Saturation transfer method

Selective inversion experiments for the determination of slow exchange rates are analogous to the saturation-transfer method in that they involve selective manipulation of one of the exchanging sites, while observing the subsequent effect on the second site as a function of time [48, 69, 70]. Chemical exchange, if present, will provide an alternative route to normal spin relaxation processes which a spin system undergoes, if perturbed at the start of an experiment. The rate of relaxation will depend on both the exchange rate k and the spin-lattice relaxation rate (Ti) (fig. 5). 

In order to examine the effect of the Lewis acid on the rate of the chain transfer reaction, the NMR saturation transfer method [65] was applied to the model system, a mixture of (2-propanolate)aluminum porphyrin [(TPP) AlOCH(CH3)2, l,X=0 Pr] and 2-PrOH (1 13) in theabsence (Fig. 39) and presence (Fig. 40) of the Lewis acid 3e (3 equiv) in CgDg at 21 °C. 

The free energy of activation for the proton shift process in the interconversion between tautomeric forms 8 and 9 was measured by the Fors6n-Hoffman double resonance saturation transfer method and determined to be 16.2 kcal mof1 (Figure 7) <20040BC1227>. This value was an average over three different structures (R = Me, Et, Pr1). 

The 1,3-diazepine 83 (R = CF3, Alk = CH3) has been shown to have the NH structure in the solid state <20040BC1227>. The average free energy of activation for interconversion of the tautomers 83 and 84 (R = H, Aik = Me, Et, Pr1) has been determined to be 16.2 kcal mol1 by the ForsenHoffman double resonance saturation transfer method <20040BC1227>. The most stable tautomeric forms of a variety of diazepine derivatives have been documented in CHEC-III(13) and earlier editions. 

The more theoretical aspects of double resonance experiments in systems undergoing chemical exchange are referred to in Section II. Here we deal with the use of the Hoffman-Forsen saturation transfer method for the study of slow rates of exchange. The basis of this method (62) is simply that since Tj is often longer than T an experiment that relates the exchange rate to the longitudinal relaxation time is more suitable for studying slow processes than one (e.g. band-shape analysis)... 

In the fast tumbling domain, the theory of Stone et al. [46] (and see Ref. 47) is used to obtain motional information. In the slow tumbling domain, Freed s [48] contributions are fundamental. In the very slow tumbling domain, saturation transfer methods are used [49] to obtain motional information. In theoretical analysis in this latter domain, the intrinsic linewidth is often treated as an adjustable parameter. Hyde and Hyde [50] have suggested an experimental way to determine it independently. 

In addition to the dnmr technique, there are other somewhat more specialized nmr methods for determining intramolecular exchange rates. The saturation-transfer method (Hoffman and Forsen, 1966 Mann, 1976, 1977a,b) enables the measurement of rates that are of the same magnitude as the rate of longitudinal relaxation. The method is based on observation... 

Regioselective ligand exchange of in-plane acetate ligands on octakis(/t-acetato-0,0 )tetraplatinum(II) complex shown in Eq. (75) is characterized by the two-term rate law (76) where, in CDCI3 at 298.2 K, ko = 0.36 s and k = 4.4 dm" mol s, as determined by a H NMR spin saturation transfer method, but no exchange is observed for the out-of-plane acetates. " Molecular orbital... 

The n.m.r. line width and saturation transfer method has been used to determine the thermodynamic and kinetic parameters for ring-opening and closing of aldo- and ketofuranoses and their phosphate esters. 

A comparison between the saturation-transfer method with exchange broadening is possible in a system where no causes other than chemical exchange are expected to contribute to the line-broadening of the eacr-changlng species. Such a system is the acid catalyzed exchange reaction between acetaldehyde and its hydrate. Indeed, as can be seen from Table 2, the specific rate constants calculated from the two methods are the same within the experimental error, nnd furthermore the ratio of the forwards and reverse rate constants agrees in the two methods with K 1.15 whldi was measured Independently from the ratio of the areas under the peaks of the two species. 

LB line broadening 1/t Is calculated according to the relation 1/t-v(Av-Av(o)) where Av(o) Is the llnewldth at neutral pH. ST saturation transfer method as explained In the text. 

The signals of the proton of a phenolic hydroxyl and water approach each other as chemical exchange between them increases as, for example, by an increase in the temperature. The chemical exchange is detectable by the saturation transfer method (421). Data of saturation... 

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