Coupling constants exchange, effect

The most characteristic coupling constant in indazoles is the cross-ring Vs, present both in indazoles and in isoindazoles unsubstituted in positions 3 and 7. 2-Methyl isomers show an additional Vmc.h coupling which can serve to identify an isoindazole unsubstituted in position 3. In 3-azidoindazole, as in 3-azidopyrazole (56), the prototropic exchange is slowed down sufficiently to allow the measurement of a zig-zag /i,4 coupling constant. The deshielding effects observed in A-acetyl derivatives, e.g. 1-acetyl (60) on H-7 and 2-acetyl (61) on H-3, are related to a preferred E conformation (Section 4.04.1.4.3). 

Gillespie, R. J., Hartman, J. S., Parekh, M. Solvent Effects on the Boron-Fluorine Coupling Constant and on Fluorine Exchange in Tetrafluoroborate Anion. Can. J. Chem. 46, 1601 (1968). 

In summary, nuclear spin exchange effects are to be expected in the NMR of the heavier elements in the solid state. From the exchange coupling constants information on the nature of electronic wave functions in solids may be obtained. 

Only when the rate of alcohol exchange is lowered by cooling is the nonequivalence of fluorine atoms evident in the NMR spectrum. These exchange effects are observed because at room temperature the chemical shift between fluorine atoms in the two environments is comparable to the exchange rate. Chemical shiftsf and coupling constants of nuclei in molecules are comparable to the rates of many chemical processes. Consequently this possible complication must always be kept in mind. Exchange phe-... 

In the coupled metal systems that will be discussed in Chapter 6, the effect of temperature is generally more complicated. Besides the effects described above, the overall temperature dependence will obviously also depend on the sign and magnitude of the exchange coupling constant J. The reader is referred to Chapter 6 for more details. 

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