Two Examples of Tautomerization

Before going into the details about the necessary modifications of TST, it is probably a good idea to introduce a few model systems, and show in some detail why the above-mentioned requirements are incorrect for protic tautomerization reactions. 

The two examples given set the paradigm for an enormous class of proton tautomerization reactions. In both cases, a proton is moved from one position in the molecule to another. Sometimes, this can be accomplished by a direct transfer as in the case of o-HBA and derivatives, and sometimes solvents, in particular the presence of H or OHor a number of bridging solvent molecules are actively involved. As was shown in Chapter 1 of this book, reactions of the first type have been the subject of study since the early 1900s, and mainly in the ground state, while reactions of the second type are a major topic of interest as the advance of fast pulsed lasers, and mainly in the excited state. But both cases have a number of things in common, which are relevant for the study of the kinetics of these reactions. 

Comparing these observations to the requirements of TST, we can immediately see a number of problems. Even apart from the fact that the mass of the proton requires it to be treated as a quantum mechanical particle, so that even if there were a well-defined barrier, we would still need to take the possibility of tunneling into account. Transfer of the proton is directly coupled, or may even be driven by a redistribution of electron density in the molecule. In excited-state intramolecular proton transfer (ESIPT) reactions, the redistributed charge almost certainly provides the driving force. The generic picture for such is reaction is due to WeUer [7, 8], who was the first to realize that the enormous Stokes shift of about 10 000 cm he observed in the fluorescence of salicylic acid (X = OH) could be a consequence of a rapid proton transfer in the excited state. 

The experimental situation with enoHzation reactions of acetaldehyde compounds is worse. But from the theoretical side it is obvious that an electronic rearrangement drives, accompanies, or follows the proton transfer. From our point of view, this is important because it means we go to a completely different molecule, as aU the nuclei must move. This, again, has serious consequences for the molecular properties of the compound, for instance its dipole moment This also appHes to both types of molecules, and this, again, has serious consequences for the interaction with the environment, which, in the more interesting cases such as reaction sin water, is polarizable. 

This in itself leads to another compHcation. The dipole moment of a molecule has two contributions nuclear and electronic. The nuclear part can to a good 

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Figure 2.20 Two examples of tautomerization from p-iminoketone to aminoenone.

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