Electron resonance field position

Fig. 14. Electron spin resonance spectrum of a frozen solution of rubidium in HMPA, at high machine amplification. The full lines show the variation of resonant field position with A for ge = 1.99800, and a microwave frequency of 9.1735 GHz. The lines are anchored at the crossovers of the MG species (A = 251.3 G). Positions of the Mc, M , ME, Mg, Mh, and M, absorptions are indicated. Reprinted with permission from R. Catterall and P. P. Edwards, Journal of Physical Chemistry, 79, 3010 (1975). Copyright 1975 American Chemical Society.

EPR spectrometers use radiation in the giga-hertz range (GHz is 109 Hz), and the most common type of spectrometer operates with radiation in the X-band of micro-waves (i.e., a frequency of circa 9-10 GHz). For a resonance frequency of 9.500 GHz (9500 MHz), and a g-value of 2.00232, the resonance field is 0.338987 tesla. The value ge = 2.00232 is a theoretical one calculated for a free unpaired electron in vacuo. Although this esoteric entity may perhaps not strike us as being of high (bio) chemical relevance, it is in fact the reference system of EPR spectroscopy, and thus of comparable importance as the chemical-shift position of the II line of tetra-methylsilane in NMR spectroscopy, or the reduction potential of the normal hydrogen electrode in electrochemistry. 

Because of the diverse and complex structures of organic molecules, the shielding effects of electrons at various positions are generally different. A careful measurement of the field strengths required for resonance of the various protons in a molecule provides us with two important types of information ... 

In metals 4f ions are much better ESR probes than 3d ions due to fact that the exchange parameter Jie is about ten times smaller for the exchange interactions between 4f electrons and the band states. The solubility of the 4f ions in many metallic elements is sufficient to perform ESR experiments and at least at low temperatures, narrow absorption lines can be expected. In most cases line positions and linewidths are determined by CF effects in addition to the well-known Korringa behavior. Again the ions with an S ground state like Gd " and Eu " are the preferred probes for ESR investigations. They warrant small crystalline-field influences on the linewidth and on the resonance field. This is the reason that by far most of the publications deal with Gd ". ... 

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