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NMR Theory: Applied Magnetic Field and Resonance Frequency

Understand the basics of NMR theory, including the relationship between the applied magnetic field and resonance frequency. Learn about different types of nuclei, chemical shift, and the effects of electronic shielding.

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NMR Theory: Applied Magnetic Field and Resonance Frequency

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  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 NMR—An 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 = ν in Hz relative to ref peak/instrument ν 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

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