2D NMR Worksheet

CHEM 30CL2D NMR Worksheet
Name: _______________________________________ Section: __________________
α-Ionone
In this exercise, you will analyze the COSY, HSQC and HMBC spectra of α-ionone. You will see
how, with the aid of 2D NMR, organic chemists can unambiguously assign 1H and 13C NMR
chemical shifts in complex natural product molecules.
Both this Worksheet and Post-Lab 3 are due at the beginning of your regular lab sections.
1. In TopSpin, open the 1H spectrum of α-ionone (file “1 – zg” —> “1”). Integrate all peaks.
2. On the structure of α-ionone below, draw all the protons that you would expect to show up in
the alkene region (4–6.5 ppm). Find these peaks on the 1H spectrum, but do not assign them
(yet!).
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3. Open the COSY spectrum of α-ionone (file “2 – cosygpmfqf” —> “1”). Find the diagonal
line that runs from the upper-right to the lower-left corner of the COSY spectrum.
Important: signals that show up on this diagonal line do not indicate J-coupling
correlations, and should be disregarded in your analysis.
4. Observe the COSY spectrum of α-ionone. Which of the alkenyl protons that you identified in
step 2 are correlated on the COSY? _________ ppm and __________ ppm.
5. Does this COSY correlation peak show up on both sides of the diagonal line? ___________
6. Based on the information you got from the COSY and the 1H spectra, assign all the 1H
alkenyl peaks. Write down their chemical shifts in ppm (e.g. “5.08 ppm”) in the appropriate
boxes on the structure provided on page 5.
7. Singlets tend to be better resolved than multiplets, and easier to pick out from a forest of
peaks. On the structure of α-ionone below, identify all the protons that you would expect to
show up as singlets. (They may not all be singlets as you predict; that would be okay!)
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CHEM 30CL
2D NMR Worksheet
Name: _______________________________________ Section: __________________
8. Observe the 1H spectrum of α-ionone. Do you find all the singlets that you expect to see?
___________
9. Typically, the J-couplings we observe on NMR are 2J couplings (over 2 bonds) or 3J
couplings (over 3 bonds). Proton couplings over more than 3 bonds are usually too small to
detect by NMR. There are, however, a number of exceptions to this. 4J couplings, and
sometimes even longer range couplings, can be detected if a π system intervenes, or if the
geometry is “W”. This is due to the back-lobe interactions of sp2 or sp3 CH and CC orbitals.
W (4J ) coupling in aromatics
Allylic (4J ) coupling
Homoallylic 5J coupling
Hb
Ha
Hb
Hb
Ha
Ha
4J coupling of H
4J coupling of Ha to Hb
due to aromatic pi system
(4J = 2–3 Hz)
can often be observed
due to allylic π system
(4J = 1–3 Hz)
a to Hb
can often be observed
5J coupling of Ha to Hb
can sometimes be observed
due to homoallylic pi system
( 5J = 0–8 Hz)
“W” 4J coupling
Ha
Hb
4J coupling of Ha to Hb
can sometimes be observed
in a saturated system
due to ideal geometrical alignment
(4J = 1–3 Hz)
10. Some of the protons that you would expect to be “singlets” may not actually be singlets!
They could be split due to the longer-range J-couplings shown above. On the structure of αionone below, identify all pairs of protons that might engage in allylic 4J coupling.
O
11. Find the 1H NMR peak that most likely corresponds to Ha: ____________ ppm.
Ha
Ha
Ha
O
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CHEM 30CL
2D NMR Worksheet
Name: _______________________________________ Section: __________________
12. Observe the COSY spectrum. Does the Ha peak you identified show COSY correlation
peak(s) that are due to allylic 4J coupling? __________. If yes, which peak is it correlated
to? ____________ ppm.
13. Assign all the singlets in the 1H spectrum. Write down their chemical shifts in ppm in the
appropriate boxes on the structure provided on page 5.
14. Observe the COSY spectrum. Besides Hb, what other peak does Ha correlate to?
___________ ppm. Based on this COSY correlation, assign that peak to a proton on the
structure provided on page 5.
Ha
O
Hb
15. There are only a few unassigned signals left! Now try assigning them based on the molecular
structure and your chemical intuition.
16. Observe the COSY spectrum. Based on the correlation peaks you see in the COSY, do your
assignments in step 15 make sense? If they don’t, try a different combination of assignments,
and see if the COSY agrees with those better.
17. After you have assigned every signal in the 1H spectrum, have your TA check your work
before moving on to the next step.
18. Open the HSQC spectrum of α-ionone (file “3 – hsqcedetgpsp.3” —> “1”). If the projections
are low quality, change the projections to higher quality 1H and 13C spectra before you
proceed. For HSQC, the projection along F2 should be a 1H spectrum, and the projection
along F1 should be a 13C spectrum (both of which can be found under the same folder as the
2D NMRs; 1H — EXPNO 1, PROCNO 1; 13C — EXPNO 5, PROCNO 1). Adjust the
intensity levels/peak heights of both the 2D spectrum and the projections as necessary.
19. HSQC correlates protons to the carbons that they are directly bonded to. Based on your 1H
NMR assignments for α-ionone, assign all of the 13C signals that are correlated with proton
signals on this HSQC. Write down their chemical shifts in ppm (e.g. “126.8 ppm”) in the
appropriate boxes on the structure provided on page 5.
20. There are a few carbons in α-ionone that do not have protons on them. Assign those carbons
based on the structure and your chemical intuition. (Note: there is one carbon signal at 198.3
ppm that does not show up on the HSQC because it is out of range.)
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2D NMR Worksheet
CHEM 30CL
Name: _______________________________________ Section: __________________
21. Open the HMBC spectrum of α-ionone (file “4 – hmbcetgpl3nd” —> “1”). If the projections
are low quality, change the projections to higher quality 1H and 13C spectra before you
proceed. For HMBC, the projection along F2 should be a 1H spectrum, and the projection
along F1 should be a 13C spectrum (both of which can be found under the same folder as the
2D NMRs; 1H — EXPNO 1, PROCNO 1; 13C — EXPNO 5, PROCNO 1). Adjust the
intensity levels/peak heights of both the 2D spectrum and the projections as necessary.
22. HMBC (Heteronuclear Multiple Bond Correlation) shows proton-carbon correlations over
multiple (usually 2–4, sometimes 5) bonds. Proton-carbon 1-bond correlations (i.e. those you
observe in HSQC) do not show up on HMBC. Observe the HMBC spectrum carefully, and
use it to confirm that the 1H and 13C assignments make sense.
23. For extra credit, draw out all the COSY correlations you observe for α-ionone on structure A
of page 6. If a COSY correlation is due to any of the long-range (4J or 5J) coupling types we
introduced on page 2, clearly label it as such (e.g. “homoallylic 5J”).
24. For more extra credit, draw out all the HMBC correlations you observe for α-ionone on
structure B of page 6. Label each correlation as either 2-, 3-, 4- or 5-bond correlations.
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CHEM 30CL
2D NMR Worksheet
Name: _______________________________________ Section: __________________
1H and 13C Assignments for α-Ionone
* All shifts shown in ppm. Because C–H bonds are omitted from this structure, 1H shifts next to
a carbon indicate the shifts of protons attached to that carbon.
1H: ______
13 C: ______
O
13 C: ______
1H: ______
13 C: ______
13 C: ______
1H: ______
13 C: ______
1H: ______
1H: ______
13 C: ______
1H: ______
13 C: ______
1H: ______
13 C: ______
13 C: ______
13 C: ______
1H: ______
13 C: ______
1H: ______
13 C: ______
1H: ______
13 C: ______
(you don’t need to distinguish
between these two)
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CHEM 30CL
2D NMR Worksheet
Name: _______________________________________ Section: __________________
Extra Credit 1: COSY Correlations for α-Ionone
Link carbons with double-headed arrows to indicate that the protons attached to those carbons
are correlated on COSY. One such correlation is already indicated for you in the structure below.
If a COSY correlation is due to any of the long-range (4J or 5J) coupling types we introduced on
page 2, clearly label it as such (e.g. “homoallylic 5J”).
O
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CHEM 30CL
2D NMR Worksheet
Name: _______________________________________ Section: __________________
Extra Credit 2: HMBC Correlations for α-Ionone
Link carbons with double-headed arrows to indicate that the proton(s) attached to one carbon is
correlated with the other carbon on HMBC. One such correlation is already indicated for you in
the structure below. Label each correlation as either 2-, 3-, 4- or 5-bond correlations.
O
2-b
ond
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CHEM 30CL
2D NMR Worksheet
Name: _______________________________________ Section: __________________
1H and 13C Assignments for α-Ionone
* All shifts shown in ppm. Because C–H bonds are omitted from this structure, 1H shifts next to
a carbon indicate the shifts of protons attached to that carbon.
1H: ______
1.54
22.7
13 C: ______
13 C: ______
131.9
O
1H: ______
6.59
13 C: ______
149.0
13 C: ______
198.4
5.48
1H: ______
13 C: ______
122.7
1H: ______
1H: ______
602
13 C: ______
132.3
13 C: ______
54.4
1H: ______
2.02
1H: ______
2.22
26.9
13 C: ______
13 C: ______
13 C: ______
1H: ______
0.83
n8
13 C: ______
1H: ______
1.44，1.20
13 C: ______
31.1
1H: ______
0.90
13 C: ______
26.8
(you don’t need to distinguish
between these two)
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