Time-Dependence of NMR Phenomena

The time scale of the NMR phenomenon is best realized when it is recalled that NMR transitions occur at the low-frequency end of the electromagnetic spectrum. 

Consider two protons situated in different environments. They will give rise to two separate resonances in the NMR spectrum, say Av Hz apart (Fig. 12.9C). However, if, by one of the mechanisms discussed below, the two protons exchange their environments at a rate faster than Av times per second, one obtains only one signal, at an intermediate frequency (Fig. 12.9A) the two nuclei are equivalent on the NMR time scale. 

The most common observable mechanisms for averaging the environments of protons, or of groups of equivalent protons, that can be observed on the NMR time-scale are proton exchange, conformational changes, and rotation about partial doublebonds. An example of each follows  

Proton exchange. In dilute solutions in aprotic solvents, the hydroxylic protons of mixtures of ethanol and water give rise to separate signals. However, an increase in temperature or concentration, or a change in pH, speeds up the prototropic exchange so that only one signal for the -OH protons is observed. 

Conformational changes. At room temperature, the NMR spectrum of cyclohexane consists of a single sharp line, because the rate of conformational inversion between the two equivalent chair forms, which is associated with the interchange between axial and equatorial positions, is fast compared with the difference (in Hz) between the chemical shifts of axial and equatorial protons. At about — 160°C, this inversion slows down enough to make separate signals for the axial and the equatorial protons observable. 

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If this were all NMR had to offer, it would not be considered particularly useful in chemical investigations, since all one achieves is a costly and inconvenient estimate of the total hydrogen, fluorine, etc., content in a sample. In practice, all applications of NMR to chemistry are from three secondary phenomena the chemical shift the time-dependence of NMR phenomena and spin-spin coupling. These effects will now be discussed. 

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