Exchange broadening

Kleier D A and Binsch G 1970 General theory of exchange-broadened NMR line shapes. II. Exploitation of invariance properties J. Magn. Reson. 3 146-60... 

The water nucleus chosen also has an impact on relaxation data. Proton relaxation is affected by chemical exchange and cross-relaxation, 2H by chemical exchange, and lyO by proton-exchange broadening (scalar spin-spin coupling between lH and lyO nuclei affects T2, but not 7)) (Glasel,... 

De Jong et al. (1976c, 1977) have studied the rate of decomplexation of t-BuNHjPFj (AM) complexes of crown ethers. The exchange broadening of the t-butyl nmr signal was used to determine the rate of cation exchange between a kinetically stable (X.AM) and a kinetically unstable (Y.AM)... 

Exchange broadening, see Electron paramagnetic response spectroscopy Exchange-coorelation effects, 34 213 Exchange reactions, 26 711-279, 31 106-107 on alloys, 26 294—296 carbon-14, from benzene- C to C - hydrocarbons, 23 127... 

Nuclear spin exchange has also been observed (80) between nuclei in semiconductors of the heavier elements, such as InSb, GaSb, GaAs, and InAs, and considerable exchange broadening of these NMR lines has been observed. Anderson (81) has given a treatment of the exchange effect in semiconductors. 

Exchange broadening is frequently observed in amides due to restricted rotation about the N-C bond of the amide group. 

Myrtaceae), from which the related 2, 4, 6 -trihydroxy-3 -formyl-5 -methyldihydrochal-cone was also obtained. The NMR spectra of the synthetic formyl derivative show evidence for exchange broadening, which was attributed to conformational exchange between two stable hydrogen-bonded rotamers from molecular mechanics and ab initio calculations ... 

The contribution of reaction (a) to the H1 line width may be estimated from the line width at very low pH where [OH-] and [NH3] are very small and accordingly kb, kc, and kd are relatively unimportant. This sets an upper limit to ka of <0.6 X 10-2 sec-1 M at 21°. Since the analysis of exchange was studied in the pH range 1.5-2.5, reaction (b) is relatively unimportant because [OH-] is very small and the upper limit for kb was estimated to be 10-12 sec-1 Thus, under these experimental conditions, only kc and kd contribute significantly to the overall exchange broadening, and these are related to t, the proton lifetime in a given NH4+ ion by Eq. (35). 

Fig. 6. Part of the spectrum of lysozyme in the presence of the inhibitor (G1cNAc)3. At the concentration used, two resonances from 1 proton of trp 63 can be observed at 6.25 ppm and 6.05 ppm. These correspond to the resonance of this protein in the unbound and inhibitor-bound protein, respectively. The shift indicates that a conformational change occurs in the protein on binding (G1cNAc)3. At low temperature (c) the rate of the conformational change is slow on the time scale of the nmr experiment, but at higher temperature (a) the two resonances coalesce as the rate of the conformational change increases. At 45°C (b) exchange broadening occurs. The rate of the conformational change was measured from these data, and at 45°C is 20 sec-1 (see Ref. 22).

The temperature dependence of the 1H NMR spectra of many [Cr(S2CNR2)3] complexes has been investigated.1000,1017 There were broad double resonances from the N—CH2 protons indicating that inversion between the A and A isomers is slow on the HNMR time-scale. No exchange broadening was observed up to 84 °C when [Cr(S2CNEt2)3] decomposed. Dithiocar-bamato complexes of other tervalent metal ions do not exhibit this stereochemical rigidity. 

Since acetone is a reasonable model for benzaldehyde, the presence of Complex I provides support for Step 3a. (A resolved A2B2X pattern cannot be reached using benzaldehyde since the solvent freezes before the slow exchange limit is achieved however, exchange broadening is evident.) Although additional experiments are needed to establish the presence of Complex I at higher temperatures, the fact that Complex I is the only species observed at low temperature... 

In the intermediate region, where r 1 is comparable with A com, Rip niay be larger than / ip even if R2m = R m because of exchange broadening effects. 

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