A Real Example Chemical Exchange and Cross Relaxation

A Real Example Chemical Exchange and Cross Relaxation 

The proton NMR spectrum of N, N-dimethylformamide (DMF) was shown in Fig. 2.15 as an illustration of the exchange of nuclei that are chemically nonequivalent. As the rate of internal rotation about the C—N bond increases, the spectrum changes drastically. However, even near room temperature, where two distinctly separate resonances occur, there is exchange—too slow to alter the ordinary one-dimensional spectrum but fast enough to show effects in a suitably designed 2D study. 

FIGURE 10.4 Pulse sequence for measuring chemical exchange and cross relaxation (NOESY). 

FIGURE 10.5 Application of the pulse sequence of Fig. 10.4 to dimethylformamide in DMSO-d6. The diagonal peaks from M, M2, and MF are indicated the other diagonal peaks arise from H20 impurity and DM SOU. (very weak). The cross peaks close to the diagonal result from methyl exchange the weaker peaks near (2.9, 8.0) are from the NOE. Shown at the top is a projection of intensity in the F2 direction. Courtesy of Herman J. C. Yeh (National Institutes of Health). 

The pulse sequence shown in Fig. 10.4 is sometimes used to study exchange, but more than 99% of the use of this sequence is to study the effect of dipolar cross relaxation via the NOE. As a result, this type of study is given the name nuclear Overhauser effect spectroscopy, NOESY, and the pulse sequence of Fig. 10.4 

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