Section A

Answer Question 1

Question 1 - answer each question on a separate page

(a)         The 19F NMR spectra of 3,4,5-trifluoropyridine shown below were acquired using two

NMR spectrometers with 19F Larmor frequencies of (a) 470.516 MHz and (b) 94.183 MHz. All 1H-19F couplings were removed using 1H decoupling.  Peak frequencies are given in Table 1.

Note: the gyromagnetic ratio for 19F is γ/2π = 40.078 MHz T一 1 .

Figure 1

Table 1

Larmor                                                peak frequency / ppm

frequency                                                                                                                       

/ MHz                  1                      2                      3                      4                      5

(a)         470.516          一152.34          一152.38          一154.42          一 154.46          一154.50

(b)          94.183           一152.26          一152.45          一154.27          一 154.47          一154.66

(i)          Calculate the magnetic field strengths, B0, of the two NMR spectrometers used      (2) to obtain the 19F NMR spectra in Figure 1.

(ii)         Identify and briefly explain two differences between the 19F NMR spectra of          (4)

3,4,5-trifluoropyridine in Figure 1 due to the difference in magnetic field.

(iii)        Using the principles of active and passive spins, explain the splitting pattern           (2)

observed for F4 in Figure 1a.

(iv)        Explain how the relative strength of the J coupling interaction between F4 and        (4)

F3/F5 influences the splitting patterns observed in Figure 1a and Figure 1b.     Use the data in Table 1 to provide a quantitative justification for your answer.

(v)         Explain the effect of the removal of 19F-1H couplings via 1H-decoupling in             (2) Figure 1 on the chemical and magnetic equivalence of F3 and F5 .

(b)         Identify the type of relaxation, transverse (T2) or longitudinal (T1), that is responsible           (6)

for the following experimental observations.  Give an account of the role of relaxation in each case.

●    Broad, poorly resolved peaks are observed in a 1H NMR spectrum of a sample containing an air bubble.  Once the air bubble is removed, a high resolution 1H NMR spectrum with narrow peaks is obtained.

●    Two 13C NMR spectra, A and B, are acquired of the same sample.  Both 13C NMR spectra are recorded using signal averaging of n = 128 scans and using the same parameters except A was acquired using 30o RF pulses and B was  acquired using 90° RF pulses.  The peaks in A are found to have a higher      signal-to-noise ratio (SNR) than in B.

 Section B

Answer THREE questions from this section

Question 2 - answer each question on a separate page

A solution of dye 1 is being studied. Figure 1 shows its longest wavelength UV-visible absorption band (solid line), along with the emission band (dashed line) observed on excitation at 495 nm.

The absorption band has been assigned to a transition from the S0 ground state to the S1 excited state of 1; it has a maximum absorption coefficient of εmax = 1.0 × 105 dm3 mol−1 cm−1 and an   oscillator strength off ≈ 1.  The emission has a quantum yield of Φem = 0.93 and it has been      assigned to fluorescence from the S1 state, which has an observed lifetime of τobs = 4.16 ns.

Figure 1

(a)         Explain why the absorption band in Figure 1 has been assigned to a transition between           (4)

the S0 and S1 states of 1, illustrating your answer with simple MO occupancy diagrams, and briefly discussing the extent to which the spin and orbital selection rules are            obeyed for this transition in 1.

(b)         Explain why excitation of 1 at 465 or 440 nm, corresponding to shoulders within the              (3)

absorption band, result in the same emission profile as that observed on excitation at 495 nm.

Figure 2 shows the longest wavelength UV-visible absorption band of dye 2 in solution      (εmax = 1.1 × 105 dm3 mol−1 cm−1), along with the emission spectrum obtained on excitation at 525 nm (Φem = 0.95; τobs = 4.08 ns).

Figure 2

(c)

Giving your reasons, assign the absorption and emission bands of 2 in Figure 2, and               (3)

explain why they occur at slightly longer wavelengths than those of 1.

(d)         The following observations were made when 2 was added to the solution of 1:

    the observed lifetime of the S1 state of 1 decreased from τobs = 4.16 ns in the

absence of 2 to τobs = 0.60 ns at [2] = 4.00 × 10−3 mol dm−3 ;

    on excitation at 495 nm, the very strong emission peak at 510 nm observed

from 1 in the absence of 2 (Figure 1) became weak when 2 was added at     4.00 × 10−3 mol dm−3, and a very strong emission peak at 550 nm appeared.

(i)          Assuming that the following relations apply, deduce the first-order rate             constant k for the decay of the S1 state of 1 in the absence of 2, and the second- order rate constant kq for quenching of the S1 state of 1 by 2.

τobs   =  1 / kobs                    and            kobs   =  k + kq [2]

(ii)         The observations have been attributed to long-range Coulombic energy

transfer.

Explain what is meant by long-range Coulombic energy transfer and why the  observations indicate that it occurs in this case.  Include a simple state diagram of 1 and 2 to illustrate your answer, and comment on whether your calculated  value of kq in part (d)(i) is consistent with this mechanism.

Figure 3 shows the longest wavelength UV-visible absorption band of dye 3 in solution          (εmax = 1.1 × 105 dm3 mol−1 cm−1), along with the emission spectrum obtained on excitation at 575 nm (Φem = 0.90; τobs = 4.50 ns).

Figure 3

(e)          Giving your reasons, suggest what might be observed in an emission spectrum excited           (3)

at 495 nm from the solution of 1:

    when 3 is added at [3] = 4.00 × 10−3 mol dm−3;

    when both 2 and 3 are added, each at 4.00 × 10−3 mol dm−3 .

Question 3 - answer each question on a separate page

(a)

The partial structure of natural product 1 is shown below and the associated resonances in the 500 MHz 1H NMR spectrum recorded in CD3OD are given in Table 1.

Table 1

(i)          Giving your reasons, assign the resonances due to each of the three methyl               (3) groups in 1 and account for their multiplicity.

(ii)         Giving your reasons, assign the resonance due to proton HA in 1 and account            (3)

for its 6H value, multiplicity and Jvalues.  Explain why its multiplicity is simpler than might have been expected.

(iii)        Giving your reasons, assign the resonance due to proton HB in 1 and account            (3)

for the multiplicity and Jvalues.

(iv)        Giving your reasons, assign the resonances due to two protons HC and HD in 1          (3)

and account for their multiplicity and Jvalues.  Outline a NMR experiment     that could allow assignment of each of these protons to a particular resonance.

(v)         Giving brief reasons, sketch the appearance of the DEPT- 135 13C NMR                    (4) spectrum of this partial structure of 1.

(b)         One of the 1 H integration signals in the 400 MHz 1H NMR spectrum of acid 2 recorded

in CD3OD is shown in Figure 1.

Note: It is not possible to assign the signal in Figure 1 unequivocally to a proton in 2.

Figure 1

State the multiplicity of the signal in Figure l and, showing your working, calculate the          (4)

J value(s) in Hz to the nearest 0.5 Hz.  Giving your reasons, suggest a possible assignment for the signal in Figure 1, explaining why it cannot be assigned      unequivocally.

Question 4 - answer each question on a separate page

The heterogeneously catalysed reaction of acetophenone, 1, at a Ni surface in the presence of an alcohol yields phenylethanol, 2.  The yields of 2 for this reaction in the presence of two different alcohols and for a 1:1 mix of them are given below.

(a)         Write a balanced equation for the conversion of 1 into 2 in the presence of propan-2-ol

and comment on the role of propan-2-ol in the catalysis.

(b)         Draw a plausible mechanism to illustrate the steps that take place at the nickel surface

during the conversion of 1 into 2 in the presence of propan-2-ol.

(c)         Briefly discuss reasons for the absence of product when the reaction is carried out with propan- 1-ol.

(d)         When propan- 1-ol is replaced with a mixture of propan- 1-ol and propan-2-ol, 30% conversion is seen.  However, if the same catalyst is isolated and reused with         propan-2-ol alone, 99% conversion results.  Explain this behaviour.

(e)         A sample of this Ni catalyst was found to adsorb 50 cm3 of Kr at a pressure of 15 mbar,

and 62.5 cm3 of Kr at a pressure of 20 mbar.  Assuming that the Langmuir adsorption isotherm applies, calculate the monolayer capacity of this sample of the catalyst.

(f)          The same sample of the Ni catalyst was found to adsorb 43 cm3 of methanol at a

pressure of 10 mbar.  Deduce whether the Langmuir adsorption isotherm is followed for the adsorption of methanol.

Question 5 - answer each question on a separate page

(a)       (i)

Write down the Hückel determinants for the allyl radical shown below, using the atom numbering shown below and using x = (c)/ where  is the Coulomb     integral,  is the resonance integral and c is the orbital energy.

(ii)      Expand the determinant you obtained in part (a)(i) to obtain a cubic equation and

hence solve for x.

(iii)     Use the roots you obtained in part (a)(ii) to find the resulting three Hückel molecular

orbital energies in terms of α and β . Hence sketch molecular orbital diagrams for    both the radical and cation allyl, indicating the bonding character and occupancy of each orbital.

(iv)      Using your answers from part (a)(iii) comment on the site selectivity of:                           (4)

.    the allyl radical to a radical-radical reaction;

.    the allyl cation to nucleophilic attack.

(b)                   The HeI photoelectron spectrum of trans-but-2-ene (1) is shown below:

10         12         14         16         18

Ionisation energy (eV)