Question 1 - answer each question on a separate page

The metal carbonyl complex Re(CO)5Br belongs to the C4v point group.

O

C

O C          CO

Re

C                      C O

Br

(i)          Deduce the reducible representation spanned by the basis of 3N Cartesian               (7) displacement coordinates for Re(CO)5Br, where N is the number of atoms in               the molecule, and hence find the sum of irreducible representations that define            the symmetries of the 3N degrees of freedom.  Explain your reasoning.

(ii)         Use your answer to part (a)(i) to deduce the symmetries of the vibrational               (3)

modes of Re(CO)5Br.  Explain your reasoning and confirm that your answer is consistent with the expected number of vibrational modes for this molecule.

(i)          Find the reducible representation spanned by the basis of CO stretching                  (1) coordinates.

(ii)         Reduction of the reducible representation found in (b)(i) reveals that the                 (3)

symmetries of the five CO stretching modes are 2a1+b1+e.  Predict the          infrared and Raman activities of the CO stretching modes of Re(CO)5Br and hence predict the number of fundamental bands you would expect to see in  each of the infrared and Raman spectra.  Explain your reasoning.

(iii)        Choose one of the four in-plane CO displacement vectors as a reference and           (3)

use the projection operator method to identify the forms of three of the CO stretching mode symmetry coordinates.

(iv)        Find the form of the second a1 mode symmetry coordinate by using the CO            (1)

vector on the C4 axis as reference.

(v)         Use your answers to (b)(iii), (iv) to predict which of the two a1 CO stretching         (2) vibrations would be expected to show the stronger IR activity.  Explain your               reasoning.

Question 2 - answer each question on a separate page

A 1H NMR spectrum of a solution of NaBH4, acquired in a magnetic field of B0 = 1.02 T, is shown in Figure 1.  Each peak is labelled with the associated chemical shift value in ppm.  Selected NMR         parameters for three nuclei are presented in Table 1.

Figure 1

Table 1

(a)         Using the information in Table 1 assign the two sets of peaks in Figure 1, the intense              (6)

quartet and the weaker septet, to the two ions: 11BH4 and 10BH4 .  Give three reasons to justify the assignment.

(b)         Showing your workings, calculate the 11B-1H and 10B-1H J coupling constants, in Hz.             (3)

Using the information in Table 1, briefly explain the relative magnitudes of these coupling constants.

Question 3 - answer each question on a separate page

(a)       (i)          The photoelectron spectrum of the H2 molecule (peaks 0-9) is shown in Figure 1.

The vibrational features seen in the spectrum are separated by 2322 cm一 1, while the stretching frequency of H2 is 3115 cm一 1 .

Figure 1

Deduce whether the intermolecular bond is longer in H2 or H2+ .  Explain your reasoning.

(ii)         The v = 2 vibrational feature is the most intense feature in the photoelectron

spectrum of H2.   Plot Morse curves for H2 and H2+, and use them to show how the Frank Condon principle explains this observation.

(i)          A molecular orbital (MO) energy-level diagram for HBr is shown in Figure 2.

(4)

Figure 2

For each MO shown on Figure 2, state the nature of the bonding character and             (2)

which atomic orbitals contribute to it.

Question 3 continued

(ii)         The photoelectron spectrum of HBr is shown in Figure 3.

Note: the spectrum was recorded using a HeI lamp (21.22 eV).

Figure 3

Assign each of the two main photoelectron bands, labelled Band 1 and Band 2,            (4)

with reference to your MO diagram from part (b)(i).  Explain the presence of any vibrational structure in terms of MO bonding character and geometry changes     accompanying ionization.

(iii)        Calculate the approximate kinetic energies of the photoelectrons associated with          (2)

the features labelled Band 1 and Band 2 in Figure 3.  Identify any key assumptions you make in performing your calculation.

(iv)        Explain how the photoelectron spectrum of HBr shown in Figure 3 would be               (2)

expected to change if a NeI lamp was used instead of the HeI lamp.

(v)         Explain the origin of the splitting of the features evident in Band 2, and comment        (2) on the fact that this splitting is not observed for Band 1.

(vi)        A similar Band 2 feature is observed in the photoelectron spectrum of the related         (2)

molecule, HI.  Comment on whether the splitting of the Band 2 features is larger or smaller for HI compared to HBr.

Question 4 - answer each question on a separate page

The UV-visible absorption spectrum (solid line) of trans-trans- 1,4-diphenylbutadiene 1 in hexane solution at room temperature is shown in the figure below, along with the emission spectrum        (dashed line), which has been attributed to fluorescence from the S1 excited state.

(i)       Explain why the absorption spectrum of 1 shows a series of fine-structure peaks, sketching a potential energy diagram to illustrate your answer and identifying the transition that gives the strongest fine-structure peak.

(ii)      Compound 1 belongs to the C2h point group, and the S0 ground state has Ag

symmetry.  The strong absorption band in Figure 1 has been assigned to the         transition to the S1 excited state, which has Bu symmetry.  The transition to the S2 excited state, which has Ag symmetry, does not result in an observable absorption band even though the transition energy corresponds to a wavelength of ≈ 300 nm.

Explain why the transition to the S1 state gives a strong absorption band whereas that to the S2 state does not.

(b)       The S1 excited state of 1 decays by a combination of radiative (fluorescence) and

non-radiative routes, with rate constants kr and knr, respectively:

(i)       Define the fluorescence quantum yield, Φf, and observed lifetime, τ, of the S1                  (3)

state of 1 in terms of kr and knr.  Hence, deduce values of kr and knr for 1 in hexane, given that the measured values are Φf = 0.35 and τ = 465 ps in this solvent.

(ii)      The absorption and fluorescence emission spectra of 1 do not change on going

from hexane to hexadecane solvent, which has a higher viscosity, but the     fluorescence quantum yield and lifetime change to Φf = 0.59 and τ = 780 ps, respectively, in hexadecane.

Propose an explanation for these changes, drawing on your knowledge of alkene       (4)

photochemistry.

(c)       Compound 2 is a monobrominated derivative of 1.

Giving your reasons, suggest:                                                                                                    (5)

    how the UV-visible absorption spectrum of 2 in hexane might differ from

that of 1 in hexane;

    how the decay of the S1 state of 2 in hexane might differ from that of 1 in

hexane, commenting on how the fluorescence quantum yield may change as a result.

Question 5 - answer each question on a separate page

The structure of compound 1 and the 6H values assigned to each aromatic proton are   shown below.  Giving your reasons and using appropriate diagrams, explain why each of the 6H values is lower than that in benzene (6H 7.30).

(3)

The structure of compound 2 is shown below and the associated resonances in the 500 MHz 1H NMR spectrum recorded in CDCl3 are given in the Table.

Table

Question 5 continued

(i)          Giving your reasons, assign the resonances due to the alkene protons in 2 and account for their multiplicity and Jvalues.

(ii)         Giving your reasons, assign the resonance due to the proton on C- 1 in 2 and

account for its multiplicity and Jvalues.

(iii)        Giving your reasons, assign the resonance due to the proton on C-2 in 2 and

account for its 6H value, multiplicity and Jvalues.

(iv)        Giving your reasons, predict the theoretical appearance (multiplicity and J

value(s)) of the protons on C-3 in 2.

(v)         The 13C NMR spectrum of 2 at 6C <100 had the following signals:

6C 96.6, 81.0, 65. 1, 61.2, 53.8, 53.3, 45.5, 36.5, 14.2.

In the DEPT- 135 spectrum, all of these signals except that at 6C 96.6 were present.

The DEPT-90 spectrum had the following signals: 6C 81.0, 65. 1, 53.8, 53.3.

Giving your reasons, assign as far as possible the resonances in the 13C NMR            (4)

spectrum at 6C <100 to carbons in 2.

Question 6 - answer each question on a separate page

HCo(CO)4 acts as a hydroformylation catalyst.  Draw out the mechanism for the hydroformylation of propene by CO and H2, indicating the aldehyde products    that form.

HCo(CO)2(P2), where P2 represents a strongly ligating bis phosphine, reacts with 1-octene under a 1:1 H2 to CO atmosphere.  Identify the three types of reaction   product, in addition to aldehydes, that might form in this reaction.

Rationalise the following specific observations for the HCo(CO)2(P2) reaction: