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¹H NMR

01¹H NMR

Proton environments

This section covers equivalent protons and how different proton environments produce separate ¹H NMR signals.

Equivalent hydrogen atoms

Hydrogen atoms that experience the same chemical surroundings are chemically equivalent. In a 1H NMR spectrum, each set of equivalent protons produces one signal.

Protons in different environments produce separate signals at different chemical shifts. The number of signals can therefore be used to determine the number of distinct proton environments in a molecule.

Ethanol, CHX3CHX2OH\ce{CH3CH2OH}, has three proton environments:

  • three equivalent CHX3\ce{CH3} protons
  • two equivalent CHX2\ce{CH2} protons
  • one OH\ce{OH} proton

A low-resolution spectrum of ethanol therefore contains three sample signals.

Using symmetry

Symmetry can make hydrogen atoms equivalent even when they occur in separate groups in a displayed formula. Always consider molecular symmetry when counting proton environments.

2-methylpropane

Symmetry makes the three methyl groups equivalent, so their nine protons form one environment. The proton on the central carbon forms the second environment.

Number of proton environments: 2

1,2-dichloroethane

For ClCHX2CHX2Cl\ce{ClCH2CH2Cl}, the four hydrogen atoms are equivalent at this level of analysis, so they produce one proton signal.

Number of proton environments: 1

Exam Tip: Count sets of equivalent protons, rather than individual H atoms. Check for symmetry before treating apparently separate groups as different environments.

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