Spin concentration measurements

ESR measurements were carried out using a Varian V-4502 X-band spectrometer with 100 kHz field modulation. Yarian dual cavity permitted g values and spin concentration measurements by comparison with a standard DPPH (diphenyl picryl hydrazine) sample. Measurements were carried out at room temperature or at 77°K. 

Thus the relative concentration is directly proportional to the "peak-to-peak" height of the derivative, provided the line is not saturated. In such cases, an accuracy of 2% can be expected. This is in marked contrast with the poor accuracy of "absolute spin concentration" measured by the comparison method, which can involve errors ranging from 20 to over 100%, depending on the system and the standard used. 

Benzene is most difficult to convert to an anion radical nevertheless, with potassium at — 80°C in DME the anion radical of benzene can be identified by ESR. Spin-concentration measurements on benzene in 2 1 by volume of THF DME with Na-K alloy at — 83°C show that at equilibrium only ca. 0.1 % benzene is converted to radical anion. Benzene also forms an anion radicaF by ESR with Rb and Cs in THF and DME, but not with Na or Li in THF-DME. Alkyl substitution destabilizes the radical anions as prepared with Na-K in THF-DME at — 100°C, with destabilization increasing with size [CH3 < CH CHj < CH(CH3)2 < C(CH3)3] and with numbers of such groups . In contrast, Si(CH3)3, Ge(CH3)3, CN and NO2 groups stabilize their benzene radical anions. Radical-anion formation even in benzene, toluene, or mesitylene with potassium can be brought about by addition of dicyclohexyl-18-crown-6 , 18-crown-6 or [2.2.2]cryptate at RT or lower. Radical anions may be prepared similarly from Rb and Cs, but not from Na with 18-crown-6 or [2.2.2]cryptand . The yield or how long such solutions are stable is not known. 

Spin Concentration Measurements, g-Values and Spectral Linewidths... 

A different approach for analyzing the absorption edge of a-Si films was suggested by Brodsky etal. (1972b). These authors used the spin concentration measured by ESR as a measure of the film perfection, and extrapolated the optical spectra to zero spin concentration. For a-Si they found a sharp edge at 1.8 eV. 

Using this ESR approach, Manivannan et al. (1999) arrived at important and relevant conclusions. They examined three activated carbons, made from coconut and a wood, of known surface areas, supplied by PICA (USA Inc) and measured their free spin concentrations. Measurements were made both in air and in evacuated specimen tubes. Results are summarized in Table 2.4. 

The absolute number of unpaired spin concentration measurements are made by comparison of the integrated intensitiy of the signal from the unknown sample with that from a standard sample with a known number of unpaired electron spins. 

The dual cavity allows the use of samples with similar filling factors and positions in the radio-frequency field. The change of sample is also simple. Differences in the two signal measuring channels are eliminated by exchange of samples. Spin concentration measurements made by relative comparison with a secondary standard can be carried out with an accuracy of 5-10%. 

First, there is a definite correlation between the spin concentration measured by ESR and the chemical structure of monomers. Glow discharge polymers of monomers that... 

Experiments on catalysts often require the measurement of an absolute spin concentration. Although this is a somewhat tedious task, it can be carried out to a moderate degree of accuracy. The number of spins per gram of unknown n is given by... 

ESR Studies. Jackson and Wynne-Jones (9) studied the ESR spectra from the chars of a number of polymers and found a correlation between d.c. resistivity and the free electron spin concentration but no correlation with C/H ratio. The g-values they measured were observed to be quite close to that of the free electron, i.e. 2.0026 for cellulosic char vs 2.0023 for the free electron. They concluded that it was not possible to determine whether the observed free spin was due to a or -n electrons. 

Because field-gradient spin-echo measurements of D depend on no driving force such as a concentration, temperature, or velocity gradient, etc., they reflect Brownian motion of the molecules in the laboratory reference frame, and are usually referred to as self-diffusion. These attributes are further discussed in Sections 2 and 3. 

Spectra. All EPR measurements were made with a Varian 100 kc. modulation spectrometer. Spin concentrations were estimated by comparison with solid diphenylpicrylhvcurazyl. Numbers of radicals were assumed to be proportional to signal height times width squared. Temperature dependence plots were obtained by controlling the temperature of the resonance cavity with cooled or heated nitrogen. Signal intensities of the temperature plot were estimated by comparing with the intensity of the sample at 25°C. 

In order to obtain absolute measurements of spin concentrations, a series of standards was made up, which consisted of known weights of diphenyl-picrylhydrazyl (DDPH) diluted with potassium chloride (the mixtures were mended in a vibrating ball mill). A suitable standard was run immediately after each unknown at identical settings of instrument gain so that recorder traces of standard and unknown could be directly compared. 

Tests for the skin effect were made on a few of the samples by making measurements on macerals diluted to various extents with potassium chloride (16). The number of spins per gram of coal was found to be independent of dilution, and so there is no evidence of a skin-effect. It was established that saturation of the energy transitions was not occurring by varying the micro-wave power by a calibrated attenuator the spin concentration found did not depend on power in the range covered (5-30 milliwatts). 

The EPR spectra, recorded at — 196°C or at 25°C, were taken with Varian E6S and V4502 spectrometers for X-band (9.1 GHz), and Q-band (35 GHz) measurements, respectively. The g values were evaluated by using a 2,2-diphenyl-l-picrylhydrazyl (DPPH) standard, with a g value of 2.0036. Spin concentrations were obtained by using a single crystal of freshly recrystallized CuS04-5H20 as a standard. The estimated error in spin concentration is 30%. 

Fig. 5.20 Changes in drug proton relaxation rate (1 /T2) as a function of increasing concentration of bovine brain phosphatidylserine (BBPS) alone and after addition of CaCI2- The spin systems measured are indicated by arrows on the drug structures. (Reprinted from Fig. 6 of ref. 117 with permission from Bertelsmann-Springer.)...

Although quantitative measurements of spin concentrations are very important in kinetic studies, the determination of this absolute parameter by ESR remains most difficult. Nevertheless, this should not discourage a photochemist who is accustomed to similar problems, such as measurements of photoexcited triplet-state concentrations in flash photolysis. 

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