Exchange crosspeak

The NOESY experiment has also been very useful for revealing the presence of rotational conformers of dimeric flavonoids and flavone C-glycosides (Figure 2.3). Strong exchange crosspeaks between equivalent protons of each conformer revealed the rotational equilibriums. This NOE phenomenon was first noted by Hatano et al. in two conformers of procyanidin dimers. The volume of the NOESY crosspeaks is related to the distance... 

The hexaanionic state of reduction of 65s was established by finding a ROESY exchange crosspeak between the two types of para protons, which indicated that the ABCDE pattern in the NMR spectra corresponds to a phenyl group, and not to a fixed benzo substituent in a hypothetical intermediate119. Thus, it was ruled out that the third species is an intermediate of a cyclization reaction. Attempts were made in parallel to prove the high reduction state by quenching reactions with electrophiles. However, these attempts failed to yield a hexa derivative, probably due to a combination of the thermal instability of the hexaanion, re-aromatization and disproportionation reactions. 

Figure 14 illustrates the presence of exchange crosspeaks in the NOESY spectra of a self-assembled tetraurea calix[4]arene capsule 5. For this capsule, the correlation time is long and NOE cross-peaks are of the same... 

Figure 5.47 Two-dimensional exchange spectrum of N,Af-dimethylacetamide and its generation, (a) The first set of spectra results from the first series of Fourier transformations with respect to The modulation of signals as a function of t is observed, (b) The second set of spectra is obtained by the second series of Fourier transformations. The unmodulated signals appear on the diagonal at (v, v ), (vx, Vx), and (v, v ), whereas the modulations due to exchange show up as crosspeaks on either side of the diagonal at (vx, Vx) and (vx, Vx). (c) A contour plot representation of (b). (Reprinted from Science 232, A. Bax, et ai, 960, copyright (1986), with permission from Science-AAAS, c/o Direct Partners Int., P.O. Box 599, 1200 AN Hilversum, The Netherlands)...

The broadness of the 5 -G H(l) signal was coupled to weak NOE crosspeaks from 5 -G H(l) for PtR, PtM, and PtC [72][107], The PtW, PtC, PtL, and PtK samples were prepared in the presence of 3-50 mM phosphate, while desalted samples were used for PtR and PtM. Phosphate anion is known to be a good catalyst of NH-proton exchange [120], Therefore, the sharper 5 -G H(l) signals in PtR and PtM could be due to the absence of salt effects rather than to sequence-related structural effects. 

Fig. 8 Encapsulation of cyclopropane in 1.24.1. Crosspeaks (circled in red) indicate exchange of guest positions within the capsule...

The 180° proton pulse exchanges the ZQ and DQ frequencies in the middle of the evolution period and thus eliminates the proton chemical shift term yielding, after Fourier transform in t, crosspeaks at the frequency tox on the Fi scale of the heteronucleus. 

Li EXSY spectra [13,64,94] yield important information with respect to the dynamic process responsible for the lineshape changes and the exchange mechanisms involved. On the basis of appropriate equations [173-176], rate constants can be obtained from the relative intensities of the crosspeaks and diagonal peaks. Alternatively, the saturation transfer experiment introduced by Hoffman and Forsen [177] can be applied using selective pulses. 

An analogous approach has been applied to a number of binary and ternary phosphate glasses, resulting in very insightful quahtative and quantitative information [67-77]. First of all, the observation of crosspeaks in 2D-exchange, RFDR, as well as DQ-ejqjeriments unambiguously reveal the occurrence of Q< >-and Unkages. 

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