Electron Paramagnetic Resonance with a 2.8 MeV Van de Graaff Electron Accelerator

Electron paramagnetic resonance (EPR, also named electron spin resonance, ESR) is a spectroscopic technique in which free radicals absorb microwave energy when immersed in a static magnetic field. The discrete field strengths producing resonance at a fixed microwave frequency provide a high resolution description of radical structure by detailing the interactions between the radical's unpaired electron and the neighboring magnetic nuclei. This method of visualizing free radicals serves as an important adjunct to spectrophotometry, since the sensitivity of EPR does not rely upon the optical characteristics of the free radical. This is of great importance for radicals whose strong optical absorption bands are masked by spectra of nonradical species or are located in instrumentally inaccessible regions of the spectrum. EPR is inherently a selective methodology as well, since non-radical molecules containing only paired electrons are invisible to EPR. In resolution, sensitivity, and specificity, EPR is an excellent complement to continuous and kinetic optical techniques.

The Notre Dame Radiation Laboratory has been in the vanguard of EPR development for over three decades. In 1963 Fessenden and Schuler developed the in situ radiolysis EPR technique, in which a continuous electron beam from a Van de Graaff electron accelerator impinged directly upon a hydrocarbon sample contained within the sample cavity of the EPR spectrometer. The observation of intense alkyl radical signals provided the first proof that this spectroscopic technique could provide detailed structural information on transient free radicals that disappear in diffusion-controlled reactions. The continuous irradiation experiment was subsequently extended (1971, Eiben and Fessenden) to allow the use of aqueous solutions as the parent material from which the radiation-produced radicals were derived. This facilitated the study of radicals of biological interest. In addition, the use of aqueous solutions provides a well-defined environment for the production of free radicals, since the types and relative yields of radicals (hydroxyl, hydrogen atom, hydrated electron) produced by water radiolysis are extremely well defined. Time-resolved in situ radiolysis ESR studies were developed in these laboratories (1973, Fessenden) to permit the study of radical formation and decay kinetics. The high spectral resolution of EPR facilitates the deconvolution of complicated free radical systems into their components.

Supported by the Division of
Chemical Sciences
Office of
Basic Energy Sciences
at the
U.S. Department of Energy

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Radiation Laboratory
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Last Modified: 02/02/2006