A compact general theory for the effect of Heisenberg spin exchange on ESR linewidths and saturation parameters is detailed. The effects of Heisenberg exchange on the linewidths of the tetracyanoethylene anion (TCNE-) radical and the di-tert-butyl nitroxide (DTBN) radical in both dimethoxyethane (DME) and tetrahydrofuran (THF) are investigated. From comparative studies of linewidth as a function of temperature and of radical concentration, TCNE-in DME is shown to undergo strong exchange with a secondorder rate constant of 4.1±0.6X10 9 M-I· sec-I at 15°C. The TCNE-radical in THF exhibits an anomalous concentration-dependent linewidth effect when compared to the theory and to the experiments employing DME as the solvent. The uncharged DTBN radical shows similar spin-exchange properties in both solvents. Possible mechanisms for the anomalous linewidth effect are discussed. The effect of spin exchange on the saturation parameters of the TCNE-radical in DME is investigated in detail, and the experimental results are shown to agree, within experimental error, with the theory developed. Electron-nuclear dipolar and electron-electron dipolar relaxation effects are discussed in terms of their (small) contributions to the experimentally determined relaxation times. Present address:

Careful continuous saturation measurements, which include corrections for nonuniform microwave and modulating fields, have been used to examine, at different temperatures, the ESR linewidths and saturation behavior of the benzene anion in a tetrahydrofuran: dimethoxyethane solvent and the tropenyl radical in molten bitropenyl. It has been found that the ratios of the longitudinal to transverse electron-spin relaxation times (T1 / T2) for these free radicals in solution are about 1.1 ± 0.1 for the benzene anion below − 60°C and for tropenyl at 165°C. Analysis of the results has shown that at 9.1 GHz, g-tensor, anisotropic dipolar, and spin–rotational relaxation mechanisms do not contribute appreciably to the observed times. The anomalously small relaxation times associated with these radicals have been attributed to effects involving the degenerate ground states of these radicals. The spin-relaxation behavior of peroxylamine disulfonate anion in aqueous solution has also been studied.

The detailed requirements for the accurate determination of electron spin concentration by electron paramagnetic resonance spectroscopy are examined, and the use of the dual-sample cavity for measuring spin concentrations is reviewed.The lack of adequate solution-phase concentration standards is examined, and a new method based on photometric titration techniques is given for determining piperidine nitroxide concentrations in polar solvents. The titration procedure is used to determine the molar absorptivity of several common piperidine nitroxides in water and in water-methanol solutions. INTRODUCTIONOne important, but thorny, experimental problem in electron paramagnetic resonance (EPR) spectroscopy is the determination of the concentration or of the absolute number of electron spins in a sample. All methods for this require a standard or known sample (I, p. 443), yet no universal spin concentration standard is available. This problem was highlighted at the recent 14th International Symposium on EPR, at which an entire session was devoted to issues involved in arriving at standards in EPR (2).Methods to determine the number of electron spins in a sample by EPR have been in place since the discipline matured in the early 1960s (I, p. 443); however, many of these techniques and the associated lore have been lost to the expanded EPR community. Here we give a review of the principles of spin concentration determination, and we offer a new technique for the establishment of convenient solution concentration standards that is of special significance to biochemical applications. 145Kooser, Kirchmann, and Matkov GENERAL CONSIDERATIONSThe fundamental principle involved in determining the number of spins in a sample is the well-known fact that the integrated intensity of a spin resonance (the area under the resonance curve, or the first moment) is proportional to the spin density of the sample. The problem in EPR is that the intensity of a resonance is a complex function not only of the spin concentration of the sample, but it is also at the mercy of a host of instrumental and sample conditions (3, 4 ) . The general relationship can be given by Eq. [l] (3):[XI is the spin concentration, M is the first moment, G is the apparent amplifier gain as determined from the instrumental settings, A is a function of the true amplifier gain and the modulation field, and B is a factor related to the microwave bridge alignment. It is the terms A and B that present the,experimental difficulty. For instance, it is known that the sample tube causes a distortion of the microwave magnetic field seen by the sample inside (5) so that different sample containers will produce different microwave fields at the sample. Other variables affect the B term in Eq. 111, such as the cavity quality factor (I, p. 167) and the cavity filling factor ( 4 ) . The quality factor depends on what type of sample is inserted into the EPR cavity. Also, most EPR experiments are done using microwave cavities that have modulation coils that are embedded in their walls...

The chemically modified surface plays an Important role In retention In reversed-phase liquid chromatography. One surface factor that might affect retention Is solute expulsion due to partial ordering of the grafted chains at sufficiently high grafting densities. This study has shown by the use of the electron paramagnetic resonance technique of spin probing that spectroscopic evidence exists for an Increase In molecular order of the bonded octadecyl phase as a function of grafting density. The spin probe, DOXYL-cholestane, shows significant Increase In the order parameter, S, as a function of Increased surface coverage. In addition, DOXYL-cholestane undergoes a decrease In rotational freedom and an Increase rotational anisotropy as the grafting density Increases.

An alteration of the classic complexometric titration of calcium and magnesium with EDTA was made to incorporate a laser photometer for endpoint detection. The photometer utilized a He-Ne laser, which allowed the red to blue transition of the calgamite indicator to be followed by monitoring the change in beam absorbance. A photodiode and an inexpensive computer interface were used to record the intensity of the transmitted laser beam as a function of time. This method makes the experiment much more accessible for students suffering from color blindness.