Tautomerizations without Requiring Reorganization of the Environment

A consequence is that such tautomerizations can not only take place in the liquids but often also in the solid state. The reaction barriers depend strongly on the heavy atom distances 2 and the energy needed for their compression. The occurrence of major electrostatic changes in the transition or intermediate states will only lead to a barrier increase which might, however, be substantial. In the following examples, different reaction types are discussed, which have been studied using dynamic liquid and solid state NMR. 

Examples of Intramolecular Tautomerizations without Requiring Reorganization of the Environment 

In this section we will discuss some typical examples exhibiting intramolecular tautomerizations whose kinetics have been studied by dynamic NMR in various environments. Often, the gas phase and liquid state degeneracies were found to be slightly disturbed in the solid state. In most cases, nonlinear Arrhenius curves were observed, which indicated the role of vibrationally assisted tunneling, supported in almost all cases by the observation ofnonclassical kinetic isotope effects. 

Bottom Arrhenius diagram of the H-, D-, T-tautomerization of the porphyrin anion 2. (Reproduced from Braun et al. [19]. Copyright (1994), with permission of Wiiey.) 

As illustrated by the dashed lines, porphyrin 3 constitutes an 18 jc-electron system with two peripheral double bonds that move during the tautomerization. 

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Exampies of intermoiecuiar Tautomerizations in the Absence of Pre-Equilibria without Requiring Major Reorganization of the Environment... 

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