ESR spectrometer

The spin adducts of free radicals and MNP or DMPO were observed by means of an ESR spectrometer. The data of hyperfine splitting constants were compiled in Tables 9 and 10 [40-42,44,45]. ESR studies on the initial free radicals revealed that the monoalkylamino radical RHN-, dialkylamino radical R2N-, and aminomethyl radical -CH2N< or aminoethylidene radical >N( CHCH3) were obtained from the corresponding primary, secondary, and cyclic tertiary amine. In case of a tertiary diamine such as TMEDA, formation of... 

The initial radicals formed from the reaction of Ce(IV) ion and reductant systems can be trapped by MNP. The spin adducts of the initial radicals and MNP were observed by means of an ESR spectrometer. The structure of the initial radicals and the hypeiTme splitting constants of the spin adduct of the radical with MNP and tt-phenyl-N-/er/-butylnitrone (PBN) are compiled in Tables 5 and 6, respectively. 

Figure 3. Controlled-potential electrolysis cell for generation of radical ions in the cavity of esr spectrometer [from (16) by permission of the authors and the American Chemical Society]. 

ESR spectra were determined with a Bruker ER-420 ESR Spectrometer using accessories for solid samples, ultraviolet irradiation in a room temperature. 

Although X-band is by far the most common, ESR spectrometers are available commercially or have been custom built in several frequency ranges (Table 1.1). 

This chapter concludes with a brief description of one advanced technique, Electron Nuclear Double Resonance (ENDOR), the capabilities for which, unlike pulsed methods, may be added as a relatively minor modification to commercial CW ESR spectrometers. 

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... 

Noda Y, Anzai K, Mori A, Kohno M, Shinmei M and Packer L. 1997. Hydroxyl, end superoxide anion radical scavenging activities of natural source antioxidants using the computerized JES-FR30 ESR spectrometer system. Biochem Mol Biol Int 42(1) 35—44. 

Electrochemical methods for the generation of anion-radicals consist of potential-controlled electrolysis. The control of a potential allows one to detain rednction just after a one-electron transfer to a depolarizer. The one-electron natnre of the electron transfer is coincidentally inspected by means of coulombometry. One molecnle mnst consnme one electron. If less than one electron is consumed in the framework of the one-electron rednction, it means that the yield of an anion-radical is not quantitative. The electrolysis in a special amponle placed into a resonator of the electron spin resonance (ESR) spectrometer permits one to identify many nnstable anion-radicals. The electrochemical methods of anion-radical generation employ an electrode as an electron donor. 

The first intermediate to be generated from a conjugated system by electron transfer is the radical-cation by oxidation or the radical-anion by reduction. Spectroscopic techniques have been extensively employed to demonstrate the existance of these often short-lived intermediates. The life-times of these intermediates are longer in aprotic solvents and in the absence of nucleophiles and electrophiles. Electron spin resonance spectroscopy is useful for characterization of the free electron distribution in the radical-ion [53]. The electrochemical cell is placed within the resonance cavity of an esr spectrometer. This cell must be thin in order to decrease the loss of power due to absorption by the solvent and electrolyte. A steady state concentration of the radical-ion species is generated by application of a suitable working electrode potential so that this unpaired electron species can be characterised. The properties of radical-ions derived from different classes of conjugated substrates are discussed in appropriate chapters. 

The decay of a radical-anion can be followed directly by generating the intermediate within the cavity of an esr spectrometer through application of a controlled potential pulse to the cathode of a thin electrochemical cell [46]. Loss of the radical-anion is then followed by decay of the esr signal. Decay is second order in radical-ion concentration for dimethyl fumarate (k = 160 M s ) and for cin-namonitrile (k = 2.1 x 10 M s ) in dimethylformamide with tetrabutylammonium counter ion. Similar values for these rate constants have been obtained using purely electrochemical techniques [47]. 

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... 

The hyperfine interactions of proton nuclei with the unpaired electron in a radical splits the proton signals into doublets with large coupling constants (up to 100 MHz) in comparison to the J values of 10 Hz observed in NMR spectroscopy. For the small magnetic field employed in an ESR spectrometer, all uncoupled protons absorb at effectively the same rf. The ENDOR spectmm appears as a series of doublets with lines spaced equally upheld and downfield from the expected absorbance of an uncoupled proton. 

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. 

A wholly different setup is required for ESR smdies of matrix isolated open-shell species, because the cavities of ESR spectrometers are too narrow to accept the standard vacuum shrouds. Therefore, these shrouds have to be fitted with a small extension mbe at the bottom into which a sapphire rod can be lowered after the gas sample mixture is deposited on that rod. This modification requires in turn a special shroud that can be raised and lowered without breaking the vacuum. Such equipment is available commercially and is described in various books and reviews on matrix isolation. 

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

Development of Dosimeter Materials. - 4.2.1 Quantitative ESR and Intensity Standards of Mn1+ and CuS04.5H20. An ESR spectrometer consists of vacuum tube amplifier and a Klystron oscillator with thermal noise and frequency drift. Hence, an inherent standard of Mn2+ was used in ESR dating of carbonate stalactites,8 and patented as a standard in ESR radiation dosimetry in 1980.102 The standard sample of MgO with Mn2+ is frequently used for calibration of -factor and the magnetic field as well as for radiation dosimetry.103... 

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