Alternating Line Width Effects

Another example of alternating line width effect was found in the spectra of durosemiquinone (6),9,10 where the effect is due to alkali metal ions hopping back and forth from one oxygen atom to the other. The rates depend on the alkali metal as shown in Table 5.3. 

Another example is the m-dinitrobenzene anion radical in aqueous solution,11,12 where the effect is due to asymmetric solvation (one nitro group solvated, the other not), an effect very similar to that with dinitrodurene anions. In this case the mean lifetime of one solvation state was 0.8 ps at 291 K and 4.5 ps at 282 K. Still more examples are mentioned in the reviews by Atkins in the early 1970s.13 

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P. D. Sullivan and J. R. Bolton, Adv. Magn. Reson. 4 39 (1970). The Alternating Line-Width Effect. ... 

The spectrum of the dvuenesemiquinone radical-cation exhibits a marked alternating line-width effect which is temperature-dependent it is ascribed to cis-trans isomerism of (68) and (69), the life-time of each isomer being comparable with the inverse frequency separation between the methyl proton-splittings (Bolton and Carrington, 1962). Carrington (1962) has discussed the theoretical line-shapes to be expected for different rates of isomerization and has estimated that the life-times of the isomers at room temperature are about 10 sec. 

Time-dependent phenomena, including the alternating line width effect [52], [82] and asymmetric line broadening [79], [80], require simulated spectra to obtain the ESR parameters and rates. 

Figure 1. 35 GHz ESR spectra of -CFtCONHi at 290°K (upper) and 77 K (lower) in irradiated crystals of CFsCONH, The center line at 290°K separates into a resolved doublet of doublets at 77°K owing to the freezing out of the torsional oscillation of the CFg group. A, B, and C identify the Mi — 0, Mi = l,and aM, = 2 transitions, respectively. The large difference in heights of the low and high field A lines at 77°K relative to the central A line doublets is caused by an alternating line width effect caused by a rapid in-phase motion of the CF group above and below the CON plane. From Reference 5.

Avdievich N I and Forbes M D E 1995 Dynamic effects in spin-correlated radical pair theory J modulation and a new look at the phenomenon of alternating line widths in the EPR spectra of flexible biradicals J. Phys. Chem. 99 9660-7... 

Alternating linewidths. ) Discussion of line width effects by ring inversion. ... 

Discussion of temperature dependence and of line width effects. Alternating line width. 

The presence of an alternating line width at 77 K, but not at higher temperatures which are independent of magnetic field suggests an additional motion is also present. Proper analysis of the line width effect shows that it is largely dipolar in nature and requires the motion of two fluorines to be in-phase with one another ( ). The particular type of motion which the radical undergoes has been suggested by an INDO... 

Asymmetric broadening effects in solids have been discussed above, especially in Section V, so here we are primarily concerned with solutions. However, it is often possible to trace line-width alternations in spectra from solutions to far greater alternations and asymmetries in frozen solutions of the radical concerned. Indeed, if fluid solutions can be made progressively more viscous, it will be found that a complete spectrum of line shapes will result, linking the narrow lines which are due to rapidly tumbling radicals to the ultimate set of lines and shoulders which demark the total g- and hyperfine-asymmetries. 

The preparation and the crystal structures of single crystals of alkali aromatic ion pairs are discussed. A close relationship was found between the ion-pair structures in solution and in the solid state. The physical properties of the pseudo-two-dimensional magnetic alkali biphenyl crystals are reviewed. The effect of spin diffusion manifests itself clearly in the line width and line shape of the exchange-narrowed electron spin resonance (ESR) line. The monoanions of cycloocta-tetraene, produced by X-ray irradiation, rotate rapidly about their eightfold axes. At 20 K this rotation is frozen, and an alternating spin density distribution is found around the ring. The equilibrium position of the monoanion is rotated by 22.5 compared with the equilibrium position of the dianion. 

Just as chemical exchange can broaden proton NMR spectra (Section 13.5), electron exchange between two radicals can broaden EPR spectra, therefore the distance between two spin probe molecules maybe measured from the line widths of their EPR spectra. The effect can be used in a number of biochemical studies. For example, the kinetics of association of two polypeptides labeled with the synthetic amino acid 2,2,6,6,-tetramethylpiperidine-l-oxyl-4-amino-4-carboxylic acid (6) can be studied by measuring the line width of the EPR spectrum of the label as a function of time. Alternatively, the thermodynamics of association may be studied by examining the temperature dependence of the EPR line width. 

An alternative method of analysis of NOESY data, which is usually sufficient for resolved peaks with a digital resolution much greater than the intrinsic line width and coupling constants, is to measure the maximum peak amplitude or to count the number of contours. NOESY cross peaks can then be classified as strong, medium or weak and can be translated into upper distance restraints of around 2.5, 3.5 and 5.0 A respectively. The lower distance constraint is usually the sum of the van der Waals radii (1.8 A for protons). This simple approach is reasonably insensitive to the effects of spin diffusion or non-uniform correlation times and can usually lead to definition of the global fold of the protein, provided a sufficiently large number of NOEs have been identified. 

An alternative technique for the removal of dipolar broadening is by introducing a time dependence into the spin system by applying a train of rf pulses while the sample remains stationary. Pulse repetition rates can easily be made greater than the static dipolar line width so in effect line narrowing can be achieved with pulse spacings of a few microseconds equivalent to rotation speeds in excess of 100 kHz. The first line... 

Two types of motion can influence the dipolar coupling and broaden the line widths. In one case, the nuclei undergo random isotropic motion at a rate such that all angles are not averaged within the time interval T2. Alternatively, the motions may be quite rapid, but the accessible angular motion is severely restricted. The effect of molecular motion on the line widths behaves the same as chemical exchange, which will be discussed in the next section. 

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