Diotropic systems

The double proton migrations can occur also in intramolecular reactions, for example, in diotropic systems [52]. These comprise the compounds whose transition into a tautomeric form requires the transfer of two migrants (such as the protons), and, in terms of the classic structural notions, this transfer must be concerted since a single transfer would result in the formation of a nonclassic structure of the zwitterionic or biradical type. A simplest example of such systems is provided by the molecule of the oxalic acid for which, depending on the nature of the stationary point on the PES that corresponds to a structure with one transferred proton, two in principle different reaction mechanisms may be realized ... 

At the same time, in the diotropic systems with a stereochemistry more favorable for a proton transfer, there are no minima on the PES that would... 

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