Nanoballetdancer.....How many signals would be seen in total in the 13C NMR spectrum

How many signals would be seen in total in the 13C NMR spectrum

 

3 different methyl signals: (1, 16) (40, 41) (27, 28, 29, 33, 34, 35)

4 different methylene signals: (17) (2, 15) (3,14) (37, 39)

1 aliphatic CH signal: (36)

2 different aliphatic quaternary C signals: (26, 32) (38)

4 different triple bond C signals: (24, 30) (25, 31) (4, 13) (5, 12)

8 different aromatic carbons: (7) (10) (6, 8) (9, 11) (18) (21) (19, 23) (20, 22)

22 different signals

Just looked at a model, methyls 40 and 41 are different; one is on the same side of the 6-membered ring as the benzene ring, the other is on the same side of the 6-membered ring as the hydrogen at C-36

23 different signals!






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Decoupling

Decoupling

  Most ¹³C NMR spectra are very complex. The methyl carbon of an ethoxy group will appear as a large quartet, with each line further split into triplets. Even in fairly simple molecules each carbon may be coupled to a number of different protons. In complicated molecules, these multiplets overlap badly, and may be impossible to analyze.

    ¹J_CH = 100-250 Hz
    ^2,3J_CH = 2-10 Hz

  To simplify ¹³C spectra, we usually use some form of broadband decoupling (noise decoupling) to remove the effect of proton couplings. This also dramatically increases signal intensity, since now all carbons appear as singlets (assuming absence of other spin 1/2 nuclei like ³¹P or ¹⁹F). The increase is actually greater by a factor of 2-3 than would be predicted on the basis of simply combining the ¹³C multiplet intensities because the Nuclear Overhauser Effect causes additional increases in signal intensity.

  

Figure  75.4 MHz ¹³C NMR spectrum of isobutyrophenone in CDCl₃. Identify all of the C-H couplings in the expansions of the coupled spectrum. Assume the coupling within the aromatic ring follow those given for benzene itself. Note that the very closely spaced ortho and meta carbons can be assigned based on the ³J_CH couiplings observed.

  

Figure  25 MHz ¹³C NMR spectrum of diphenyl selenide in CDCl₃.

  Figure  shows the fully coupled and decoupled ¹³C NMR spectra of diphenyl selenide. Although the large ¹J_C-H splittings are easy to identify, the fine structure of the individual multiplets is not first order (e.g., only the para carbon has an approximately centrosymmetric pattern, the others do not). This is because we are looking at the X part of an AA'BB'CXpattern (^JABC^j are protons, X is carbon). Since the AA' and BB' parts are strongly coupled, we see the usual complex effects of "virtual coupling" on the X resonance. When noise {¹H} decoupling is applied, the spectrum becomes much more intense, and only 4 lines remain, one for each carbon.

  In this compound we have a second magnetically active nucleus (⁷7Se, natural abundance 7.5%, I = 1/2), so each of the ¹³C peaks has ⁷7Se satellites, although coupling between C-4 and the selenium is too small to detect (the satellites are under the main peak).

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J-Modulated Spectra.

J-Modulated Spectra. This is the most primitive form of spectral editing. By placing a suitable delay time between the pulse and the beginning of the acquisition, spectra are obtained in which C and CH₂ groups are positive, and CH and CH₃ are negative.

  In this experiment, after the pulse there is a short delay, during which the decoupler is turned off, and the ¹³C NMR spectrum becomes modulated by the CH coupling frequency. After the delay the decoupler is turned on, and the FID is recorded. If the delay is 1/J then the quaternary and CH₂ carbons are positive, and the CH and CH₃ signals are negative. If the delay is 1/2J all peaks except quaternary are nulled.

  

DEPT (Distortionless Enhancement of Polarization Transfer)

DEPT (Distortionless Enhancement of Polarization Transfer): The DEPT technique has proven superior to others in providing information on attached protons reliably, efficiently and with high selectivity. It is a proton-carbon polarization transfer method, so DEPT spectra are actually more sensitive than normal acquisitions. A set of spectra with pulse delays adjusted for π/2 (DEPT-90) and 3π/4 (DEPT-135) are taken. 

The DEPT-90 spectrum shows only CH carbons, the DEPT-135 shows positive CH₃ and CH, and negative CH₂ signals. It is important to understand that the appearance of positive and negative signals can be reversed by phasing, so it is necessary to have some way of determining whether the spectrum has been phased for CH₂ positive or negative. Quaternary carbons are invisible

  

  Figure The normal ¹³C NMR spectrum and a typical set of DEPT spectra of an alkyne. Note the absence of the quaternary alkyne carbons in the DEPT spectra, and the presence of small peaks for the CH₂ and CH₃ signals in the DEPT-90 spectrum, which, in principle, should have only CH signals.

  "Leakage" can occur in DEPT-90 spectra because ¹J_C-H varies as a function of environment, and the technique assumes that all ¹J_C-H are identical. This can result in small peaks for CH₂ and CH₃ signals, which should have zero intensity. For similar reasons the C-H of terminal acetylenes (C≡C-H) will show anomalous intensities in DEPT spectra (either nulled or very small in DEPT-90, or present in DEPT-135) because the C-H coupling is much larger (around 250 Hz) than the normal value of 125 Hz for which the DEPT experiment is usually parameterized. Of course, leakage can also result from an incorrectly calibrated pulse width for the spectrometer.

........... ..............

¹H and ¹³C NMR spectra of the abcc-monomer and the ABCC-sequence-regulated copolymer.

FromSequence-regulated vinyl copolymers by metal-catalysed step-growth radical polymerization

• Kotaro Satoh,
• Satoshi Ozawa,
• Masato Mizutani,
• Kanji Nagai
• & Masami Kamigaito

Nature Communications

 

1,

 

Article number:

 

6

 

doi:10.1038/ncomms1004

 http://www.nature.com/ncomms/journal/v1/n1/fig_tab/ncomms1004_F3.html

 

 (a) ¹H and ¹³C NMR spectra of 7. (b) ¹H and ¹³C NMR spectra of poly(7) obtained with CuCl/N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) ([CuCl]₀=200 mM; [PMDETA]₀=800 mM) in bulk at 100 °C. All spectra were measured in CDCl₃ at room temperature The brackets for repeating units in poly(7) are positioned differently from those in Figs. 1 and 4 so that the signals originating from the terminal groups can be assigned to the chemical structures.