Fast exchange

Equation (B2.4.16) then says that the signal is given by equation (B2.4.22), regardless of slow or fast exchange. 

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.

These experiments yield T2 which, in the case of fast exchange, gives the ratio (Aoi) /k. However, since the experiments themselves have an implicit timescale, absolute rates can be obtained in favourable circumstances. For the CPMG experiment, the timescale is the repetition time of the refocusing pulse for the Tjp experiment, it is the rate of precession around the effective RF field. If this timescale is fast witli respect to the exchange rate, then the experiment effectively measures T2 in the absence of exchange. If the timescale is slow, the apparent T2 contains the effects of exchange. Therefore, the apparent T2 shows a dispersion as the... 

Figure 4-8. NMR absorption by a hypothetical two-identical site system with chemical exchange (/I) Slow exchange limit. (B) Moderately slow exchange. (D) Coalescence. (F) Fast exchange limit.

From the invariance of the coupling constant 2J(SiP) between HMPA-P and Si over a wide temperature range (compounds 4-7, 9-11) a rigid coordination of the HMPA to silicon can be deduced. Only in the case of the methyl complex 6 above 25 °C is the beginning of exchange of HMPA observed. However, a fast exchange of the coordinated acetonitrile at room temperature has been found for 12. 

The usable time ranges can be estimated from the following considerations. The slow and fast exchange limits correspond to r-1 relaxation times are roughly 10-3 < r < 1 s for protons, or less for other nuclei with larger (8 v) values. 

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... 

In the fast exchange limit ft2 A2, a Lorentzian at the centre d>, is observed with full width at half height 2/T + A2/Q. In the ultrafast exchange limit discussed in the previous section, A2/fi 2/T, the line shape becomes independent of the exchange... 

Fig. 1. 13C-NMR fast exchange-slow exchange transition for the conformational interconversion of cyclododecane in solution of propane-d, (left side) and in the solid by CP-MAS techniques (right side) at 75.47 MHz. The temperature decreases from top to bottom as indicated at the spectra. Chemical shifts are given at the signals and refer to TMS = 0 ppm. (Ref.7))...

Fig. 3a and b. Fast exchange-slow exchange NMR transition for the conformational interconversion of octamethyltetrasiloxane. a, MAS 13C-NMR solid state spectra on the left side in comparison to solution spectra in propane-di on the right side (at 75.47 MHz), b. MAS 29Si-NMR spectra at 59.63 MHz. Temperatures are indicated in K, shift positions refer to TMS = 0 ppm and correspond to the scale at the bottom. (Ref. I0))... 

Electronic Effects in Metallocenes and Certain Related Systems, 10, 79 Electronic Structure of Alkali Metal Adducts of Aromatic Hydrocarbons, 2, 115 Fast Exchange Reactions of Group I, II, and III Organometallic Compounds, 8,167 Fluorocarbon Derivatives of Metals, 1, 143 Heterocyclic Organoboranes, 2, 257... 

Time-resolved luminescence quenching measurements using the probe Tb(pyridine-2,6-dicarboxylic acid)i and the quencher bromophenol blue show the existence of micellar clusters in AOT-based w/o microemulsions. The fast exchange appearing over several microseconds was attributed to intracluster quenching, whereas the slow exchange on the millisecond time scale was attributed to intercluster exchange [243]. 

Micelles the mostly spherical nanoscale aggregates formed by amphiphilic compounds above their critical micelle concentration in aqueous solution have a narrow size distribution and are dynamic, because there is a fast exchange of amphiphiles in solution and those incorporated in micelles. However, micelles are defined as self-assembled structures, since the structure is in thermodynamical equilibrium. 

Since the ligand is in fast exchange with respect to its bound and free state, every molecule experiences an averaging over time where the fraction L... 

Exchange-transferred spectroscopy was introduced with the finding of the etNOE [97] and its theoretical explanation in terms of fast exchange several years later [98] laid the basis for the large variety of applications being present today. The core element of etNOE is the dependence of the cross-relaxation rate 0 ° on the correlation time T,-. The overall cross-relaxation rate is defined by ... 

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