4.   Low energy – the radio region

Direct measurements of microwave absorption give indication of molecular structure. Those measurements have very little application in chemical analysis.

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.

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