Nuclear Magnetic Resonance

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1H NMR
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Introduction - NMR

• NMR is the most powerful tool available for
  organic structure determination.
• It is used to study a wide variety of nuclei
• 1H
• 13C
• 15N
• 19F
• 31P

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Nuclear Spin

• A nucleus with an odd atomic number or an odd
  mass number has a nuclear spin.
• The spinning charged nucleus generates a magnetic
  field.

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External Magnetic Field

• When placed in an external field, spinning
  protons act like bar magnets.

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Two Energy States

• The magnetic fields of the spinning nuclei will
  align either with the external field, or against
  the field.

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Magnetic Shielding

• If all protons absorbed the same amount of energy
  in a given magnetic field, not much information
  could be obtained.
• CH3 - CH2 - CH2 - Cl
• CH3- CH2- CH- -OH -NH

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Protons in a Molecule

• Depending on their chemical environment, protons
  in a molecule are shielded by different amounts.

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The NMR Graph
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NMR Signals

• The number of signals shows how many different
  kinds of protons are present.
• The location of the signals shows how shielded or
  deshielded the proton is.

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NMR Signals

• The intensity of the signal shows the number of
  protons of that type.

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NMR Signals

• Signal splitting shows the number of protons on
  adjacent atoms.

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Standard

• TMS is added to the sample.
• Since silicon is less electronegative than
  carbon, TMS protons are highly shielded. Signal
  defined as zero.
• Organic protons absorb downfield (to the left) of
  the TMS signal.

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Chemical Shift

• Measured in parts per million.
• Ratio of shift downfield from TMS (Hz) to total
  spectrometer frequency (Hz).
• Same value for 60, 100, or 300 MHz machine.

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Location of Signals

• More electronegative atoms deshield more and give
  larger shift values.

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Location of Signals

• Effect decreases with distance.
• CH3-Cl 3.5 ppm
• CH3-CH2-Cl 3.0 ppm
• CH3-CH2-Cl 2.6 ppm
• CH3-CH2-CH2-Cl 2.0 ppm

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Location of Signals

• Additional electronegative atoms cause increase
  in chemical shift.
• CH3-CO-Cl 2.8 ppm
• Cl-CH2-CO-Cl 4.1 ppm

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Typical Values
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Spin-Spin Splitting

• Non equivalent protons on adjacent carbons always
  interact each other.
• Equivalent protons do not not split each other.
• CH3 CO - CH3 Do not split
• CH3 CH2 - Cl Split each other

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Spin-Spin Splitting
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Spin-Spin Splitting
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Spin-Spin Splitting
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Spin-Spin Splitting

• If a signal is split by N equivalent protons, it
  is split into N 1 peaks.

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Spin-Spin Splitting

• Equivalent protons do not split each other.
• Protons bonded to the same carbon will split each
  other only if they are not equivalent.
• CH3-C-H

H
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Spin-Spin Splitting

• Protons on adjacent carbons normally will couple.
• Protons separated by four or more bonds will not
  couple.
• CH3-CH2-CH2-CH2-Cl

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Coupling Constants

• Distance between the peaks of multiplet measured
  in Hz called coupling constant.
• Not dependent on strength of the external field

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Typical coupling constant
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Spin Decoupling

• Its a powerful tool for determining
• 1. The connectivity of the protons.
• 2. Assigning proton peaks

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Spin Decoupling

• Irradiation of one proton in a spin coupled
  system removes its coupling effect on the
  neighboring protons to which it had coupled.

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