The ESR spectrometer

The ideas sketched in this section are treated in depth in the numerous textbooks on In particular, the book by Symons provides 

This section is concerned with describing in detail contemporary methodology for electrochemical ESR. Experimental designs for two aims will be considered the detection of short-lived electrogenerated intermediates and the investigation of the kinetics and mechanism of their decay. 

A particular problem arises when aqueous solutions are studied because of the high dielectic absorption of water in the microwave region. In order to prevent extensive microwave absorption, and hence loss of sensitivity, a region of the cavity is selected where the electric component is low but the magnetic component is high. For a standard H0i2 cavity, thin cells holding ca. 0.1 ml are used. These can be made demountable for tissue studies or can be part of a flow system. 

As with NMR spectroscopy it is sometimes helpful to measure spectra over a range of frequencies. The most popular second choice is Q-band (ca. 35 000 MHz, i.e., ca. 3.5-times X-band frequencies). In principle, going to higher frequencies should increase the sensitivity, because of the Boltzmann factor, but in practice this is about compensated by the small size of the cavity resonator, and hence the need for small samples. This can, of course, be a great advantage if only small samples are available, as, for example, is often the case if isolated me-tallo enzymes are being studied. The next most popular frequency is S-band (ca. 3000 MHz, or ca. 1 /3 that of the X-band). Sensitivities are lower, but for aqueous systems, using loop-gap resonators, this may not be a serious factor. 

Spin-echo methods are less widely used, but certainly rival the ENDOR method, and the sensitivity can be much greater. The method has been most widely developed in studies of transient species produced in flash photolysis or pulse radiolysis experiments. At this stage, the great advantages found in NMR spectroscopy have not been fully realized in ESR spectroscopy, but time will tell. 

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For routine studies with the ESR spectrometer, it is most convenient to work at X-band frequencies ( 9.5 MHz or 3 cm). The sample is usually contained in a 4 or 5 mm diameter quartz tube having a sensitive region about 2 cm in length. An alternative frequency is at Q-band ( 35,000 MHz or 1 cm). Here, the cavity dimensions are much smaller and the diameter of the sample tube is less than 2 mm. This creates some problems in handling and degassing powder samples. By varying the frequency it is possible to determine which features in a spectrum are due to Zeeman interactions... 

The radical-anions of aliphatic nitrocompounds are detectable in aqueous solution as transient intermediates formed during continuous electrolysis in the cavity of the esr spectrometer [4], Decay of the species occurs by protonation and then further reactions. 2-Methyl-2-nitropropane has no acidic hydrogens so that it can be examined in aqueous alkaline solution where the radical-anion is not protonated. Over the pH range 9-11, this radical-anion decays by a first order process with k = 0.8 0.1 s at 26 C. Decay results from cleavage of the carbon-nitrogen bond to give a carbon centred radical and nitrite ion. Ultimately, the di-(ferr,-butyI)nitrone radical is formed in follow-up reactions [5],... 

Acknowledgments This research was supported by a grant from the Research Corporation and by an NSF equipment grant DMR-8501362 for the purchase of the ESR spectrometer. Support of the ESR measurements at S-band was provided by a grant NIH-RR-01008 to the National Biomedical ESR Center. We thank Drs. J.S. Hyde and C Felix for their assistance with these measurements. 

Radical Generation. The ESR spectrometer, flow system, and general procedure have been described (46). The apparatus was calibrated with freshly prepared diphenyl picrylhydrazyl (DPPH) solutions. The peroxy radical concentrations were determined by double integration of derivative spectra. A standard coal sample in the dual cavity allowed corrections to be made for changes in cavity Q. The rates of decay of the less reactive radicals were determined by stopped-flow techniques with manually or electrically operated valves. The decay was recorded... 

A six- or four-line ESR spectrum that can be fitted to a triplet spin Hamiltonian is strong evidence that the species in the sample embodies two unpaired electron spins. Support for the presence of a triplet spin system often can be found in the weak Ams = 2 line, which appears at one-half the field strength of the center of gravity of the Ams = 1 six-line pattern. This nominally forbidden Amj = 2 resonance results when the ESR spectrometer field and frequency produce a micro-wave quantum of energy just sufficient to jump the gap between the uppermost and lowermost triplet substates, that is, a transition over two quantum levels. 

The ESR spectrometer used is the property of The National Aeronautics and Space Administration and was kindly made available by the Langley Research Center. 

Appropriate modifications of the ESR spectrometer and generation of free radicals by flash photolysis allow time-resolved (TR) ESR spectroscopy [71]. Spectra observed under these conditions are remarkable for their signal directions and intensities. They may be enhanced as much as one hundredfold and may appear in absorption, emission, or in a combination of both modes. These spectra indicate the intermediacy of radicals with substantial deviations from equilibrium populations. Significantly, the splitting pattern characteristic for the spin density distribution of the intermediate remains unaffected thus, the CIDEP (chemically induced dynamic electron polarization) enhancement facilitates the detection of short-lived radicals at low concentrations. 

From the point of view of the solvent influenee, there are three features of an electron spin resonance (ESR) speetrum of interest for an organic radical measured in solution the gf-factor of the radical, the isotropie hyperfine splitting (HFS) constant a of any nucleus with nonzero spin in the moleeule, and the widths of the various lines in the spectrum [2, 183-186, 390]. The g -faetor determines the magnetic field at which the unpaired electron of the free radieal will resonate at the fixed frequency of the ESR spectrometer (usually 9.5 GHz). The isotropie HFS constants are related to the distribution of the Ti-electron spin density (also ealled spin population) of r-radicals. Line-width effects are correlated with temperature-dependent dynamic processes such as internal rotations and electron-transfer reaetions. Some reviews on organic radicals in solution are given in reference [390]. 

Figure 6.8 Orientation of the Al foils sample in the TE 102 microwave cavity of the ESR spectrometer and the composition of the sample as a sandwich of the Al foils and the PARAFILM layers.

Several strips of coated film were exposed to light in the sample tube of the ESR spectrometer and developed. Figure 5 shows... 

Thus second-order corrections are important in ENDOR studies. For example, if (a/h) =10 MHz and the ESR spectrometer operates at 9.5 GHz, then the second-order term is 0.1 percent of the value of a in magnitude so that a precision of 0.01 MHz in the determination of a requires attention to second-order corrections. 

These values of quantum yield are indicative of a very efficient charge generation process, since they do not include corrections for the decay by recombination. Decay curves for the photosignal can be followed by setting the magnetic field of the ESR spectrometer at the derivative peak maxima of the signal and... 

Since the axis of the ion source is parallel to the axis of the electromagnet of the ESR spectrometer, 12, the electrons are collimated in such a way that, after passing through the channel, the intensity of the electron beam is still high even at low energies. For example, for a 5-e.v. electron beam, with very clean electrodes, we obtain currents as high as 50 / amp. If there is an insulating deposit on Electrode 7, the intensity is considerably decreased. 

After crossing the electron beam, the gas flow, which contains both the reaction products and the unreacted molecules, is condensed on a cold finger, 1, located in the cavity of the ESR spectrometer, 2. The temperature of the cold finger may be varied to 77 °K. The ratio of the pressures in the two compartments of the ion source is approximately 30, so that the amount of pyrolysis products trapped on the cold finger is very low. 

In-situ experiments where the generation of radicals is carried out within the cavity of the ESR spectrometer. 

Received January 24, 1975. Work supported by NSF grants GP-10538 and -40991X. The ESR spectrometer was purchased with the aid of NSF equipment grant GP-29184. 

For electrochemical applications, the experimental arrangement is rather simple. Because of the broad application of ESR, this method is treated first. Some additional information on NMR in electrochemistry can be found at the end of this section. In ESR experiments the spectrum can be recorded when the species under investigation is created either just inside the spectrometer (intra muros generation, subsequently treated as the in situ method) or outside the spectrometer (extra muros). In the latter case the sample has to be transferred by means of a flow apparatus or by removal of a small sample from the electrochemical cell, which is put into a standard ESR cuvet. For reasons already outlined in Chap. 4, the latter procedure, which is similar to an ex situ experiment, carries some inherent sources of error because of the limited lifetime or subsequent chemical reactions of the species initially created by the electrochemical reaction. Since no particular design of the cuvet is necessary with respect to the ESR spectrometer, the latter procedure will not be discussed in detail. 

Mu and Kadish have described an ECESR cell with a thin layer design suitable for measurements at both ambient and low temperatures [617]. The thin layer of electrolyte solution is enclosed by the quartz tube inserted into the microwave cavity of the ESR spectrometer and a solid quartz rod fixed in the center of the tube. An expanded platinum mesh in the gap is used as the working electrode. At low electrode potential scan rates, the cell shows an acceptable electrochemical response. 

A combination of ECESR and in situ UV-Vis spectroscopy has been proposed by Petr et al. [643]. In the case of a cell that was designed to be similar to the ECESR cell proposed by Piette [613], a UV-Vis spectrometer is coupled with the cell via fiber optics. The working electrode is of a minigrid type. A cell design with an electrochemical cell directly coupled with a cuvet fitting into the ESR spectrometer has been described by Friedrich and Baumgarten [644]. 

Although the detection limit for ESR spectroscopy per se is extremely low, the use of electrochemical cells filled with solvents that have high dielectric constants results in considerable losses in the cavity of the ESR spectrometer. This in turn increases the limit of detection. In the case of electrode reactions that have only very small stationary concentrations of radicalic intermediates, detection may be impossible. The use of spin traps may help. These compounds are rather simple organic molecules that react easily with radicals forming adducts (see Fig. 5.118). The molecular structure of the intermediate may be deduced from the known structure of the spin trap and the observed ECESR spectrum. Unfortunately, this technique doesn t necessarily trap the major reaction intermediate rather, it only traps those which react easily with the spin trap. Consequently, misinterpretations are possible. 

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