Wednesday, November 2

Carbon-13 NMR

In this post we will discuss about 13C NMR. This topic is well covered under Spectroscopy. It is one of the most important topic as per CSIR syllabus. Under this post we will discuss some basics of 13C NMR and at the end of the post there are some Problems related to the topic asked previously in CSIR Examinations.


What is 13C NMR?

Nuclear Magnetic Resonance (NMR) spectroscopy has become the dominant method of analysis for organic compounds, because in many cases it provides a way to determine an entire structure using one set of analytical tests. It is also increasingly used in inorganic chemistry and biochemistry, where it also provides a lot of valuable structural information.

Nuclear Magnetic Resonance (NMR) Spectroscopy is not limited to the study of protons. Any element with a nuclear spin (13C, 17O, 19F, 31P and many others) will give rise to an NMR signal.Carbon-13 NMR (13C NMR or referred to as carbon NMR) is the application of nuclear magnetic resonance (NMR) spectroscopy applicable to carbon. It is similar to proton NMR (1 H NMR) and allows the identification of carbon atoms whereas in other identification of H. As such 13C NMR is an important tool in chemical structure elucidation in organic chemistry. 13C NMR detects only the 13C isotope of carbon, whose natural abundance is only 1.1%, because  the main carbon isotope, 12 C, is not detectable by NMR since it has zero net spin.



Some of the properties of 13C NMR:

  • 13C has only about 1.1% natural abundance (of carbon atoms)
  • 12C does not exhibit NMR behaviour (I=0)
  •  13C nucleus is also a spin 1/2 nucleus
  • 13C nucleus is about 400 times less sensitive than H nucleus to the NMR phenomena
  • Due to the low abundance, we do not usually see 13C-13C coupling
  • Chemical shift range is normally 0 to 220 ppm
  • Chemical shifts are also measured with respect to tetramethylsilane, (CH3)4Si (i.e. TMS)
  • Similar factors affect the chemical shifts in 13C as seen for H-NMR
  • Long relaxation times (excited state to ground state) mean no integrations
  • "Normal" 13C spectra are "broadband, proton decoupled" so the peaks show as single lines
  • Number of peaks indicates the number of types of C


Additional Detailed Notes on this topic is provided below:


Note: these notes belong to MIT University and These are posted here just for a detailed overview of the topic.


MUST VIEW:
Practice Problems on Spectroscopy
Isolobal Analogy
18 el
This is sample image of Notes, Full notes can be downloaded from below link


Download Notes on 13C NMR Spectrascopy  Click Here



Practice Problems on 13C NMR Spectroscopy

 Ques.1: Predict the number of 13C NMR signals in the following compounds:
  1. Benzene
  2. naphthalene
  3. Anthracene
  4. pentacene
  5. cyclohexane
  6. salicylic acid
  7. beta napthol
  8. camphor
  9. alpha napthol
  10. di benza anthracene
  11. 7-norborane
  12. resorcinol
  13. 2-pentanone
  14. pinene
  15. menthol
  16. aspirin
  17. alpha-terpenol
  18. citral
  19. vanilin
  20. toulene
  21. 4-chloro benzonitrile

 Questions Asked In CSIR NET JRF, GATE and other Examinations:

[CSIR JUNE 2012]
The number of signals that appear in broad band decoupled 13C NMR spectrum of  phenanthracene and anthracene is:  

 

[CSIR JUNE 2012]

 [CSIR DEC 2014]
  [CSIR JUNE 2014]


 [CSIR JUNE 2013]





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