examination question paper - uit20172408100622/kje-8303.pdf · 3c. 3d “triple-resonance”...

EXAMINATION QUESTION PAPER

Exam in: KJE-8303 Nuclear Magnetic Resonance

Date: 30.05.2017

Time: 4 hours

Place:

Approved aids:

Ruler, pen

Type of sheets (sqares/lines):

Any

Number of pages incl. cover page:

16

Contact person during the exam:

Johan Isaksson

Phone:

41354726

NB! It is not allowed to submit rough paper along with the answer sheets. If you

do submit rough paper it will not be evaluated.

UiT / Postboks 6050 Langnes, N-9037 Tromsø / 77 64 40 00 / [email protected] / uit.no 2

Question 1. Theory (30 p)

E B0

α

β

ω

ω

-½

+½

µ = γ I h/2π

ω0 = -γ Bo v0 = -γ Bo /2π

ω0 = larmor frequency (rad-1)γ = gyromagnetic ratioµ = magnetic moment

γΗ = +2.67 × 108 rad s−1 T−1

µ

µ

1a. Above is a schematic representation of what happens when a spin ½ nuclei is put in a strong magnetic field (B0). Briefly describe equilibrium, and what characterizes the energy levels. Define polarization. (5 p)

1b. Briefly explain the vector model (vector representation) and use it to describe the magnetization in a sample after a 1H 90 degree rf pulse. (5 p)

B0

z

x

y

M

Equilibrium

z

x

yM

Excited

ω1=γB1

B1

ω0=γB0

B1

z

xy

ωz

x

ymany nuclei

Mo - net magnetization vector allows us tolook at system as a whole

z

xy

ωz

x

y

Mo - net magnetization vector allows us tolook at system as a whole

many nuclei


1.01.52.02.53.03.54.04.55.05.56.06.57.07.58.0f1 (ppm)

-100

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

12

3

45

6

7

OO OO HH

OO HH

OO HH

OOHH

12

3

45

6

7

OO OO HH

OO HH

OO HH

OOHH

7’ 7’’

4

3

HO5

HO4HO6

DMSO

H2O

56

600003900 Hz 600001200 Hz

1c. What is chemical shift of protons? Briefly explain why different protons in a molecule

have different chemical shifts and three factors that greatly affect the observed chemical shift. (6 p)

1d. Draw in the figure above, what is upfield and downfield, which part of the spectra is

shielded, which is deshielded and which part resonates at higher larmor frequency and which resonates at lower larmor frequency. (6 p)

Preparation

Evolution

Detection

Mixing

2.02.53.03.54.04.55.05.56.06.57.0f2 (ppm)

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

f1 (p

pm)

2.02.53.03.54.04.55.05.56.06.57.0f2 (ppm)

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

f1 (p

pm)

f1 f2

d1 t1 t290x 90x

1e. Explain briefly how the 2D dimensions in the spectra above is constructed using the

functional elements to the right. Explain very briefly the origin of crosspeaks (in a general form) (8 p)


Question 2. 1D NMR (30 p)

2a. i. Assign the peaks based on their multiplicity and their chemical shifts. (5p) 2a. ii. The doublets at 7.1-7.2 PPM are leaning towards each other. Briefly explain the phenomena. (5p)

2b. Above you can see two phosphorous spectra of triphenylphosphate, with and without proton decoupling. Explain what i) proton decoupling is and ii) why is the peak so much sharper and iii) why the peak has (probably) increased in intensity in the proton decoupled spectra? (7 p)


2c. Zooming in on the phosphorous peak in the decoupled spectra you can see several satellites (magenta, cyan and yellow boxes). Can you assign which couplings they arise from? (3p) 2d. i. In the 1D carbon spectra to the right there are two peaks: a) an aromatic CH and b) an aromatic tertiary carbon. Denote the peaks in the figure. (1p)

ii. Explain why it is like this. (2p)

iii. What could you do to make a carbon spectra quantitative? (1p)


2e. Analyze the coupling patterns in the following multiplets and report them using the Hz scale below the multiplets (6p)

475480485490495500505510515520525f1 (Hz)

480490500510520530f1 (Hz)

485490495500505510515520f1 (Hz)


Question 3 – KJE-3303. 2D/3D NMR (20 p)

E B0

S

S

I

I ∆

∆

N-∆/2

N+∆/2

N-∆/2N+∆/2 W0

W2

W1

W1

W2

W0

3a. Use the figure above to very briefly explain why Protein size is important for the quality of the NMR data you can acquire. You can also state other reasons. (3p) 3b. Name at least two techniques/tricks used to improve the spectral quality. (2p) 3c. 3D “triple-resonance” spectra are the foundation of protein NMR. Very briefly explain the principle of triple-resonance experiments and how they are used to assign the protein backbone. (5 p)

N

H3C

H3C CH3

O1

2 3


3d. This is a ROESY of N,N-dimethylacetamide. i) What are the peaks marked by arrows (same sign as the diagonal peaks), ii) how do they arise and iii) what does it tell us about the behavior of the molecule? (5 p)

0.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.01H

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

f1 (

ppm

)

2

23

2037 11’/’’21’

10

24

21’’

15

MeO

H

3e. In the structure above, the stereochemistry around the H20 has been removed. Use the assigned ROESY to say if H20 points up or down and provide the argument you used to decide its configuration. (5 p)

N

N

Br

O

NHN

O

H

Cl

12

34

567

89

10

1112

13141516

17

18 19 20

21 22

23

24

Br

Br


Question 4. 2D NMR (20 p) Proton

6.6.16.26.36.46.56.66.76.86.97.07.17.27.37.47.57.67.77.87.98.08.18.28.38.4.51H (ppm)

0.96

0.92

1.03

1.06

2.00

Carbon

10010511011512012513013514014515015516016517017518018519019520013C (ppm)

110.

69

126.

5212

6.73

129.

4113

2.93

133.

1513

5.31

156.

28

181.

94

184.

94


DQF-COSY

6.06.16.26.36.46.56.66.76.86.97.07.17.27.37.47.57.67.77.87.98.08.18.28.38.48.51H (ppm)

6.0

6.5

7.0

7.5

8.0

8.5

1H (

ppm

)

ROESY 300ms (poor quality)

6.06.16.26.36.46.56.66.76.86.97.07.17.27.37.47.57.67.77.87.98.08.18.28.38.48.51H (ppm)

6.0

6.5

7.0

7.5

8.0

1H (

ppm

)

HSQC+HMBC


6.06.16.26.36.46.56.66.76.86.97.07.17.27.37.47.57.67.77.87.98.08.18.28.38.48.51H (ppm)

100

110

120

130

140

150

160

170

180

190

13C

(pp

m)

4a. The spectra above are for an unknown molecule with the molecular formula: C10H6O3. Elucidate the structure and report the 1H and 13C assignments in the HSQC+HMBC. All spectra are acquired in CDCl3 (δ1H=7.26 PPM, δ13C=77.16 PPM). (20p)

Good luck!

Johan


Compendium – Chemical shift tables and coupling constants

examination question paper - uit20172408100622/kje-8303.pdf · 3c. 3d “triple-resonance”...

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