Equilibrium Problems Studied by

Recently there have been critical re-appraisals of the n.m.r. methods for examining equilibrating systems, since results from different 

Boum describe their use of the line shape method in the temperature range —24° to — 82°C (deuterium decoupling was used so that only the unperturbed single proton resonance was observed) and of a double-resonance method in the temperature range —97° to — 116°C. This involved the observation of recovery of magnetization of one of the two lines in the spectrum after a saturating r.f. field applied to the other line was removed. Consistent rates of inversion were found from both methods as evidenced by linearity of the Arrhenius plot. The results do not agree with the spin-echo results of Allerhand et al In this type of work, while fairly consistent results of rate constants may be obtained, there is dispute as to how the thermodynamic parameters should be derived, even in the relatively simple case of cyclohexane.  

A comparison of the line-shape method with an equilibration method has shown good agreement in results (the Arrhenius plots for both methods are colinear). The example chosen was the interconversion of rotamers in A7 -methyl-A(-benzylformamide, which between 90° and 170°C shows averaged spectra. One of the rotamers can be obtained substantially pure at low temperatures, and the return to equilibrium was studied in the range —2 5° to 20°C. A similar study using AT-benzyl-A -2,4,6-tetramethylbenzamide and N,AT-2,4,6-penta-methylbenzamide has also been made. ° 

For most purposes, organic chemists are satisfied with less rigorous treatments of equilibrium processes than those exemplified by the above. In the remainder of this Section are listed some of the many such treatments, some of which are more rigorous than others. They are classified according to the nature of the equilibrium involved. For convenience, those systems in which an equilibrium process was shown to be absent, and those, such as keto-enol systems, where equilibration is slow, are included here. 

---