, not cylindrically symmetrical about the M-L bond The former is normally stronger, and may be provided by a lone pair (one of the most studied cases is the lone pair orbital on the CO ligand in metal carbonyl complexes). The F MOs can be classified in different point groups with different ligand arrangements. For example, in octahedral ML 6, it can be shown that the 6 ligand F MOs are split into a 1g, e g and t 1u orbitals. The effect of these ligand MOs is to interact with d orbitals of the same symmetry. The crystal field orbital increases in energy, and the ligand orbital energy decreases. The net effect is an increase in ) 0, which leads to low spin complexes rather than high spin complexes.

23 Electronic Transitions A d 6 T.M. complex might have an MO diagram like this: The F bonding orbitals are filled by the 12 valence e-, and remaining 6 valence e- fill t 2g and e g orbitals accordingly. All of this gives rise to very complex manifolds of states which will not be derived here - however there is a simpliflication that can be used in interpreting electronic spectra of octahedral t.m. complexes: All higher energy orbitals which may be occupied are t 2g or e g : since g means symmetric w.r.t. inversion through the centre of the molecule, all states must also be g (so all excited and ground states from t 2g and e g occupancy are g states: however - all g-g transitions are forbidden, as in homonuclear diatomic molecules. How do such complexes absorb radiation???

24 Electronic & Vibronic Selection Rules The answer is that interaction may occur between the motion of electrons and vibrational motions, so that some of the vibronic transitions are allowed. Recall that for diatomics a large number of good quantum numbers exist such that selection rules can be expressed solely through these numbers. However, in non-linear polyatomic molecules, the goodness of quantum numbers deteriorates because of the complicated number of motions that are present, such that there is only one number remaining: the total electron spin quantum number, S Thus, the selection rule )S = 0 still applies, unless there is an atom with high nuclear charge in the molecule (i.e., tripletsinglet transitions are weak in benzene, but much more intense in iodobenzene). For the orbital part of the electronic wavefunction, the selection rules for transitions between two electronic states depend completely on the symmetry of the states. Electronic transitions involve the interaction between the molecule and the electric component of the electromagnetic radiation - so selection rules are the same as for IR vibrational transitions in polyatomic molecules. So the electronic transition intensity is proportional to *R e * 2, where: R e ' R ) ( )) m e µr e dj e

25 Selection Rules, 2 For an allowed transition, *R e * 0, and the symmetry requirement for this is: '(R ) e ) '(µ) '(R)) e ) ' A or for transitions between non-degenerate states: '(R ) e ) '(µ) '(R)) e ) e A The components of R e along the Cartesian axes are: R e,x ' R ) ( m e µ x R )) e dj e R e,y ' R ) ( m e µ y R )) e dj e R e,z ' R ) ( m e µ z R )) e dj e and since R e ' (R e,x ) 2 % (R e,y ) 2 % (R e,z ) 2 electronic transitions are allowed if any of the above terms are non-zero. So, for a transition to be allowed: '(R ) e ) '(T x ) '(R)) e ) ' A '(R ) e ) '(T y ) '(R)) e ) ' A '(R ) e ) '(T z ) '(R)) e ) ' A If the product of two symmetry species is totally symmetric, those symmetry species must be the same, thus: '(R ) e ) '(R)) e ) ' '(T x ) and/or '(T y ) and/or '(T z ) is the general selection rule for a transition between two electronic states (remember, replace = with e if degenerate states are involved).

26 Selection Rules, 3 If the lower state of the transition is the ground state of a molecule with all of its MO s filled (no unpaired e-: a closedshell molecule), then the ground electronic state is totally symmetric and the selection rule simplifies to: '(R ) e ) ' '(T x ) and/or '(T y ) and/or '(T z ) If vibrations are excited in either the upper or lower electronic state (or both), the vibronic transition moment is: R ev ' m R ) ( ev µ R)) ev dj ev Which has the same selection rules as for electronic trans.: '(R ) ev ) '(R)) ev ) ' '(T ) and/or '(T ) and/or '(T ) x y z and since: '(R ev ) ' '(R e ) '(R v ) then: '(R ) v ) '(R)) v ) '(R) e ) '(R)) e ) ' '(T x ) and/or '(T y ) and/or '(T z ) It so happens that very often that the same vibration is excited in both states, '(R v N) = '(R v O), and the selection rule is the same as the electronic selection rule. If no vibrations are excited in the upper or lower state, then '(R v N) or '(R v O) will be totally symmetric.

27 Vibrational & Rotational Structure Vibrational coarse structure is seen in the electronic bands of some molecules. For example, the A 1 B 2u - X 1 A 1g absorption spectrum of benzene has this coarse structure below: Rotational fine structure can be observed, and in principle is similar to that observed in IR vibrational spectra - though greater changes in rotational constants are seen between electronic states. Below are spectra of type B (left: top experimental, bottom calculated) and type A and C (right, both calculated) showing rotational fine structure in the A 1 B 2u -X 1 A g system of 1,4-difluorobenzene.

28 Summary of Chromophores The absorption of a photon which causes an electronic transition can often be traced to specific electrons in the molecule. For example, if there is a C=O group in the molecule, an absorption around 290 nm will be observed (though the precise location will depend on the nature of the molecule). Functional groups that have characteristic optical absroptions are called chromophores, their presence often accounts for the colouration of substances. All of the examples in this summary are different types of chromophores. Group ῡ/cm &1 λ max /nm ε max /(L mol -1 cm -1 ) C=O (π*-π) 61, ,000 57, ,500 C=O (π*-n) 37,000-35, H 2 O (π*-n) 60, ,000 (1) d-d transitions These transitions involve the promotion of electrons between non-degenerate d orbitals on transition metal atoms (e.g., [Ti(OH 2 ) 6 ] 3+, e g - t 2g transition near 20,000 cm -1 (500 nm), and the wavenumber of the absorption maximum suggests ) 0 20,000 cm -1 )

29 Summary of Chromophores, 2 (2) vibronic transitions Major problem with interpretation of visible spectra of octahedral complexes is that d-d transitions are forbidden. The Laporte selection rule says that the only allowed transitions are those accompanied by a change in parity (u : g and g : u only allowed). The d-d transition is parity forbidden b/c it corresponds to a g-g transition -but vibration of the molecule can destroy the inversion symmetry of the molecule, and then the g, u classification no longer applies. Removal of the centre of symmetry gives rise to a vibronically allowed transition (though it is weakly allowed). (3) charge-transfer transitions Absorption of radiation may occur as the result of transfer of an electron from a ligand MO into the d orbitals of the metal atom, or vice versa. Such transitions are associated with the e- moving over large distances, meaning the transition moment dipoles are very large and absorptions very intense. For example, in MnO 4 - ions, the intense violet colour results from an electron transfer from an MO largely confined on O to an MO confined on Mn - this is a ligand-metal charge transfer (LMCT) transition. The reverse, a metal-ligand charge transfer (MLCT) transition, is also possible (e.g., e- from d orbitals into B* MOs of aromatic ligands is a very common example).

30 Summary of Chromophores, 3 (4) B*-B and B*-n transitions B*-B transitions Absorption by a C=C double bond excites the B e- into an anti-bonding B* orbital, and the chromophore activity is therefore due to a B*-B transition. For unconjugated double bonds, absorption occurs around 180 nm (UV), or 7 ev for the transition to occur. When the C=C bond is in a conjugated chain, the energies of the MOs are closer together, and the B*-B transition absorbs at longer wavelengths (can even move to visible region for really long conjugated chains). B*-n transitions The transition responsible for absorption in carbonyl compounds involves the lone pairs on the O atom in the C=O bond. The concept of the lone pair means that there is an MO in which the e- are largely confined to the O atom. One of these electrons in the n MO may be excited into an empty B* MO, giving rise to a B*-n transition, with typical absorption energies around 4 ev (290 nm). These are symmetry forbidden transitions, and as a result, are very weak.

31 Key Concepts 1. It is possible for polyatomic molecules (notably, AH 2 type molecules) that have linear ground states to change to bent conformations in the excited states, thereby changing the symmetry species associated with each of the MO s. 2. AH 2 molecules are examined in details, with the Walsh diagram applied to monitor the relationship between MOs in linear and bent molecules in the ground and excited states. 3. Formaldehyde is examined in detail, as being on of the basic C=O chromophores. The B*-n transition is responsible for absorption of visible radiation. 4. Hückel theory is developed to treat molecules containing C=C chromophores, where the B*-B transition is important for strong absorption. 5. Transition metal complexes can be treated with crystal field or ligand theory, depending on the interactions between the ligands and transition metals. Most of these complexes have d-d transitions, and the metal centre are chromophores absorbing in the visible region of the EM spectrum. 6. Electronic symmetry selection rules are defined, with the only quantum number selection rule being )S = 0. g - g and u - u transitions, which are symmetry forbidden, may weakly occur as the result of accompanying vibronic transitions