Time scale: NMR

One of the most important characteristics of micelles is their ability to take up all kinds of substances. Binding of these compounds to micelles is generally driven by hydrophobic and electrostatic interactions. The dynamics of solubilisation into micelles are similar to those observed for entrance and exit of individual surfactant molecules. Their uptake into micelles is close to diffusion controlled, whereas the residence time depends on the sttucture of the molecule and the solubilisate, and is usually in the order of 10 to 10" seconds . Hence, these processes are fast on the NMR time scale. 

Although not a heteroaromatic compound, the case of citrinin studied by Destro and Luz ([97JPC(A)5097] and references therein) is so significant that it deserves mention here. Citrinin exists in the crystal as a mixture of the p-quinone 5a and o-quinone 5b tautomers (Scheme 3). The equilibrium ii temperature dependent and by using CPMAS NMR (Section VI,F) and, more remarkably. X-ray crystallography, the authors were able to determine the AH and AS values (the rate is extremely fast on the NMR time scale, >10 s ). 

Similar isomerization reactions were observed in CDCI3 solutions of N-( -aminomethyl)-l,2,4-triazoles and -tetrazoles (90T633). As for the analogous benzotriazoles, these reactions are-intermolecular and slow at 20°C in the NMR time scale. 

Tautomeric equilibrium in the symmetrical phenoxy-substituted derivative 136 (R = Ph, r = R = OPh) is fast at ambient temperature on the NMR time scale however, at —84°C the proton exchange becomes frozen and both annular tautomers 136a and 136b can be observed (Scheme 40). The similar exchange was also found for P-aryl-substituted 136 (R = Me, Ft, Ph R = R = Ph). In these cases, the equilibrium is very slow, even at ambient temperature, which was attributed to increased steric demands of four phenyl substituents. Unsymmetrically substituted azaphosphorinanes (R R ) provide even more interesting examples. These compounds (R = Ph R = Me, -Pr R = MeO, -PrO) were found to... 

The exchange of the iV-proton between the 1-N and 3-N sites is fast on the H NMR time-scale at ambient temperature, as found by Vernin et al. (1977). In CDC13 two AA BB systems are observed only at low temperatures ( — 80 °C). 

Under conditions where the rotation about the C-N bond of dimethylformamide is slow relative to the NMR time scale, the two methyl resonances will be separate singlets. Conversely, if the rotation is made to be very fast, the two methyl groups will be chemically equivalent. Their resonance will then appear as a sharp singlet. In between these extremes, kinetic information can be extracted from the line shapes. In most systems the parameter that is changed to go between these limits is the temperature. In some systems, pH or pressure has the same effect. 

Compounds 225a-f showed interesting dynamic phenomena on the NMR time scale with broad lines at room temperature and appearance of two sets of sharp peaks at -50 °C corresponding to conformers 226 and 227 (Fig. 3). By contrast, 225 g-1 exist essentially as one conformer. These results show that the presence of a Me substituent adjacent to the 0 atom in ring B and syn to the ring junction hydrogen (see 225 g) prejudices the molecule in favor of conformer 226, thus placing the Me substituent pseudoequatorially (cf. 226, = Me). Sim-... 

The gold(I) exchange occurs rapidly on the NMR time scale. The phosphine dependence of the signal collapse establishes an associative mechanism. By contrast, the gold(III) exchange occurs slowly on the NMR time scale. 

However, despite the rather dramatic change in coordination geometry that is observed upon comparing [TpBut Me]CuCl and [TpBut]CuCl (41), only rather minor perturbations are observed in comparing the structures of the Cud) dimers [TpBut]Cu 2 (37) and [TpBut,Me]Cu 2 (22). Thus, both the average Cu-N bond lengths and also the Cu - Cu separations in [TpBut Me]Cu 2 and [TpBut]Cu 2 are very similar. Nevertheless, although the coordination environment about each copper center is similar, the 5-methyl substituent does influence the fluxional nature of the molecule in solution. Thus, whereas [TpBut]Cu 2 is fluxional on the NMR time scale at room temperature, with a static structure that is only observed at -56°C, [TpBut Me]Cu 2 exhibits a static H NMR spectrum at room temperature. Furthermore, a static spectrum for... 

The structure of [pzTp]2GaMe, determined by x-ray diffraction, demonstrates that the [pzTp] ligands adopt both bidentate and tridentate coordination. However, in solution, the complexes [Tp]2GaMe and [pzTp]2GaMe are fluxional on the NMR time scale at room temperature, exhibiting a single set of resonances for the pyrazolyl groups. 

Fig. 43. H NMR evidence that the reversible reaction between [TpBu Me]ZnOH and C02 is rapid on the NMR time scale ( = internal standard). Reprinted with permission from Ref. (151). Copyright 1993 American Chemical Society.

In 1935, Naumov and Manulkin 7) tried without success to isolate (—)-methyl-ethylpropyltin iodide, (—)-(/). In fact, triorganotin halides are generally configurationally unstable, as shown in 1968 by Peddle and Redl8), whereas tetraorganotin compounds appeared to be configurationally stable within the NMR time-scale (see below). 

Table 1. Molecules of type M, i.e. U(CH3)A(CH3)BC—V—SnXYZ used to determine the configurational stabpity at tin. (st. stands for configurationally stable within the NMR time-scale (Avab) u., for configurationally unstable RT stands for room temperature)... 

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