energy – the radio region
Direct measurements of microwave absorption give indication of molecular
structure. Those measurements have very little application in chemical
The major use of radio waves is in nuclear magnetic resonance (NMR) spectrometry.
That works because some atomic nuclei exhibit ‘nuclear spin’. Those are
primarily ordinary hydrogen (1H, protons) and secondarily 13C,
19F, and 31P. When in strong magnetic fields, they
absorb specific wavelengths of microwave radiation as the nuclear spin
axes ‘flip’ to the opposite direction.
A typical NMR spectrometer uses a fixed microwave frequency (e. g., 90
MHz). The magnetic field, around 21 kiloGauss, is varied by careful control
of the magnet current to give a record of absorbance vs. field. Because
individual protons in a molecule are influenced by adjacent atoms, they
absorb at different magnetic fields to produce the NMR spectrum. For the
convenience of the chemist, the spectrum is commonly presented as absorbance
vs. ‘chemical shift’. The chemical shift is the difference in field strength
from that at which a standard molecule absorbs. A very typical standard
is tetramethylsilane, TMS , Si(CH3)4, which has
12 identical protons. The magnitude of the shift indicates the environment
of the proton(s) inside the molecule, while the intensity of the absorption
indicates how many protons are in that environment.
Electron spin resonance (ESR) or electron paramagnetic resonance (EPR)
is a related technique used mostly for study of molecular structure rather
than for analysis.
Back to top