Proton Transfer in Systems with the Intramolecular Hydrogen Bonding

3 Proton Transfer in Systems with the Intramolecular Hydrogen Bonding 

Transformations of this type play an important role in the mechanism of quite a few reactions, in particular, it is they which lie at the root of the intramolecular tautomerism [52]. It is therefore not surprising that the theoretical investigation of the intramolecular proton transfers is represented by a wide range of calculations of methodological importance. Particularly thoroughly was studied the reaction of the 1,5-proton shift in 3-oxy-2-propenal (cis-enol of malonal-dehyde) XI which affords one of the most significant examples of the degenerate intrachelate tautomerism. 

In the case of XI, the presence of two structures of and C2y symmetry may be assumed whose nature and interconversions will determine the PES profile (Fig. 9.4)  

The nonempirical methods provide, in the main, consistent results. The size of the basis set and inclusion of correlation effects affect the quantitative estimates without altering the qualitative conclusion as to the energetic preferability of the Cg structure. 

A need for a complete optimization of geometry may be illustrated by two nonempirical calculations done with nearly equivalent basis sets. The conclusion [57] that the C2V form is more stable (the type-b PES in Fig. 9.4) was disproved through a more careful geometry optimization of the Cg structure [58]—the PES of type c in Fig. 9.4. An analysis of the force constants and the complete optimization of geometry [59] have made it possible to draw a quite definitive conclusion that in malonaldehyde there exists a symmetrical double-well potential with a transition state corresponding to an unstable structure of C2V symmetry (potential curve c in the Fig. 9.4). 

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