Field-swept EPR

Figure 16.5 (A) Field swept EPR spectrum of DNA I at 40 K with the excitation...

In powders, frozen solutions and even single crystals, many of the hyperfine and nuclear quadrupole splittings are typically not resolved in the field-swept EPR spectrum due to inhomogeneous broadening effects. In transition metal complexes, for example, often only flic largest hyperfine coupling from the metal ion is observed. This lack of resolution is mainly due to the transition selection rules, which show fliat the number of EPR lines increases multiplicatively,... 

This class of experiments involves measuring a field-swept EPR spectrum, either with CW excitation or using m.w. pulses. A variety of 2D pulse field-swept EPR experiments exist that aim to increase the resolution by the addition of a second dimension to the Bq sweep Ti and T2 filtered EPR [83], forbidden-transition-labeled EPR (FORTE) [84], anisotropy-resolved EPR [85], and magic-angle spinning EPR [86]. Here only two types of experiments are discussed nutation and electron Zeeman-resolved EPR. 

It is not always possible to evaluate the electron spin quantum number 5 of a paramagnetic species from the field-swept EPR spectrum. Often only the (l- /i), I+V2)) EPR transitions can be observed, or several species with dilFcrent S values contribute to the spectrum. Under suitable conditions S can be determined from the nutation frequency nw- If the m.w. radiation excites only a single transition then... 

In this review we have concentrated on explaining the basic mechanisms behind ENDOR and ESEEM spectroscopy. These two methods, along with field-swept EPR experiments, provide a means to obtain a detailed description of the EPR parameters of paramagnetic centers in single crystals, powders, and frozen solutions. To obtain the most accurate EPR parameters requires not one technique, but a combination, and preferably applied at several m.w. frequencies. Measurements at multi-frequencies allow possible ambiguities that arise from data measured at only one m.w. frequency to be resolved. 

In 1975, Aasa and Vanngard [19] pointed out that correct integrals of EPR spectra would be obtained from frequency-swept spectra, but because in EPR the magnetic field is usually swept, the spin-packet linewidth in field-swept EPR requires additional consideration. The linewidth has proper units of frequency (MHz), and the integral of the lineshape function with respect to frequency is normalized to one. The frequency to field change of variables is given by Eq. (5), where Vo is the microwave frequency and v(So) is the frequency separation of the two levels involved in the transition at magnetic field Bo [20,21 ] ... 

PUbrow JR, Sinclair GR, Hutton DR, Troup GJ. 1983. Asymmetric lines in field-swept EPR Ci looping transitions in ruby. J Magn Reson 52(3) 386-399. 

ESE-detected EPR spectroscopy has been used advantageously for the separation of spectra arising from different paramagnetic species according to their different echo decay times. Furthemiore, field-swept ESE... 

Note that, just like for the first-order expression in Equation 5.12 also the second-order expression in Equation 5.18 applies to field-swept spectra, and a different expression found in EPR textbooks (Pake and Estle 1973) applies to frequency-swept spectra. The effect of including a second-order contribution to the central hyperfine splitting is illustrated in Figure 5.7 on the spectrum of a not uncommon contaminant of metalloprotein preparations Cu(II) ion coordinated by nitrogens of tris-hydroxy-ethyl aminomethane or Tris buffer. 

In order to identify the spin multiplicity of the tris(carbene), field-swept two-dimensional electron spin transient nutation (2D-ESTN) spectroscopy was used. This technique is based on pulsed fourier transform (FT) EPR spectroscopic methods and is capable of elaborating straightforward information on electronic and environmental strucmres of high-spin species even in amorphous materials, information that conventional CW EPR cannot provide. The nutation spectra unequivocally demonstrated that the observed fine structure spectrum is due to a septet spin state. 

In the spin-correlated RP the two radicals interact via electron-electron dipolar and exchange interaction which leads to line splitting. The ET process creates the RP in a strongly spin-polarized state with a characteristic intensity pattern of the lines that occur either in enhanced absorption (A) or emission (E).144 145 The spectrum is therefore very intense and can directly be observed with cw EPR (transient EPR) or by pulse methods (field-swept ESE).14 To study the RPs high field EPR with its increased Zeeman resolution proved to be very useful the first experiment on an RP was performed by Prisner et al. in 1995146. From the analysis of the RP structure detailed information about the relative orientation of the two radicals can be extracted from the interaction parameters. In addition kinetic information about the formation and decay of the RP and the polarization are available (see references 145,147). 

Experimentally the continuous-wave (CW) EPR experiment is a field-swept experiment in which the microwave frequency (Vc) is held constant and die magnetic field varied. Computer simulations performed in field space assume a symmetric lineshape function,/in Eq. (3) (J(B - B es), CTg), which must be multiplied by dv/dB and assume a constant transition probability across a given resonance [1,29]. Sinclair and Pilbrow [30,31] have described the limitations of fliis approach in relation to asymmetric lineshapes observed in high-spin Cr(III) spectra and the pres-... 

All interactions apart from the electron Zeeman interaction have their own units. For the hyperfine interaction these are MHz and 10" cm" and can be selected from the Units drop-down list. Since the hyperfine couplings Alg/J) measured directly from the field-swept CW EPR spectrum are dependent upon the g-value, it is far easier to determine -values if the simulation employs frequency units as the g- and -values are then independent. Consequently, we have provided a units calculator, accessible from the main Tool bar, to convert Gauss and mT into units of frequency. 

The field-swept electron spin echo (FS-ESE) spectrum (which yields a spectrum similar to that acquired by a CW experiment, but is acquired using a simple two-pulse sequence) of VO(ema)2-treated rat bone yielded spin Hamiltonian parameters quite different from those obtained with the other two vanadyl compounds (Table 4). As with VO(pic)2, it was clear that the eoordination state of the orally administered VO(ema)2 had changed [95]. Despite the difference in spin Hamiltonian parameters determined from CW-EPR (VOSO4, VO(pic)2) and FS-ESE (VO(ema)2) spectra, it appears that all three compounds share the same metabolic fate after uptake into bone mineral. 

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