c 13 NMR

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CARBON-13 NMR M.JAYASREE M.PHARMACY DEPT. OF PHARMACOLOGY 1

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CONTENTS: Introduction to 13 C-NMR Silent facts of 13 C-NMR Differences between 13 C-NMR and 1 H-NMR Problems in 13 C-NMR Chemical shifts Interpretation of 13 C-NMR 2

INTRODUCTION TO 13C-NMR: 

INTRODUCTION TO 13 C-NMR Proton NMR used often for the complete elucidation of the unknown compound. 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. 3

Silent facts of 13C-NMR: 

Silent facts of 13 C-NMR 4

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I=0 I=1/2 5

DIFFERENCES BETWEEN 13C &1H NMR: 

DIFFERENCES BETWEEN 13 C & 1 H NMR 13 C -NMR 1 H-NMR Study of spin changes of carbon nuclei. Chemical shift ranges between 0-240 ppm. Very fast process. Pulse- fourier transform technique is used Study of spin changes of proton nuclei. Ranges 0-14ppm. Very slow process. Continuous wave method is used 6

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13 C -NMR 1 H-NMR valves of coupling constant ranges from 125-250HZ. Information of total number of protons carbons is obtained. By peak splitting, possible to interpret protons directly attached to carbon atom Ranges from 0-15HZ Information like total number, types of chemical environment of protons is obtained. It possible to interpret protons attached to adjacent carbon atom 7

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13 C-NMR 1 H-NMR Effect of substituent on adjacent carbon atom cannot varies the chemical shift. It works o frequency sweep. Substituents alters the chemical shift. It works on either field sweep or frequency sweep. 8

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Problems of 13C-NMR: 

Problems of 13 C-NMR 13 C-NMR signal is 6000 times weaker than 1 H-NMR why??? NATURAL ABUNDANCE GYRO MAGNETIC RATIO COUPLING PHENOMENON 10

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NATURAL ABUNDANCE: 13 C natural abundance is very low (1.08%). GYRO MAGNETIC RATIO: 13 C nucleus gyro magnetic ratio is much lesser than proton nucleus. C-1.404; H-5.585. This shows that CMR is more sensitive than PMR which is overcome by using FT-NMR technique 11

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Coupling phenomenon: 13 C & 1 H have I=1/2 so that coupling between them probability occur. Extremely complex spectra and overlap of multiplets difficult to interprete . 13 C- 13 C coupling no! Not probable 13 C- 1 H coupling YES! Very common 12

Problems of 13C-NMR can overcome by: 

Problems of 13 C-NMR can overcome by 13

FT-NMR: 

FT-NMR FID 14

NUCLEAR OVERHAUSER ENCHANCEMENT : 

NUCLEAR OVERHAUSER ENCHANCEMENT 15

DECOUPLING: 

DECOUPLING 16

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COUPLING TO ATTACHED PROTONS

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The effect of attached protons on 13 C resonances n+1 = 4 n+1 = 3 n+1 = 2 n+1 = 1 C 13 3 protons 2 protons 1 proton 0 protons H H H C 13 H H C 13 H C 13 Methyl carbon Methylene carbon Methine carbon Quaternary carbon ( n+1 rule applies ) COUPLING TO ATTACHED PROTONS (J’s are large ~ 100 - 200 Hz)

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ETHYL PHENYLACETATE 13 C coupled to the hydrogens

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DECOUPLED SPECTRA

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DECOUPLING THE PROTON SPINS PROTON-DECOUPLED SPECTRA A common method used in determining a carbon-13 NMR spectrum is to irradiate all of the hydrogen nuclei in the molecule at the same time the carbon resonances are being measured. This requires a second radiofrequency (RF) source (the decoupler) tuned to the frequency of the hydrogen nuclei, while the primary RF source is tuned to the 13 C frequency. 1 H- 13 C RF source 2 RF source 1 continuously saturates hydrogens pulse tuned to carbon-13 13 C signal (FID) measured “the decoupler”

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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, all occuring rapidly. 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. You no longer see multiplets for the 13 C resonances. Each carbon gives a singlet, and the spectrum is easier to interpret.

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ETHYL PHENYLACETATE 13 C coupled to the hydrogens 13 C decoupled from the hydrogens in some cases the peaks of the multiplets will overlap this is an easier spectrum to interpret

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q t t s s d d d SOME INSTRUMENTS SHOW THE MULTIPLICITIES OF THE PEAKS ON THE DECOUPLED SPECTRA s = singlet t = triplet d = doublet q = quartet CODE : This method gives the best of both worlds.

Relaxation process: 

Relaxation process 25

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CHEMICAL SHIFTS OF 13 C ATOMS

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R- C H 3 8 - 30 R 2 C H 2 15 - 55 R 3 C H 20 - 60 C -I 0 - 40 C -Br 25 - 65 C -N 30 - 65 C -Cl 35 - 80 C -O 40 - 80 C C C 65 - 90 C = C 100 - 150 C N N 110 - 140 110 - 175 R- C -OR O R- C -OH O 155 - 185 R- C -NH 2 O 155 - 185 R- C -H O R- C -R O 185 - 220 APPROXIMATE 13 C CHEMICAL SHIFT RANGES FOR SELECTED TYPES OF CARBON (ppm)

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Aldehydes Ketones Acids Amides Esters Anhydrides Aromatic ring carbons Unsaturated carbon - sp 2 Alkyne carbons - sp Saturated carbon - sp 3 electronegativity effects Saturated carbon - sp 3 no electronegativity effects C=O C=O C=C C C 200 150 100 50 0 200 150 100 50 0 8 - 30 15 - 55 20 - 60 40 - 80 35 - 80 25 - 65 65 - 90 100 - 150 110 - 175 155 - 185 185 - 220 Correlation chart for 13 C Chemical Shifts (ppm) C-O C-Cl C-Br R 3 CH R 4 C R-CH 2 -R R-CH 3 RANGE /

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SPECTRA

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Proton-decoupled 13C spectrum of 1-propanol (22.5 MHz) 200 150 100 50 0 1-PROPANOL PROTON DECOUPLED HO-CH 2 -CH 2 -CH 3 c b a

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2,2-DIMETHYLBUTANE

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BROMOCYCLOHEXANE

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CYCLOHEXENE

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TOLUENE

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a a b b c c 1,2-DICHLOROBENZENE

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REFERENCES: 

REFERENCES William Kemp: Organic spectroscopy, 3 rd edition. Robert M. Silverstein: Spectroscopic identification of Organic compounds, 6 th edition. Jagmohan : Organic spectroscopy, principles and applications, 2 nd edition. Pavia: Introduction to spectroscopy, 3 rd edition. P.S. Kalsi : Spectroscopy of organic compounds, fifth edition. Y.R. Sharma: Elementary organic spectroscopy, principle and chemical applications. 39