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

NMR Theory

http://vam.anest.ufl.edu/forensic/nmr.html

• 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.

Effect of B0 on resonance frequency

Schematic of an NMR

• 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.

Resonance Frequency

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

1H NMR—An Example Spectrum

• 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.

Chemical Shift

Electronic Shielding

Shielding in Spectrum

Example C-13 Spectrum

C-13 Number of Peaks

Chemical EquivalenceWhen two nuclei give the same peak because they have the same chemical environment they are said to be chemically equivalent.

Coincidental Equivalence