Temperature Coalescence Studies DNMR

It must be emphasized that all time-dependent chemical phenomena, such as tautomerism, are sensitive to temperature changes. In this section, we treat classic DNMR (dynamic NMR) studies and cases of changes in the NMR spectra with temperature together. In Table XI we have classified these studies according to the physical state of the sample and to the nuclei 

Both tautomers at RT (unusual) Both tautomers on cooling (Kt) Annular tautomerism in azoles (AG and 4G ) 

The imbalance between and NMR studies in the solid state (Section VI,F) partly reflects the fact that it is easier to introduce N than into heterocyclic compounds, particularly azoles (DNMR in the solid state usually requires isotopic enrichment). Compared to solution studies, solid-state intermolecular proton transfer between tautomers has the enormous advantage that the structure of the species involved is precisely defined. 

Some papers (second column of Table XII) report the observation of individual tautomers on cooling. These results were used to determine the populations by integration and, therefore, of Kt and dG. 

The largest series of papers (third column) is related to the study of annular tautomerism in azoles. This class of tautomerism is especially well suited for DNMR because in many cases Kt = 1 (autotrope) and because the activation barriers are in the range easy to be measured by NMR. To 

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