Rapid chemical exchange

The occurrence of an average CH3-CO resonance line position from acetic acid and TBT acetate is a consequence of rapid chemical exchange of the tributyltin group. It represents the CH3-CO protons in a time-averaged environment. 

Tributyltin carboxylates undergo rapid chemical exchange, as evidenced by NMR. As a consequence, even the interfacial reaction between tributyltin carboxylate and chloride is fast. IR, mass spectra, gas chromatographic retention time and chloride assay show that the product of the reaction is tributyltin chloride. 

THE H-NMR spectrum of an unknown compound, C]HgO. This compound shows an OH in its IR spectrum. Its DU equals 0. Examination of its NMR spectrum indicates only alkyl-type H s.The integral provides the actual number of H s in this case, since they total eight.The broad singlet due to one H at 3.8 S is probably due to the hydroxy H, which is not coupled due to rapid chemical exchange.The two H s at 3.5 S appear as a triplet and must be coupled to two H s—the two H s at 1.5 S. 

Addition of a small amount of a paramagnetic lanthanide shift reagent to a solution of ligand results in chemical shift of the ligand NMR spectrum which is proportional to Ln/L ratio at low values. The condition of rapid chemical exchange must prevail when tb, the mean lifetime of the complex is much shorter than 1 /Ab. The condition is Tb [Pg.796]

The NMR spectra of most of the lanthanide shift reagents in solution involve rapid chemical exchange. In many situations one has 1 2 and 1 1 adducts and free ligand in solution. The observed lanthanide induced shift, S is the sum of two contributions... 

CH-OH No coupling Because of the rapid chemical exchange of the —OH proton in... 

Rapid chemical exchange is often observed in H spectra when our sample contains labile protons. Labile protons are most often those found on heteroatoms in hydroxyl, carboxyl, and amino groups. In special cases, other H s may be observed to undergo rapid chemical exchange if there is a combination of several conditions that each contribute toward making a particular especially labile, e.g., if the is alpha to several carbonyls or if there is a strong propensity for the molecule to tautomerize. 

Rapid chemical exchange means that the exchange takes place on a time scale faster than any that can be resolved by using the instrument. As an aside, the time scale that can be observed with an NMR spectrometer is referred to as the NMR time scale in fact, the NMR time scale may vary over many orders of magnitude, with the specific time scale depending on what experiment is being conducted. 

Rapid chemical exchange A chemical exchange process that occurs so rapidly that two or more resonances coalesce into a single resonance. 

A second advantage of higher fields is that rapid chemical exchange processes (for example between conformational states) are more readily apparent. Line broadening in the rapid exchange limit is given by ... 

Figure 231. The CPMG sequence performed on the pentapeptide Leu-enkephalin 2.2 in DMSO. Hie faster decay of the amide protons (left) relative to the aromatic protons of the tyrosine residue (right) results from the amide protons coupling to quadmpolar (Section 2.5). The very fast decay of the highest frequency amide proton occurs because this is in rapid chemical exchange with dissolved water, broadening the resonance significantly. The numbers show the total T2 relaxation period 2xn.

FIGURE 9.28 The 300-MHz NMR spectrum of ordinary ethanol. There is no signal splitting by the hydroxyl proton due to rapid chemical exchange. Expansions of the signals are shown in the offset plots. 

Protons attached to electronegative atoms with lone pairs such as oxygen (or nitrogen) can undergo rapid chemical exchange. That is, they can be transferred rapidly from one molecule to another and are therefore called exchangeable protons. 

Protons that undergo rapid chemical exchange (i.e., those attached to oxygen or nitrogen) can be easily detected by placing the compound in D2O. The protons are rapidly replaced by deuterons, and the proton signal disappears from the spectrum. 

In this case the excess relaxation rate has become appreciable - having reached about half its limiting value (P3/T23) t very rapid chemical exchange ( >> 1) as we shall see below. 

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