Malonaldehyde, intramolecular transfers

The reorientation of partially deuterated rotors, as discussed in Section IV.A.5, is an example of proton displacements between positions that are distinguishable. Even when these positions are equivalent in the isolated molecule, the interaction with an environment of lower symmetry usually makes the positions inequivalent so that the PES becomes asymmetric. This situation applies also for proton translations in intra- and intermolecular PT processes, examples of which are discussed below (Sections IV.B.l and IV.B.2). Prototype examples for intramolecular transfers are the tautomer-ization of free base porphyrines and phthalocyanines and the PT in malonaldehyde, tropolone, 9-hydroxyphenalone, and so on. The tautomer-ization of carboxylic dimers is the best studied example of intermolecular transfer in a (near) symmetric PES. 

One of the most thoroughly studied examples of intramolecular tunneling is isomerization of malonaldehyde involving the transfer of an H atom in an OH O fragment. 

Carrington, T., Jr., Miller, W. H., 1986, Reaction Surface Description of Intramolecular Hydrogen Atom Transfer in Malonaldehyde , J. Chem. Phys., 84, 4364. 

Coe JD, Martinez TJ (2006) Ab initio molecular dynamics of excited-state intramolecular proton transfer around a three-state conical intersection in malonaldehyde. J Phys Chem A 110 618-630... 

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. 

One of the most well-known examples of intramolecular tunneling is the isomerization of malonaldehyde consisting in hydrogen transfer in the fragment OH O ... 

Figure 6.3. Geometries of (a) the reactants and (b) the transition state for the intramolecular proton transfer in malonaldehyde. The bond length is given in angstroms. From Bosch et al. [1991].)...

Tautomeric equilibrium in aqueous cw-malonaldehyde, see reaction 1 in Figure 8-4, is a prototypical reaction extensively studied in the gas phase but still relatively unknown in solution. In fact, despite the large number of NMR experiments [52,53,54] and quantum chemical calculations [55] with the polarized continuum model (PCM), [1] the actual stability of czT-malonaldehyde is not well clarified, although the trans isomer should be the predominant form in water. Secondly, the involvement of the light proton in the reaction may in principle provide relevant quantum effects even in condensed phase. All these complications did not prevent this reaction to be used as a prototypical system for theoretical studies of intramolecular proton transfer in condensed phase by several investigators [56,57,58,59,60] including ourselves. 

Figure 8-4. Schematic picture of tautomeric intramolecular proton transfer in malonaldehyde...

We have applied this method to saddle crossing dynamics in intramolecular proton-transfer reaction of malonaldehyde [38,39] and isomerization reaction of Aig [40-44]. The former is a reacting system, involving a typical chemical bond breaking-and-forming the latter is the smallest inert gas cluster in which no saddle dynamics more regular than the dynamics within... 

The previous studies of concerted hydrogen atom and proton transfer in hydrogen-bonded systems have been limited to studies of reaction pathways for simple model systems [4-8] with simple, reduced dimensionality methods for including quantum tunneling [13,14,34,35]. The applicability of modem computational methods to such systems is exemplified by more recent studies of the energetics of intramolecular hydrogen atom transfer in molecules such as malonaldehyde [36-39] and models of... 

Fig. 9.4. Possible energy profiles of the intramolecular proton transfer reaction of Eq. (9.8) in malonaldehyde XI...

Table 9.4. Barriers to intramolecular proton transfer (energy differences between the Q and C2V structures) in malonaldehyde calculated in different appproximations...

With the geometry relaxation taken into account, the calculated barrier of Eq. (9.15) (using the basis set STO-3G) is lowered from 48kcal/mol to 14kcal/mol. Characteristic is the reduction of the nonvalent distance O - O from 2.73 A in the starting dimer to 2.30 A in the transition state. This trend toward compression of the reactive site and, as a consequence, lowering in the activation barrier was noted earlier in connection with intramolecular proton transfer in malonaldehyde. 

Inclusion of an additional water molecule leading to a six-membered cycle, as in the malonaldehyde XI, provides more favorable steric conditions in XXI, XXII for proton transfer. Thus, for formamidine hydrate the calculated barrier of tautomerization XXIa XXIIb, when intermolecular mechanism is operative, (17 kcal/mol with the 3-21G and 21 kcal/mol with the 6-31G basis set) is three times lower than in the case of the intramolecular reaction Xllla Xlllb (see Sect. 9.2). 

Yagi K, Taketsugu T, Hiiao K (2001) Generation of full-dimensional potential eneigy surface of intramolecular hydrogen atom transfer in malonaldehyde and tunneling dynamics. J Chem Phys 115 10647... 

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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