Proton Transfer in Malonaldehyde Enol

The enol form of malonaldehyde is favored over the tautomeric aldehyde due to the presence of an intramolecular hydrogen bond. It constitutes one of the smallest model systems 

64 Similar findings have been reported already in the literature (Sim et at, 1992). 

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Figure 13-4. Energy profile for the proton transfer in malonaldehyde enol. 

Table 13-6. Computed total energy differences [kcal/mol] and distances [A] for the proton transfer in malonaldehyde enol (6-311++G(d,p) basis). 

Malonaldehyde, CH2(CHO)2, exists as an intramolecularly hydrogen-bonded enol (86) in the vapour phase. Molecular dynamics calculations suggest that while a short 0—0 distance favours proton transfer to an (identical) tautomer, such proximity is neither a sufficient nor a necessary condition. 

Fig. 5.7 Chemical structure and proton tunneling within the enol form of malonaldehyde. The proton of the hydroxyl group (a) is transferred via a tiansition state (b) to the oxygen of the (former) aldehyde group (c). During the transfer the single- tmd double-bonds as well as hydroxyl and aldehyde groups are interchanged resulting in a mirrored variant of the original molecule...

A new ab initio semi-classical technique for the calculation of tunnelling effects has been demonstrated for the case of the interconversion of the (degenerate) enols of malonaldehyde via intramolecular proton transfer the method incorporates tunnelling into first principles molecular dynamics. The use of density functional theory to model transition states in this reaction has also been described even the most advanced levels of theory meet substantial difficulties in such systems. 

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