Introductory discussion to 13

C-NMR

PRESENTED BY: SHEAMA FARHEEN SAVANUR

INTRODUCTION

The study of carbon nuclei through nuclear magnetic resonance (NMR) spectroscopy is a important technique for determining the structures of organic molecules along with infrared and proton NMR.

It can be used to determine the number of non equivalent carbons and to identify the types of carbon atoms that may be present in a compound. Thus, 13C-NMR provides direct information about carbon skeleton of a molecule.

THE 13C NUCLEUS

Carbon-12 nucleus, the most abundant isotope of carbon is NMR inactive since it has a spin (I) of zero. Carbon-13,however has odd mass and does have nuclear spin I=1/2

But the resonance of 13C are most difficult to observe than those of protons mainly because of 2 reasons namely The natural abundance of carbon-13 is very low. The magnetogyric ratio of a 13C nucleus is smaller than that of hydrogen

For a given magnetic strength, the resonance of a 13C nucleus is about one-fourth the frequency required to observe proton resonances.

CORRELATION CHARTS

Correlation charts for different functional groups

Proton-coupled 13C spectra-spin spin splitting of carbon-13 signals

Spectra that show the spin-spin splitting, or coupling, between carbon-13 and the protons directly attached to it are called Proton-Coupled Spectra or sometimes Nondecoupled Spectra.

Proton-coupled spectra for large molecules are often difficult to interpret. The multiplets from different carbons commonly overlap because the 13C-H coupling constants are frequently larger than the chemical shift differences of the carbons in the spectrum. Sometimes interpretation of spectra for some molecules become difficult.

Proton decoupled 13C spectra

Most modern spectrophotometers have proton decoupler, i.e. a second tunable radiofrequency generator.

Here the decoupling of spin-spin interactions take place, and all the spin interactions are averaged to zero.

The decoupling technique obliterates all interactions between protons and 13 C nuclei; therefore only singlets are observed are observed in the decoupled 13 C NMR spectra. This technique although simplifies the spectrum and avoids overlapping multiplets, it has its own disadvantage that the information on attached hydrogen is lost.

Example: Ethyl phenylacetate

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