Q-band microwave

FIGURE 9.3 Linewidth as a function of microwave frequency. The linewidth (FWHH) of the low-field gj-feature is plotted versus the frequency in L-, S-, X-, and Q-band. The left-hand panel is for the ferric low-spin heme in horse heart cytochrome c, and the right-hand panel is for the [2Fe-2S] cluster in spinach ferredoxin. (Data from Hagen 1989.)... 

In many systems ESR line widths of Cu(ll) depend on the microwave frequency. It was shown in some early papers that lines are broader at Q-band (35 GHz)" and narrower at S-band" 7 compared with those at X-band. This was our initial motivation for... 

Frozen-solution ESR spectra of Tc2G in mixed aqueous hydrochloric acid and ethanol provided data consistent with equal coupling of the unpaired electron to both technetium nuclei (101). IsotopicaUy pure "Tc (/ = 9/2) in 99Tc2Cl leads to a large number of lines in the X-band spectrum owing to second-order effects, in addition to the hyperfine lines presence for this dimeric axially symmetric system. The Q-band spectrum obtained at 77°K with a microwave frequency of 35.56 GHz exhibited fewer lines, and computer-simulated spectra were generated to correspond to the experimental spectrum withgit = 1.912, gi = 2.096, An = 166 x 10 4 cm"1, IAL = 67.2 x 10 4 cm 1, and gav = 2.035. 

Figure 18. Q-band EPR spectrum of the g perpendicular region of spinach plastocyanin. Field sweep was between 11,275 and 12,275 gauss while the microwave frequency was 34.282 GHz.

Traditionally, the available microwave frequency determines the resonance field For g = ge = 2.0023 (the isotropic g factor of a free electron) and a frequency of 9 GHz, the resonance field equals He = 0.3211 T (X-band), while 35 GHz correspond to He = 1.2489 T (Q-band). Note that EPR spectra are usually evaluated using a spin Hamiltonian, and orbital contributions are effectively accommodated in the g tensor. 

In Figure 12 we show the Q-band EPR spectra of Co-deuterolysin measured at 10 to 5 K. Preliminary observation of the X- and S-band EPR (9 and 3 GHz, respectively) showed an axial feature but line-broadening prevented us from identifying the g principal values from the spectrum. Resolution improvement with the respect to g values is one advantage of use of a high microwave frequency. From the spectra at 33.9570 GHz, g principal values could be unambiguously measured. 

When EPR data are taken at X-band (9,500 MHz) the intensities of the so-called forbidden transitions, d here, are small and often neglected. This amounts to using only the first two terms in Eq. (18-1). However for small samples it is often necessary to use higher microwave frequencies (K-band, 24,000 MHz, Q-band, 35,000 MHz, or V-band, 75,000 MHz). In these experiments it is important to include the third term in Eq. (18-1), the nuclear Zeeman term. 

Once work described here was completed on the nucleotides and nucleosides, it was not easy to extend this work to oligonucleotides. This step required years of work to produce even very small single crystals. The crystals turned out to be too small to use in the X-band Cryo-Tip apparatus described in Section 18.3.1. Thus, a great deal more time had to be devoted to building helium temperature apparatus at higher microwave frequencies (Q-band). This task has only recently been completed. Now we have the exciting new results discussed in Section 18.5.2. However, there is much that remains to be done. 

Several techniques allow further elucidation of the information contained in an EPR spectrum. Through single-crystal EPR it is possible to determine the orientations of the g and A tensors relative both to each other and to the internal coordinates of a structurally defined active site. The use of several microwave frequencies can be particularly informative. While the spectrum shown in Fig. 2 (middle) was taken with an X-band spectrometer (u = 9 GHz), Q-band (v 35 GHz) and S-band (u = 3 GHz) should also be employed. The high microwave frequency leads to increased resolution by spreading the g values over a wider magnetic field range with little effect on hyperfine splitting. Low... 

The resulting energy levels are depicted in Fig. la,b, where the energy is plotted against magnetic field. The resonance frequencies depend, of course, on the field. For experimental reasons, in EPR spectroscopy one keeps the frequency of the microwave field constant (usually close to 9 GHz, X-band, or to 35 GHz, Q-band), and varies the field Bq. The resulting transitions (corresponding to A = hv (microwaves)) are indicated with arrows, and displayed in a so-called stick spectrum (Fig. Ic). 

Here v is a (fixed) frequency of the microwave source (e.g. ca 9 GHz at X-band, 35 GHz at Q-band). The spectrum is taken as a first derivative of the absorption versus the ramping magnetic field Bz. According to the selection rules, only those transitions are observable (intense) for which the change in the quantum number of the projection of the spin angular momentum fulfils the condition... 

Electron spin resonance (ESR) spectroscopy is a very powerful and sensitive method for the characterization of the electronic structures of materials with unpaired electrons. There is a variety of ESR techniques, each with its own advantages. In continuous wave ESR (CW-ESR), the sample is subjected to a continuous beam of microwave irradiation of fixed frequency and the magnetic field is swept. Different microwave frequencies may be used and they are denoted as S-band (3.5 GHz),X-band (9.25 GHz), K-band (20 GHz), Q-band (35 GHz) and W-band (95 GHz). Other techniques, such as electron nuclear double resonance (ENDOR) and electron spin echo envelope modulation (ESEEM) spectroscopies, record in essence the NMR spectra of paramagnetic species. 

In an applied magnetic field Bo 0, the triplet states have in addition a Zeeman energy. The Zeeman splitting then consists of three non-equidistant terms. For technical reasons, the ESR spectrum is usually measured at a constant microwave frequency, e.g. 9.4 GHz (X band) or 35 GHz (Q band). It exhibits a fine-stracture splitting between the resonance fields of the Amj = 1 transitions which is also termed the fine stmcture (Fig. 7.1) this is a direct result of the zero-field splitting. 

Combined zero-field and g-tensor anisotropy The g-tensor anisotropy can be appreciable for transition metal ion complexes, but also for some triplet state molecules. The Q-band spectrum of an (NO)2 surface complex in zeolite LTA has been analyzed to have rhombic symmetry for the g-tensor and the zero-field splitting, see Fig. 6.5 in Chapter 6. Complications due to overlap with another spectrum (in this example an NO surface complex) are common in practical applications. Variations of the experimental conditions, e.g. the sample composition (amount of nitric oxide in this example) and measurements at different microwave frequencies can then give support for the assignment. Refinement of the visual assignment by simulations as discussed in Section 3.4.1.7 is also frequently employed. 

The levels involved in the transition are referred to as a Kramers doublet having effective spin S = 1/2 and an effective g-factor that depends on the fs. The splitting can be obtained by measurements at two microwave frequencies, applying an analytic procedure developed by Aasa et al. several years ago [30]. Values of D for the Fe(III) ESR spectrum of lipoxygenase estimated by measurements of the effective g-values at X- and Q-band frequencies and by analysis of the temperature variation of the signal strength in the range 0.6 < D (cm ) < 3 were reported.The asymmetry... 

The intensity ratio between the main and satellite lines has been used to estimate the distance between a paramagnetic centre and protons at surrounding molecules. The method is based on the assumptions that the point dipole approximation applies for the anisotropic hfc, and that this coupling is much smaller than the nuclear Zeeman energy of the proton. It was found advisable to measure this ratio at as high microwave frequency as possible to achieve the latter condition, i.e. Ba < Bn in terms of the direct field model. The procedure was adequate at Q-band but not at X-band to obtain the distance between a proton and the P03 radical in a single crystal of Na2HP03-51120, see [37] for details. 

The hyperfine pattern of a sample may differ at different microwave frequencies due to the difference in the nuclear Zeeman energy, with Bn 0. 5 mT at X-band, 1.8 mT at Q-band for the H- hfc in a free radical (g 2.00). An example is shown in Fig. 4.21(a) for the hydrazine cation radical discussed in Chapter 1. For the magnetic field oriented along the N—N (Y) bond the four hydrogen atoms are accidentally equivalent giving a normal quintet of lines at X-band. The hfc due to the two... 

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