Barriers by Dynamic NMR Spectroscopy

Rate process with activation energies between 6 and 25 kcal mol-1 can be studied conveniently by the NMR method. Line shape theories have been well reviewed.103 In most cases a single rate constant k is estimated at the coalescence temperature, and AG is obtained from the Eyring equation. For exchange between two unequally populated sites, AG for the forward and reverse reactions are different. This is an important point to be considered in the case of nitrogen inversion phenomena. 

A fundamental point arises in the interpretation of NMR coalescence data.2 The rate constant obtained from an NMR experiment is always the sum of the forward and reverse rate constants, but three cases can be distinguished  

Binsch and H. Kessler, Angew. Chem., lnt. Ed. Engl. 19, 411 (1980) J Sandsirom, Dynamic NMR Spectroscopy . Academic Press, New York, 1982. 

Neglect of these three points has led to much confusion in the literature. It was therefore suggested2 that all AG1 be referred to half barriers. It is clearly necessary to state which half barrier is under discussion. 

For biased equilibria, Anet10 5 developed a method of estimating k and the population of the least stable conformer from line-broadening measurements. This method uses the expression developed by Gutowsky and Holm106 from the Bloch equations 

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