NMR Spectroscopy

Most Common And Important Nuclei

Nucleus

MHz

Abundance

Rel Sensitivity

13C

68

1.1

1.59

1H

270

100

100.00

31P

108

100

6.64

19F

256

100

83.00

15N

41.2

0.37

0.10

2H

54

0.1

0.96

*at 6.3 Tesla, or 63000 gauss

The action of magnetic field on nuclei with spin

Technically, this diagram only applies to nuclei with a spin of 1/2. When a magnetic field is applied to these nuclei, they find one of two allowed states: aligned with the magnetic field, or aligned against it. These differ in energy very slightly, with a difference which depends on the strength of the magnet. Even with a very strong magnet applied to the sample, the difference in energy is only a fraction of a kcal/mole. These energy gaps correspond to photons of light in the radio frequency range. In fact, many commonly encountered radio transmitters have frequencies in these ranges, including cell phones, cordless phones, FM radio stations, and police radios.

The magnetic field strengths shown span the currently available magnets used for NMR. The numbers in the gaps are the resonant frequencies for the nuclei, protons (1H) on the top and carbons (13C) on the bottom. Often, chemists use the appalling, but useful, shorthand of referring to the magnet's strength by mentioning its resonant frequency for hydrogens. Hence, our spectrometer, with a 4.7 Tesla magnet, is referred to as a 270 MHz instrument, even though it can look at scores of elements, all at different frequencies.

The first commercially available machines were called 60 MHz instruments; they had 1.5 tesla magnets. With the advent of superconducting magnets, field strengths grew past the 100MHz barrier, and are now pushing 10 times that in experimental systems (900 MHz is about the limit now).

Chemical Shift.

Position of signals:

d is a unitless expression designed to be independent of field strength. It is expressed in "ppm" or parts per million of reference absorbance frequency.

The reference for carbon and hydrogen is TMS, or tetramethylsilane ((CH3)4Si), set to 0.

Web References

People who want more information about NMR might try visiting these sites. Caution: I have not tried all of these, so this is not necessarily an endorsement.

[This list was last updated on 3/4/98]

For general information on nuclear magnetic resonance, explore the NMR Information Server at the University of Florida, University of Akron's MAG-NET (an impressive guide to the world of NMR), or the National Magnetic Resonance Facility at the University of Wisconsin, Madison.