2012학년도 MEET 자연과학추론II 의학계열 28번 문제
28. 다음 화합물(A~C)에 해당하는 양성자 짝풀림(proton-decoupled) 13C-NMR 스펙트럼을 <보기>에서 골라 옳게 짝지은 것은?
정답 : 1번.
C-13 NMR (CMR)
1. NMR experiments may be done with any atom which has a nucleus with a magnetic spin.
2. Examples of various isotopes with nuclear magnetic spin states are listed below.
a) The chart shows the frequency and magnetic field strength for which the nuclei have their
resonance. Note that for a given isotope, resonant frequency is proportional to applied field.
C-13 NMR (CMR)
1. NMR experiments may be done with any atom which has a nucleus with a magnetic spin.
2. Examples of various isotopes with nuclear magnetic spin states are listed below.
a) The chart shows the frequency and magnetic field strength for which the nuclei have their
resonance. Note that for a given isotope, resonant frequency is proportional to applied field.
|
isotope à
|
1H
|
2H
|
13C
|
19F
|
|||||
|
Ho,
gauss
|
10,000
|
14,100
|
42,276
|
51,480
|
10,000
|
10,000
|
42,276
|
10,000
|
42,276
|
|
Resonant
Frequency MHz |
42.6
|
60.0
|
180
|
220.0
|
6.5
|
10.7
|
45.3
|
40.0
|
169.2
|
3. A significant difference in proton NMR vs CMR is
that 1H is the most abundant isotope of hydrogen.
a) In CMR the lesser abundant isotope is observed. About 1% of all carbon atoms are C-13.
b) The lower abundance means a larger sample must be used in the instrument.
a) In CMR the lesser abundant isotope is observed. About 1% of all carbon atoms are C-13.
b) The lower abundance means a larger sample must be used in the instrument.
Proton NMR requires about .1 mg
of sample. CMR requires about 1-5 mg of sample.
4. The 1H NMR spectrum ranges typically from 0-12 ppm; the CMR spectrum ranges from 0-210 ppm.
a) Even slight differences in a carbon's chemical environment results in a significant chemical
shift so that the number of carbon atoms (of different chemical environment) in the compound
can be determined simply by adding up the number of signals in the spectrum.
5. A great advantage of CMR is that the NMR experiment can be run in two different modes.
a) The "off-resonance decoupling" experiment results in the splitting of each signal for each carbon
atom in the molecule by the hydrogen atoms attached to it.
i) This means that the number of hydrogen atoms attached to the carbon atom can be quickly
deduced simply by counting the peaks of the multiplet and subtracting 1.
4. The 1H NMR spectrum ranges typically from 0-12 ppm; the CMR spectrum ranges from 0-210 ppm.
a) Even slight differences in a carbon's chemical environment results in a significant chemical
shift so that the number of carbon atoms (of different chemical environment) in the compound
can be determined simply by adding up the number of signals in the spectrum.
5. A great advantage of CMR is that the NMR experiment can be run in two different modes.
a) The "off-resonance decoupling" experiment results in the splitting of each signal for each carbon
atom in the molecule by the hydrogen atoms attached to it.
i) This means that the number of hydrogen atoms attached to the carbon atom can be quickly
deduced simply by counting the peaks of the multiplet and subtracting 1.
 |
b) In the proton-decoupled mode each
carbon nucleus appears as a singlet.
i) The protons are kept from coupling with the C-13 nuclei by irradiating the sample with
radio waves which flip the protons: they do not spend enough time in either spin state to
couple with the C-13 nuclei.
i) The protons are kept from coupling with the C-13 nuclei by irradiating the sample with
radio waves which flip the protons: they do not spend enough time in either spin state to
couple with the C-13 nuclei.
 |
ii) Adjacent carbon atoms do
not split the signal in C-13 NMR. Why not?
Answer: Since there are so few C-13 atoms (~1%), the chances of having two C-13 atoms next to each other in a molecule are small.
Answer: Since there are so few C-13 atoms (~1%), the chances of having two C-13 atoms next to each other in a molecule are small.
6. In CMR as in NMR, electronegative atoms attached to
the carbon atom results in a downfield shift.
a) In addition, replacement of a hydrogen atom on a carbon with an alkyl group results in a
downfield shift.
a) In addition, replacement of a hydrogen atom on a carbon with an alkyl group results in a
downfield shift.
|
Chemical Shift, ppm
|
10.2 ppm
|
21.8
|
30.9
|
69.0
|
|
Answer
|
C-4
|
C-1
|
C-3
|
C-2
|
7. In the CMR spectrum of 2-butanol above, assign which
carbons are responsible for each observed
peak.
8. Coupling constants could be used in proton NMR to distinguish cis and trans isomers of alkenes.
a) The chemical shift of the allylic carbons in CMR are lower for the cis isomer than the trans
isomer by about 5 ppm, so this is another way of deciding upon which isomer is being studied.
b) Notice the chemical shifts for the isomers cis- and trans-2-butene.
cis: 11.4 ppm trans: 16.8 ppm
peak.
8. Coupling constants could be used in proton NMR to distinguish cis and trans isomers of alkenes.
a) The chemical shift of the allylic carbons in CMR are lower for the cis isomer than the trans
isomer by about 5 ppm, so this is another way of deciding upon which isomer is being studied.
b) Notice the chemical shifts for the isomers cis- and trans-2-butene.
cis: 11.4 ppm trans: 16.8 ppm
9. Unlike proton NMR, CMR integrals do not yield ratios
of nonequivalent carbons since the peak
intensities in CMR are subject to distortion.
10. Below are shown some CMR chemical shift correlations (ppm).
intensities in CMR are subject to distortion.
10. Below are shown some CMR chemical shift correlations (ppm).
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