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NMR stands for Nuclear Magnetic Resonance Spectroscopy. This is the technique used for determining the structure of the organic compounds. It is a non-destructive technique. It is of two types¹H NMR and¹³C NMR.

Proton nuclear magnetic resonance (¹H NMR) is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen-1 nuclei within in the molecule of substance in order to determine the structure of its molecules.

Carbon-13 nuclear magnetic resonance is the application of nuclear magnetic resonance spectroscopy to carbon. It is analogous to proton NMR and allows the identification of carbon atom in an organic molecule just as proton NMR identifies Hydrogen atoms.

In the terms of NMR,all three Ha protons are chemically equivalent to each other, as are all three Hb protons. The Ha protons are, however, chemically non-equivalent to the Hb protons. As a result, the resonance frequency of the Ha protons is different from that of the Hb protons. For example methyl acetate has two distinct set of protons.
The three methyl acetate protons labelled Ha below have different resonant frequency compared to three Hb protons, because the two sets of protons are in non-identical electronic environment.

If you assign R, S configuration to the carbons bonded to methyl group, it is apparent that cis-1,2-dimethylcyclohexane is a mesocompound. When the cyclohexane ring undergoes a ring-flip, the ring passes through an intermediate that has a plane of symmetry.

Plane of symmetry in cyclohexane

Types of protons

Both the ¹³C NMR spectrum and the ¹H NMR spectrum show 4 peaks.

Both the¹³C NMR spectrum and the¹H NMR spectrum show 4 peaks.