Resonance saturation effect

Another case where resonance influences carbon acidities is the comparison of toluene to diphenylmethane and lastly triphenylmethane (pK s = 41.2,33.0, and 31.5, respectively). The large shift between toluene and diphenylmethane is due to additional resonance stabilization of the conjugate base. However, the third additional phenyl ring has little effect. Several factors are involved to account for this small change. One is that the phenyl rings cannot all be planar with the anionic carbon, which is required for full resonance stabilization (examine the trityl radical discussed in Chapter 2), and instead a propeller twist develops in the anion. This is an example of a steric inhibition of resonance. A second factor is called a resonance saturation effect. Once the charge on the conjugate base is stabilized via resonance, the additional resonance is not as effective at stabilization. 

In the ionization of the p-aminoanilinium ion in 50% EtOH-H20, the apparent a value of the substituent is —0.42239. This is rather more negative than the cr° value, which would have been expected to be applicable. This is an example of resonance saturation of two opposing powerful —R groups. A careful analysis of the effect of resonance saturation on the species involved on both sides of the equilibrium is required in order to explain the apparent enhancement of the —R effect of NH2239. 

Section IV explains a new approach to high resolution spectroscopy based on various kinds of saturation effects. Some of the experiments are performed inside the laser resonator, which implies the presence of coupling phenomena between the absorbing molecules under investigation and the laser oscillation itself. These feedback effects can be used for high-precision frequency stabilization and to measure frequency shifts and line profiles with an accuracy never... 

E. L. Lim, K. G. Hollingsworth, P. E. Thelwall and R. Taylor, Measuring the acute effect of insulin infusion on ATP turnover rate in human skeletal muscle using phosphorus-31 magnetic resonance saturation transfer spectroscopy. NMR Biomed., 2010, 23,952-957. 

In mixed solutions of ferro- and ferri-cytochrome c cross saturation effects could be observed by this technique. For example when the methyl resonance at +23.2 ppm of ferricytochrome c (Fig. 19) was irradiated, saturation effects were also observed in the methyl resonance of ferro-cytochrome c at +3.3 ppm (Fig. 27). This cross relaxation was shown to arise from an exchange of protein molecules, and hence also the saturated spins, between the ferrous and ferric oxidation states. The life-time in either oxidation state then has to be comparable to or shorter than the longitudinal spin relaxation time of the observed protons. Besides... 

Fig. 9.2. Saturation effect of the modified DANTE sequence on a signal off-resonance with respect to the carrier frequency. The trajectory of the in-plane component of the magnetization is shown. The z magnetization is tilted by the first small angle pulse toward the x axis, and starts precessing toward the y axis. The duration of the pulse corresponds to the duration of a 90s precession, so that at the end of the first pulse the projection lies on the y axis. The phase of the following pulses is rotated in phase with the precession of the signal, in such a way as to follow the spin magnetization in its spiral movement toward the xy plane.

From the ratios of anions present it is possible to calculate the formal charge per polymer unit (Table 10). A resonable average figure for moderate degrees of doping is about one-quarter of an electron removed per iodine atom incorporated, although it is evident that a saturation effect also occurs (Figure 5). 

In order to obtain a Larmor resonance line we have to vary the frequency of the microwave field and count the number of spin Hips per unit time. In order to avoid saturation effects the microwave field amplitude was kept low. The resonance curve obtained in the described manner is rather asymmetric. The lineshape can be described using the known spatial configuration of the magnetic field and a thermal distribution of the axial energy. A least squares fit to the data points as shown in Fig. 9 leads to a fractional uncertainty of about 10-6 and the g factor can be quoted with the same error [9]. 

These quantities are used in the context of saturation effects in spectroscopy, particularly spin-resonance spectroscopy (see p.25-26). 

A second variation of saturation transfer experiment has been devised by Dalvit and coworkers that uses the transfer of magnetization from the water (167). Water is intimately associated with proteins being bound either within or on the surface of the macromolecu-lar structure. Saturation of the water resonance will lead to protein saturation through a variety of mechanisms, including saturation of the aH resonances, saturation of exchanging protein resonances, and NOE interactions between water and the protein. If a compound is bound to the protein it will also become saturated, and this effect can be used as an indication of ligand binding (167). 

A simple differential saturation method has been proposed (63) in which the differential saturation effects are obtained at the same overall irradiation level due to an audio-frequency modulation of the resonance frequency in the continuous-wave mode of operation. This results in the appearance of sidebands in addition to the centre band of the spectrum. By a judicious selection of the modulation index, the saturation of signals in the sidebands may be made to amount to 10 — 0-1% of those in the centre band. The lineshape of such a combination of the centre band and the two closest sidebands may be adjusted to that corresponding to the function ... 

Spectral diffusion of saturation can be detected in various ways, including electron-electron double resonance (ELDOR), effects on adiabatic rapid passage spectra, saturation recovery, and CW saturation. Of these, ELDOR is by far the most attractive in principle, because the effect is observed directly. Unfortunately, the requisite equipment is not widely available. We look forward to increased use of ELDOR, particularly time-resolved ELDOR, to study special motions of slowly diffusing spin labels. 

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