NUMBER
4 IN A S E R I E S
How can Electron Paramagnetic Resonance (EPR) provide a chem ist with information about free radical reaction mechanisms? First, he can detect transient free radicals in the course of a reaction. He can then identify the free radical by obtaining in formation concerning the molecular environment of the unpaired electron. In addition, he can make a quantitative measurement of the free radical concentration; and finally, measure this con centration as a function of time.
IDENTIFICATION OF FREE RADICALS In numbers 1-3 of this series, we have presented examples which illustrate the detection of free radicals using EPR. This example, No. 4, will discuss how EPR can identify free radicals.
Free radicals are often associated with molecules containing magnetic nuclei. One common example would be molecules con taining hydrogen. The hydrogen nucleus has a permanent mag netic moment. If the free radical electron in its orbit comes into proximity with a hydrogen nucleus, its energy is perturbed by the "additional" magnetic field associated with the magnetic mo ment of the hydrogen nucleus. The magnetic field for each hydrogen nucleus will either add or subtract from an externally applied laboratory magnetic field, thus resulting in a splitting of the electron resonance absorp tion into multiplets. The number of splittings depends upon the number of nuclei (n) interacting with the electron and the spin quantum number (I) of the nucleus. In the case where all nuclei are alike then there are (2nl + 1 ) splittings possible. The strengh of the "additional" magnetic field will, of course, de pend on how close the electron comes to the nucleus or, more correctly, the "spin" density at the magnetic nucleus. It is this interaction between the magnetic nuclei and the un paired electron which allows positive identification of the free radical. Two simple examples are illustrated in Figs. 1 and 2. Fig. 1 is the spectrum of a hydrogen atom where n = l , \=Vz yielding 2 lines with a separation between lines of — 500 gauss. Fig. 2 is the spectrum of the methyl radical where η = 3 and I=VS> yielding 4 lines and a splitting o f ~ 24 gauss.
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 or other examples in this series, please write: INSTRUMENT DIVISION.
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