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Chemistry LibreTexts

NMR Spectroscopy 3

1H-NMR Spectra

An 1H-NMR will contain a unique signal for each different type of H atom present in the compound. What do we mean by "type" of hydrogen atom? Since the amount of shielding is dependent on the local chemical environment, the exact chemical shift for H atoms can vary widely. There are three basic methods you can use to determine if H atoms are identical.

  1. Substitution method

    This is the simplest but slowest method. The idea is to replace every H atom, one at a time with another atom i.e. a Cl atom to see if you generate a different compound. Each different product indicates a different type of H atom.

  2. Vebal Description

    This requires to you describe each H atom verbally. If you have a different description then the H atoms are different.

    For example:
    • An -NH is differnt from a -CH (based on the atom the H is attached to).
    • A -CH3 is different to a -CH2- (based on the number of attached hydrogen atoms).
    • An sp3 C-H is different than an sp2 C-H which is different to an sp C-H.
    • Others include position on a ring or chain, cis / trans relationships etc.
  3. Symmetry

    The symmetry method is the most sophisticated but requires a knowledge of molecular symmetry. H atoms that are related by mirror planes, axis of rotation or a center of inversion are equivalent to one another.

The first method is the easiet but slowest, the last is the fatest but requires a good knowledge of molecular symmetry. See the thre examples below.

 

 

JMOL