NMR Theory

1
NMR Theory

• There are 2 variables in NMR an applied magnetic
  field B0, and the frequency (? ) of radiation
  required for resonance, measured in MHz.

http//vam.anest.ufl.edu/forensic/nmr.html
2
Effect of B0 on resonance frequency

• NMR spectrometers are designated according to the
  frequency required to make protons resonate. The
  modern standard is 300 MHz. However,
  manufacturers are actively pursuing stronger
  magnets. 900 MHz is currently as high as it gets.

3
Schematic of an NMR
4
Resonance Frequency

• Different types of nuclei resonate at
  significantly different frequencies. Example on
  a 300 MHz instrument (1H approx 300 MHz) 13C
  resonates at approx 75 MHz.
• However, the same type of nucleus also absorbs at
  slightly different exact frequencies, depending
  on its chemical environment.
• Exact frequency of resonance chemical shift
• The magnitude of the magnetic field actually felt
  by a nucleus (Beff) determines where it absorbs.
• Electron clouds shield the nucleus from the
  magnet
• Circulation of electrons can generate local
  magnetic fields that influence Beff
• Modern NMR spectrometers use a constant magnetic
  field strength B0, and pulse a broad range of
  frequencies to bring about the resonance of all
  nuclei at the same time.

5
1H NMRAn Example Spectrum

• An NMR spectrum is a plot of the intensity of a
  peak against its chemical shift, measured in
  parts per million (ppm).

6
Chemical Shift

• NMR resonances appear as sharp peaks.
• Chemical shift is measured in ppm
• ppm ? in Hz relative to ref peak/instrument ?
  in MHz.
• Protons absorb between 0-10 ppm. C-13 nuclei
  absorb between 0-250 ppm.
• The terms upfield and downfield describe the
  relative location of peaks. Upfield means to the
  right. Downfield means to the left.
• Reference peak 0 ppm tetramethylsilane (TMS).
  TMS is a volatile inert compound that gives a
  single peak upfield from typical NMR absorptions.

7
Electronic Shielding
8
Shielding in Spectrum
9
Example C-13 Spectrum
10
C-13 Number of Peaks
11
Chemical Equivalence
When two nuclei give the same peak because they
have the same chemical environment they are said
to be chemically equivalent.
12
Coincidental Equivalence