Carbon-13Nuclear

Magnetic Resonance

Spectroscopy

Presented By:Arif Iqbal

Shahid Roll No.:

108

Introduction

Nuclear Magnetic Resonance concern the magnetic properties of certain atomic nuclei. It concern the atoms having Spin Quantum Number ( I ).

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IntroductionSome atoms have no spin. Atoms having even atomic number and mass number (12C, 16O).Some other nuclei have spin. Atoms having either odd atomic number or mass number or both (1H,13C,19F).

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Carbon-13 NMR12C have no Magnetic

Spin and produces no NMR signal.13C have Magnetic Spin I= ½13C accounts for only 1.1% of naturally occurring carbon

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Carbon-13 NMRThe low probability of

adjacent 13C atoms leads to no detectable carbon-carbon splitting No coupling between 13C and C. 13C have weak signal13C have strong coupling with H 13C have weak coupling with C

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13C NMR Spectroscopy: Signal Averaging and FT-

NMRTwo techniques have been developed to detect the 13C isotope in an organic sample by NMR:

Signal averaging (increases instrument sensitivity)

Fourier-transform NMR (increases instrument speed)

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Signal averaging

increases instrument sensitivity. Any individual 13C NMR spectrum is extremely “noisy”, but when hundreds of individual runs are added together by computer and then averaged, a greatly improved spectrum results.

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Signal averaging

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Fourier-transform NMRincreases instrument speed.

In the FT-NMR technique, all signals are recorded simultaneously.All 1H and 13C in the sample resonate at once, and the complex composite signal is manipulated using so-called Fourier transforms before it can be displayed. 8

Fourier-transform NMR

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Fourier-transform NMR

The sensitivity of 13C is only 1/5700 of 1H; This sensitivity problem is overcome with Fourier Transform (FT) NMR instrumentation

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Spin-Spin SplittingIt is unlikely that a 13C would be adjacent to another 13C, so splitting by carbon is negligible.13C will magnetically couple with attached protons and adjacent protons.These complex splitting patterns are difficult to interpret.

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Recording of CMR Spectra

The CMR Spectra is recorded in decoupling condition.The nuclei are decoupled by double irradiation

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Spin DecouplingTwo Types of decoupling:

Homonuclear decoupling:Decoupling

of same nuclei (C and C) or (H and H).

Heteronuclear decoupling:Decoupling

of different nuclei C and H.Broad Band DecouplingOff resonance decoupling

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Broad Band Decoupling: 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 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. 14

Off resonance decoupling

The Coupling can be achieved by off setting the high power proton decoupler by about 1000-2000 Hz up field or about 2000-3000 Hz downfield from the frequency of TMS without using the noise generator.

Groups signals－ CH3 quartets－ CH2 － triplets

Doublets

A quarternary Carbon Singlet

C H

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Proton off-resonance decoupling

Broad band decoupling16

Interpreting 13C NMRThe number of different signals indicates the number of different kinds of carbon.The location (chemical shift) indicates the type of functional group.The peak area indicates the numbers of carbons (if integrated).

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Interpreting 13C NMR

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Chemical Shift in CMR Spectra

C-13 NMR has d 0 to 220 ppm (1HNMR d 0 to 12 ppm)CMR spectra is recorded under proton decoupling conditionIn the CMR multiplets, no need to determine area ratios.

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13C Chemical shifts are most affected by:

Hybridization of 13C atom: –sp3 C signal is in the range 0-90

–sp2 C signal is in the range 110-220

–C=O signal is at the low-field end, in the range 160-220

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13C Chemical shifts are most affected by:

Characteristic Carbon NMR Chemical Shifts (ppm)

(CH3)4Si = TMS = 0.00 ppm (singlet) CDCl3 (solvent) = 77.0 ppm (triplet)

RCH3 0 – 40 RCH2Cl 35 – 80 benzene ring 110 – 160

RCH2R 15 – 55 R3COH 40 – 80 C=O ester 160 – 180

R3CH 20 – 60 R3COR 40 - 80 C=O amide 165 – 180

RCH2I 0 – 40 RCCR 65 – 85 C=O carboxylic acid 175 – 185

RCH2Br 25 - 65 R2C=CR2 100 - 150 C=O aldehyde, ketone 180 – 210

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Interpreting 13C NMR

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13C Chemical shifts are most affected by:

Electro negativity of nearby atoms:

C bonded to O, N, or halogen absorb downfield because O, N, or halogen pull electrons away from nearby 13C atoms, decreasing their electron density and “deshielding” them. 23

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Example

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Example

200 180 160 140 120 100 80 60 40 20 0

H

H

H H

CCH2CH2CH2CH3

HOCH

CH

CH

CC

OCH3CH2

CH2CH2

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Thank You