Friedrich Gauss (1777-1855). In these units, Earth’s magnetic field
measures about 0.5 gauss, or 50 microteslas. A small magnet such as
those used to attach something onto a refrigerator door is about 50
gauss, or 5 milliteslas. Thus, the strengths of magnets used in NMR
spectrometers are quite substantial, well beyond anything normally
encountered on Earth.
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Figure 17.1. A toy top spinning in Earth’s gravitational field.If an organic molecule is placed in a strong magnetic field, the nu-
clear spins of the various atoms will align. Their alignment can be per-
turbed when just the right radio-frequency energy of electromagnetic
radiation is absorbed (the resonant frequency). The most abundant
type of atom in any organic molecule is hydrogen; different hydrogen
atoms in a given molecule will have different resonant frequencies,
depending on the precise nature of its electromagnetic environment
within the molecule. Thus, NMR may be used to help determine un-
known molecular structures of organic molecules, by looking at the
energies needed to perturb the alignment of the spins. This is a widely
used application of NMR spectroscopy in organic chemistry.
In a living organism, the most abundant atom is also hydrogen,
most of which will be in the form of water molecules. Thus, it is pos-