“Carbon-13 &2D Nuclear Magnetic

Resonance Spectroscopy”

Presented By: Guided By:Madhavanand Ingalageri Dr. A R SaundaneMSc IV-Sem Professor in Chemistry Gulbarga University Kalaburagi

IntroductionNuclear magnetic resonance concern the magnetic

properties of certain atomic nuclei. It concerns the atoms having spin quantum number.

12C nucleus is not magnetically active the spin number I being zero. But 13C Have I = ½

13C account for only 1.1% of naturally occurring carbon 13C- 13C coupling is negligible and not observed.

The gyromagnetic ratio of 13C is one-fourth of that of 1H.Each nonequivalent 13C gives a different signal.A 13C signal is split by the 1H bonded to it according to the (n + 1) rule.The most common mode of operation of a 13C-NMR spectrometer is a hydrogen-decoupled mode.

Principle of NMRWhen energy in the form of radio frequency applied.When applied force is equal to precessional frequency.Absorption of energy occurs.Nucleus is in the resonance.NMR signal is recorded.

Why Carbon-13 NMR required? Carbon NMR can used to determine the number of non-

equivalent carbons and to identify the types of carbon atoms(methyl, methylene, aromatic, carbonyl….) which may present in compound.

13C signals are spread over a much wider range than 1H signals making it easier to identify & count individual nuclei.

Chemical Shift

Carbon-13 chemical shifts are most affected by, Hybridiasation state of carbon & Electronegative group attached

to carbon.

Hydrogen Decoupled mode(Broad Band Decoupled) A sample is irradiated with two different radio frequencies.

One to excite all 13C nuclei.A second broad spectrum of frequencies to cause

all hydrogen's in the molecule to undergo rapid transitions between their nuclear spin states.

On the time scale of a 13C-NMR spectrum, each hydrogen is in an average or effectively constant nuclear spin state, with the result that 1H-13C spin-spin interactions are not observed; they are decoupled.

Thus, each different kind of carbon gives a single, unsplit peak.

ETHYL PHENYLACETATE

13C coupledto the hydrogens

13C decoupledfrom the hydrogens

in some casesthe peaks of the multiplets willoverlap

this is aneasier spectrumto interpret

Off-Resonance DecouplingOff-Resonance decoupling simplifies the

spectrum by allowing some of the splitting information to be retained.

In this technique only the 13C nuclei are split by the protons directly bounded to them and not by any other protons i.e., one observes only one bond coupling 13C -1H

The coupling between each carbon atom and each hydrogen attached directly to it, s observed acc to n+1 rule.

Use of off-resonance decoupled spectra has been replaced by use of DEPT 13C NMR

Example: Propanol

DEPT 13C NMR SpectroscopyDistortionless Enhancement by

Polarization Transfer (DEPT-NMR) experiment

Run in three stages

Example: 6methylhept-5-en-2-ol(a) Ordinary broadband-decoupled

spectrum showing signals for all eight of 6-methylhept-5-en-2-ol

(b) DEPT-90 spectrum showing signals only for the two C-H carbons.

(c) DEPT-135 spectrum showing positive signals for the two CH carbons and the three CH3 carbons and negative signals for the two CH2 carbons.

COSY Spectrum2D NMR spectra have two frequency axes and one

intensityThe common 2D spectra are 1 H -1H shift correlations

known as COSY Spectrum.COSY identifies pair of protons which are coupled to

each other.The compound is identified using a contour plotOne dimensional counterpart of a given peak on the

diagonal lies directly below that peak on each axisThe presence of cross peak normally indicates protons

giving the connected resonance on the diagonal are geminaly or vicinally coupled.

COSY Spectrum of m-dinitrobenzene

HECTOR Spectrum 2D-NMR spectra that displays 13C – 1H-NMR shift correlations are called HETCOR

spectra. It shows coupling between protons and the carbon to which they are attached. The HETCOR spectrum of 1-chloro-2propanol is shown in the fig.

The 2-D spectrum is composed only of cross-peaks, each one relating carbon to its directly bonded proton(s).

The methyl doublet of 1H-NMR spectrum appears at δ 1.2 when drawn cross-peak and then dropped down to the 13C spectrum axis indicates that the 13C peak at δ 20 is produced by the methyl carbon of 1-chloro-2-propanol(C-3)

  The 1H -NMR signal at δ 3.9 is due to CH-OH (C-2 proton) tracing out to the

correlation peak and down to the 13C spectrum shows the 13C NMR signal at 67 arises from the C-2 carbon of the compound i.e., the carbon carrying the hydroxyl group.

  The 1H-NMR peaks at δ 3.4-3.5 for the two protons on the carbon bearing the

chlorine, the interpretation leads us to the cross-peak and down to the 13C peak at δ 51

Hector spectrum of 1-chloro-2-propanol

Applications of 13C NMRCMR is a noninvasive and nondestructive

method,i.e,especially used in repetitive In-vivo analysis of the sample without harming the tissues .

CMR, chemical shift range(0-240 ppm) is wider compared to H-NMR(0-14 ppm), which permits easy separation and identification of chemically closely related metabolites.

C-13 enrichment, which the signal intensities and helps in tracing the cellular metabolism.

CMR technique is used for quantification of drugs purity to determination of the composition of high molecular weight synthetic polymers.

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