NUMBER
3 IN A SERIES
Chemists seeking to detect the presence of free radicals in chemical systems will find EPR Spectrometry the most sensitive and rapid technique available. The ability of Varian EPR Spec trometers to detect as low as 2 χ 10" free radicals (10" s molar concentrations), and to respond in times less than 100 micro seconds, has rendered all other techniques obsolete. This ability to detect, and in many cases to identify free radicals is inherent in the basic phenomenon of Electron Paramagnetic Resonance, since EPR Spectrometers respond only to chemical systems containing unpaired electrons.
UNIVALENT OXIDATION-REDUCTION BY ELECTROLYSIS NO, Hg
+ le
-> Calomel
NH,
Cyclic voltammetry can be an effective tool for studying univalent oxidation-reduction reactions of organic molecules, provided one observes a split wave in the typical current-potential curve. Very often these split waves are not resolved and an uncertainty arises as to whether one or multiple electron steps are occur ring. EPR has eliminated the necessity of observing split waves in such measurements by direct detection of the one-electron intermediate (free radical ion) resulting from a univalent oxida tion-reduction.
In situ generation and detection of univalent intermediates in an EPR cavity allow not only positive detection of intermediates in a complex organic electrode reaction, but also permit direct study of the nature of the intermediate. Fig. 1 illustrates typical one-electron intermediates as detected by EPR in the in situ electrolytic reduction of ortho, meta and para nitroaniline in 0.1M KC1 solutions. Attention is called to the radically different EPR spectra obtained for these three intermediates whose molecular structures differ only by the position of substitution of the amino group. It is this difference in the number of lines and spacings of lines (hyperfine pattern) that allows positive identification of these intermedi ates. How to identify free radicals by means of their "EPR hyper fine pattern" will be discussed in a later number of this series. ρ-NITROANILINE
Detection and identification of free radicals are not the only results obtainable from the EPR spectrum, however. It is also possible to measure the rate of free radical formation for studies of complete reaction kinetics. Varian EPR Spectrometer systems and accessories are designed for a wide range of applications in the fields of chemistry, biology, medicine and physics. For additional information about the example above and other chemical applications of EPR, please write: INSTRUMENT DIVISION.
VARIAN PALO ALTO
associates
4,
CALIFORNIA
