O chemical shifts

TABLE 23. 17 O chemical shifts for aromatic nitro compounds ... 

TABLE 10. "O chemical shift data (ppm) for hydroxamic acids and esters ... 

Table 1 O chemical shifts (relative to internal 1,4-dioxane) for sultines in CD2CI2 at ambient temperature (or lower)...

Adsorbent Loading solvent Relative intensity (°/o) Chemical shift (ppm from TMS) Linewidth (Hz)... 

O chemical shifts of reference compounds Me(CH2)30Ac, 363.6, 165.2 (C6D6) butanol (CD2Cl2)1.0. 

Information on the hydration state of the Gd(III) chelate in solution is indispensable for the analysis of its proton relaxivity. Several methods exist to determine q, though they are mostly applicable for other lanthanides than Gd(III). In the case of Eu(III) and Tb(III) complexes, the difference of the luminescence hfetimes measured in D2O and HjO can be related to the hydration number [15, 16]. For Dy(III) chelates, the lanthanide induced O chemical shift of the bulk water is proportional to the hydration number [17]. Different hydration states of the same chelate may also coexist in solution giving rise to a hydration equiUb-rium. Such an equilibrium can be assessed by UV-Vis measurements on the Eu(III) complex [18-20]. These techniques have been recently discussed [21]. 

Table 6.4. O chemical shift tensor components of ternary oxide compounds.

Figure 6.17. A. Relationship between the O nuclear quadrupole coupling constant xq and the cation electronegativity for the bridging and non-bridging oxygens in the alkaline earth metasilicates. B. Relation between the logarithm of the O chemical shift and the cation radius for bridging and non-bridging oxygens in the alkaline earth metasilicates. From Timken et al. (1987) by permission of the American Chemical Society.

Figure 6.27. Relationships between the O nuclear quadrupole coupling constant Xq and the O chemical shift 8 for various oxygen sites present in aluminosilicate glasses and melts. The triangles denote Si-O-Si groups, squares denote Si-O-Al, circles denote Al-O-Al and diamonds denote tricluster units. From Xue and Kanzaki (1999) by permission of the American Chemical Society.

All facets of study have been greatly aided by the ease with which crystal structures may be obtained and by the availability of sensitive Fourier transform NMR. spectrometers which allow nuclei such as O, V, Nb, Mo. and to be used for structural studies. Oxygen-17 NMR spectroscopy has proved to be particularly useful because O chemical shifts are very sensitive to environment. As a result it is po.ssible to distinguish between terminal and various kinds of bridging oxygen sites. The O spectrum of IW Om] " and its structure arc shown in Fig. 16.20a. - We see... 

O chemical shifts of phenols hydrogen bonded to heteroaromatic nitrogens in systems like o-hydroxypyridines or similar compounds with one or more nitrogens or hydroxy groups show OH chemical shifts that are very similar (94-97 ppm), with the exception of a para-substituted methoxy derivative (90 ppm) , but this can be ascribed to a simple substituent effect (see above). 

Phenols quite often take part in multiple hydrogen bonding exemplified by 28, 29, 30, 35 and 36, a system akin to a large number of dyes and indicators. The hydrogen bonding can be described by O chemical shifts (see Section n.E.2), by ( 0)... 

Zhuo investigated O chemical shifts of o-hydroxy Schiff bases. These systems are in some instances tantomeric. As described previonsly O chemical shifts are very good indicators of tantomerism (see Section n.K.l). Provided that good reference values for the two tautomeric states exist, the equilibrium constant can be determined. Zhuo used the values for simple Schiff bases as models for the phenolic form (48). For the form 48B a value from a simple enamine was chosen. This, however, is not a very appropriate choice, as it does not at all take into account the charged resonance form (48C). The equilibrium constant determined for Af-(2-hydroxy-l-naphthalenyhnethylene) amine is quite different from that derived by /(N,H) coupling constants. ... 

Fig. 6.5.10. Plots of the observed isotropic O chemical shift against the hydrogen bond length.

Fig. 9.1. C chemical shift of polyethylene and n-paraffins. ( ) C chemical shift in solution (O) chemical shift in the solid state [3, 5, 16].

Examination of O chemical shifts of several 2-substituted and 2,5-disubstituted furans <85MRC985> has demonstrated the additivity of substituent effects by plotting the O chemical shifts of 2,5-disubstituted furans versus those of the 2-substituted analogues. Failure of the electronic character of substituents in determining O chemical shifts is found, because they do not correlate with Hammet constants. Such a lack of correlation is known for C ortho chemical shifts in aromatic systems. A good correlation is obtained by plotting O chemical shifts versus a parameter Q of the substituent—a parameter which accounts for the paramagnetic contributions and is a function of the polarizability and ionization potential of the substituent. The cyano- and methylfurans, however, do not follow the Q relationship. 

Figure 10 shows the O NMR spectra of poly(isoprene) gamma irradiated in the presence of O-enriched oxygen. The authors have identified classes of peaks through comparison with model compounds it is conceivable that in future a database of O chemical shifts will aid researchers in this area. The authors were able to show that irradiation of poly(isoprene) in oxygen results in the formation largely of alcohols and ethers from hydrogen abstraction and termination reactions, respectively, of alkoxy radicals. These radicals are products of homolysis of hydroperoxy radicals. 

FIGURE 3. Plot of chemical shift (NO2) nitrobenzenes vs O chemical shift (NO2) yS-nitrostyrenes in CH3CN. Reproduced from Reference 58 by permission of Elsevier Science B.V. 

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