Slow exchange, NMR

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))... 

For the complexes under discussion, this rule appears to play a very dominant role, such that slow exchange NMR spectra are observed at elevated temperatures for the W(dcq)n(mpic)4 n complexes (5,6), even though the iso-electronic and iso-structural W(CN)8 ion has been... 

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.

To see how this method works, let us construct the NMR spectra according to Eq. (11-37). We shall consider the case where a = p = 0.5. That is, this reaction will represent an exchange process such as that in dimethylformamide. We shall use the values (8 v) = 30.0 Hz and A = 1.00 Hz and assume that they are independent of temperature. The results are displayed in Fig. 11-5 over a range of lifetimes, 10-4 = r< 1 s. The extremes of this range provide the limits of fast and slow exchange in which one or two sharp singlets are seen. 

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. 15. Splitting pattern with the assignment of the 13C-NMR shifts of meso-4,5-dimethyloctane at 100.6 MHz within the slow exchange regime of the CH—CH bond rotation. Chemical shifts refer to TMS = 0 ppm. (Ref. 30>)...

Observation by NMR of both the high-spin and low-spin forms of a complex in solution is unusual. As outlined above [27], [Fe(HB(pz)3)2] shows only averaged spectra upon cooling to 243 K. Given that the two spin states differ in their solid state Fe-N bond distances by ca. 0.2 A, slow exchange is expected. 

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