Fast exchange spectrum

Figure B2.4.5. Simulated lineshapes for an intennolecular exchange reaction in which the bond joining two strongly coupled nuclei breaks and re-fomis at a series of rates, given beside tlie lineshape. In slow exchange, the typical spectrum of an AB spin system is shown. In the limit of fast exchange, the spectrum consists of two lines at tlie two chemical shifts and all the coupling has disappeared.

In presence of molecular motion the NMR line shape will change. A particularly simple situation arises, if the motion is rapid on timescale defined by the inverse width of the spectrum in absence of motion 6 1. In this fast exchange limit, which in 2H NMR is reached for correlation times tc < 1CT7 s, the motion leads to a partially averaged quadrupole coupling and valuable information about the type of motion can directly be obtained from analysis of the resulting line shapes. The NMR frequency is then given by... 

The bicapped square pyramidal structure of Ni Pbio has two chemically distinct lead environments, but its ° Pb NMR spectrum shows a single, somewhat broad resonance at —996 ppm (25°C, 104.7 MHz) as a result of fast exchange [57]. As the Ni Pbio sample is cooled to —45°C, the signal broadens significantly due to slowing of intramolecular exchange on the NMR time scale, but the limiting spectrum has not been achieved. 

NMR longitudinal relaxation times can be used for determination of the site of protonation in polyfunctional acids and bases (93JCS(P2)283). Thus, the 14N NMR spectrum of 4-aminopyridine shows clearly from the sharpening of the signal for the ring nitrogen that protonation has occurred here. This procedure is an important innovation in the elucidation of heterocyclic tautomeric structures, especially for the cases of fast exchange. 

The NMR spectrum shows the presence of (5a) and (9b), containing a monodentate N-bonded RNSNR group in the cis,irons configuration, and the S-bonded isomer for which configurations (5b), (5c) and (5d) are in the limit of fast exchange. 

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