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C 13 NMR Contents :-

• Introduction• Background to C-13 NMR • Comparison C 13 & H NMR• Chemical shift • Coupling & Decoupling of C 13 NMR • NOE effect• FT technique• Restricted rotation • Reference

Submitted by:- ASHOK JANGID05/03/2023

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INTRODUCTION Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained. It relies on the phenomenon of nuclear magnetic resonance and can provide detailed information about the structure and chemical environment of molecules

300 mhz 900 mhz

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BACKGROUND TO C-13 NMR

C-12 C-13

Spin number = zero Even mass and even Charge NMR inactive

Spin number = ½ Odd mass & odd charge NMR active Nature abundance of C-13- -is very low

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COMPARISON OF H-1 NMR & C-13 NMRBoth are given information about the number of chemically nonequivalent nuclei. Both are given information about the environment of nuclei .

H - 1 C - 13

Abundance 99 % 1 . 1 %

Chemical shift 0 – 15 ppm 0 – 220 ppm

reference TMS TMS

Coupling Yes No

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CHEMICAL SHIFT Can use TMS (tetramethylsilane) for organic solvents; functions as both H-1 and C13 reference.

Pri. Alkyl 0 – 40 ppm

Sec. alkyl 10 – 50 ppm

Ter. alkyl 15 – 50 ppm

Alkyl halide 10 – 65 ppm

Alcohol & ether 50 – 90 ppm

Alkenes aromatics 100 – 170 ppm

Amides 150 – 180 ppm

Carboxylic acid or Aldehydes &ketones 160 – 185 or 183 – 215 ppm

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PROTON-COUPLING OF C-13 NMR

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PROTON DECOUPLING OF C-13 NMR

(Free induction decay)

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• In this method the hydrogen nuclei are “saturated”, a situation where there are as many downward as there are upward transitions .

• During the time the carbon-13 spectrum is being determined, the hydrogen nuclei cycle rapidly between their two spin states (+1/2 and -1/2) and the carbon nuclei see an average coupling (i.e., zero) to the hydrogens.

• The hydrogens are said to be decoupled from the carbon-13 nuclei.

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{ Y = gama }

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Where is the magnetogyric ratio of the nucleus being irradiated , and is the nucleus being observed NOE is the enhancement of the signal and it must be added to the original signal strength :

Total predicted intensity ( max ) = 1+ NOE max

yyirr

obs

When a specific nucleus is magnetically excited and its neighbor is at equilibrium, relaxation occurs between the two nuclei. Because of this relaxations, the intensity of the nucleaus that was at equilibrium changes. For a carbon and its neighboring proton it is possible to observe an almost four-fold increase in signal stranght resulting from decoupling.

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RESTRICTED ROTATION

At room temperature a near sample of di methyle formamid show two peaks because the rate of rotation around the Hindered partial double bond is slow . At 123 degree C , the Rate of exchange of the two groups is rapid enough so that the two peaks merge .

CH

CH

3

3

N C

O

H

……. ………_

(b)

(a)

+

O_

(b)

D M F ( dimethyl formamide )

3

3

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You don't have to know much about NMR to realize that the presence of three signals of equal intensity suggests that the three methyl groups in N,N-dimethylacetamide are different.

If the formula in the inset represented the true structure of N,N-dimethylacetamide, you should expect free rotation around the sigma bond between the nitrogen atom and the carbonyl carbon. This rotation would make the two methyl groups attached to the nitrogen atom identical. In that case, the NMR spectrum of N,N-dimethylacetamide should contain two signals, one for the methyl group attached to the carbonyl carbon, and a second for the two methyl groups attached to the nitrogen. The intensity of the second signal should be twice that of the first. Clearly that is not the case. Something must be restricting the rotation around the C-N bond.

Restricted Rotation in DMA

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In our discussion of resonance theory, we learned that one of the structural features required for resonance involved a non-bonded pair of electrons adjacent to a pi bond. This situation occurs in amides, where the lone pair of electrons on the nitrogen atom interacts with the pi bond of the carbonyl group

Structure C represents the resonance hybrid that is formed by mixing structures A and B. The partial double bond in the hybrid structure C implies a barrier to rotation around the bond between the nitrogen atom and the carbonyl carbon atom. If that barrier is great enough, the two methyl groups attached to the nitrogen atom would be Non equavilent; one of them would always be adjacent to the methyl group attached to the carbonyl carbon, while the other would be next to the carbonyl oxygen atom. Then the NMR spectrum of N,N-dimethylacetamide should contain three signals, one for the methyl group attached to the carbonyl carbon, and one for each methyl group attached to the nitrogen. The intensities of the three signals should be the same

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F T TECHNIQUE

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F T instrument :-

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REFERENCE

• Spectroscopy by Pavia ,Lampman ,Vyvyan• Spectroscopy by Kemp

NMR spectroscopy and polymer microstructure Alan Tonelli Nmr Spectroscopy of Polymers Roger N. Ibbet

Proton and Carbon NMR Spectra of Polymers, Volume. 2,. Quang-Tho. Pham, Roger Petiaud, and Hugues Waton, Wiley,

Proton and Carbon NMR Spectra of Polymers, Vol.1, Heyden, Philadelphia 1981. Silverstein rm, Bassler gc,

Carbon-13 NMR Spectroscopy of Biological Systems Nicolau Beckmann

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