Spin probe lineshapes

In this subsection we show the effect of the orienting potential on the ESR lineshape of paramagnetic species of particular practical interest, namely, the nitroxide stearic spin probe and Cu ion complex dissolved in a nematic liquid crystal. 

Figure 13 shows the absorption spectra for a nitroxide spin probe and compares the case of free diffusion (P2 0) with those obtained for various values of the orienting potential. Such spectra have been calculated for an isotropic diffusion coefficient. Note the drastic change in lineshape at the onset of molecular order. 

ESR spectra of the rigid spin probe cholestane embedded in model membranes yield information about the molecular order (order parameters and and about the rate of motion of the probe molecules in the membranes (rotational diffusion constants D and Dj ). For a correct analysis of the ESR lineshapes in the slow motion regime (10 stochastic Liouville Equation (SLE) formalism. 

In this review, we are interested in the ESR signal of stable free radicals (spin probes) dissolved in liquid environments with particular reference to water. The key question to be addressed is the effectiveness of the spin probe as transmitter to supply information on the host. First studies of paramagnetic solutes in liquids involved copper chelates in organic solvents [10] and transition ions in solution [11] with attempts to describe the ESR lineshape as being influenced by the Brownian tumbling motion of the paramagnet in the liquid state [12]. Subsequent theoretical refinements in the case of fast reorientation [13,14] (see also Refs [4,15]) with clear experimental confirmation [16] and further extension and experimental validation of the theory of the ESR lineshape to arbitrary reorientation rates [17-21] paved the way to the quantitative use of ESR to characterize the liquid state of matter. 

Figure 3. ESR lineshapes of spin probes in frozen (a) and mobile (b) hosts, (s The magnetic dipoles m of immobile spin probes in a frozen liquid have different u>o values due to their different orientations with respect to Bo, thus resulting in a broad line with width Arno (black line), usually referred to as rigid-limit or pwder lineshape. (b) If the spin probe undergoes rotation (sketched as instantaneous clockwise jumps at random times), ft>o fluctuates. When the rotational rate 1/r is larger than the width of the coo distribution Acoo, the different precession frequencies become indistinguishable and an average value is seen, that is, the ESR lineshape coalesces (motional narrowing) [4,14]. Adapted from Ref. [26].

Owing to the orientation distribution of the spin probes, and then of n, distribution resulting in a broad absorption line when sensitive signal detection. The field-swept derivative pattern of the ESR lineshape in the absence of spin probe motion (powder lineshape) shows sharp details that allow one to measure the principal values of the Zeeman g tensor (Fig. 5, top right). 

Figure 5. ESR lineshapes of a spin probe gx = 2.0093, gy = 2.0064, gj = 2.00215, A = 0) undergoing reorientation with jump angle 6 = 80° and rotational correlation times r = 900 (top), 45 (middle), and 9 ns (bottom). The lineshapes are convoluted by a Lorentzian with width 1/7J = 5.26 MHz to account for the changes of the Larmor frequency, occurring each on average, due to the...

The brackets indicate a proper thermal average, C is a constant, is the real part of z, and f = —1. The derivative takes into account that the lineshape is usually displayed in derivative mode. The correlation function SxSx t)) is evaluated by the quantum time evolution of the electron spin under the influence of the reorientation of the spin probe according to the equation of motion [31] ... 

Figure 7. Selected ESR line-shapes of the spin probe TEM-POL (see Fig. 2a) in quenched bulk water and subsequent reheating at the indicated temperature. Note that (i) for technical conve-tuence the static magnetic held Btj, and not the microwave frequency as In Fig. lb. Is swept (ii) the phase sensitive detection displays the lineshape in derivative mode. Adapted from Ref. [26].

High-spin dendrimers, like 5 (G = 1), are ideal probes for such a study since they are ESR active and this spectroscopy has the same time scale as such a dynamic process. In fact, the anisotropic components of some ESR parameters, such as those responsible for the fine structures of a high-spin molecule, are averaged out under molecular tumbling leading to noticeably lineshape effects. 

When the H- H dipole-dipole interaction can be measured for a specific pair of H nuclei, studies of the temperature dependence of both the H NMR line-shape and the H NMR relaxation provide a powerful way of probing the molecular dynamics, even in very low temperature regimes at which the dynamics often exhibit quantum tunnelling behaviour. In such cases, H NMR can be superior to quasielastic neutron scattering experiments in terms of both practicality and resolution. The experimental analysis can be made even more informative by carrying out H NMR measurements on single crystal samples. In principle, studies of both the H NMR lineshape and relaxation properties can be used to derive correlation times (rc) for the motion in practice, however, spin-lattice relaxation time (T measurements are more often used to measure rc as they are sensitive to the effects of motion over considerably wider temperature ranges. 

Crystalline amino acids have often been used as model compounds for probing functional group interactions in proteins. The 3-site 120° jump motion of the ammonium (-NH3) group in alanine has been studied using 2H NMR lineshape analysis and by considering the anisotropy of the 2H spin-lattice relaxation [182]. The activation energy for this motion was estimated to be 40.5 kj mol-1. 

Most NMR spectrometers have 12 to 18 shim controls (Churmny and Hoult 1990). Each user will adopt their own procedure but the aim is to produce the minimum linewidth consistent with a good lineshape. In practice, some shims are much more significant than others and for particular probes different shims will be important. For solid-state operation, shimming usually needs to be carried out relatively infrequently. One possible procedure for probes tuned to H is to crudely shim on H2O. If there is no proton channel most multinuclear probes will tune to D, so D2O can be used. For CP-MAS probes that tune to - C, adamantane is a useful compound which should be shimmed under spinning and H decoupling conditions. A typical resolution for in admantane of 3-4 Hz at 7.05 T and 10 Hz at 11.7 T should be achieveable. 

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