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Proton transfer, acid computational

Figure 12. Proton transfer potentials computed for formic acid - methyleneimine pair at the MP2/6-31G level. R(N-O) is held fixed at 3.25 A. The dielectric continuum of the surrounding medium is indicated by e. Figure 12. Proton transfer potentials computed for formic acid - methyleneimine pair at the MP2/6-31G level. R(N-O) is held fixed at 3.25 A. The dielectric continuum of the surrounding medium is indicated by e.
The detailed mechanism of this enantioselective transformation remains under investigation.178 It is known that the acidic carboxylic group is crucial, and the cyclization is believed to occur via the enamine derived from the catalyst and the exocyclic ketone. A computational study suggested that the proton transfer occurs through a TS very similar to that described for the proline-catalyzed aldol reaction (see page 132).179... [Pg.139]

There has been considerable interest in the last two decades in the site of acidity of hydroxamic acids. There are two possible acidity sites the NH and OH fragments in the C(0)NH0H group. It was originally assumed that these compounds were acidic due to the OH group. Since then, much effort has been expended on the determination of the site of proton transfer under various conditions by both experimental and computational studies. The proton transfer leads to the following possible products ... [Pg.588]

In the next chapter by Ibon Alkorta and Jose Elguero, applying computational approaches presents interrelations between aromaticity and chemical and physicochemical properties of heterocycles. The following problems and properties are considered tautomerism, conformation analysis, acid-base equilibria, H-bonding and proton transfer, energetics, reactivity, IR-, NMR-, and MW-spectroscopies. At the end is a discussion of problems related to supramolecules and macrocycles. [Pg.353]

Prior to 2001, when the first serious computational approaches to the problem appeared in print, four mechanistic proposals had been offered for understanding the Hajos-Parrish-Wiechert-Eder-Sauer reaction (Scheme 6.8). Hajos and Parrish proposed the first two mechanisms Mechanisms A and B. Mechanism A is a nucleophilic substitution reaction where the terminal enol attacks the carbinolamine center, displacing proUne. The other three mechanisms start from an enamine intermediate. Mechanism B invokes an enaminium intermediate, which undergoes C-C formation with proton transfer from the aminium group. Mechanism C, proposed by Agamii to account for the nonlinear proline result, has the proton transfer assisted by the second proline molecule. Lastly, Mechanism D, proffered by Jung, proposed that the proton transfer that accompanies C-C bond formation is facilitated by the carboxylic acid group of proline. [Pg.408]

Houk employed SMD(water)/M06-2x/6-31-l-G(d,p) computations to model these histidine-catalyzed reactions. For the reaction of isobutyraldehyde with formaldehye, eight TSs were located, four with the carboxylic acid group as the proton donor and fonr with the imidazole as the proton donor. The lowest energy TS for each case has the Houk-List geometry with the proton transfer occurring with the C-C formation. In this case, the two lowest TSs are of equal energy. [Pg.416]

Zielinski, T. J. and Poirier, R. A., Examination offormamide, formamidic acid, amidine dimers, and the double proton transfer transition states involving these dimers, J. Comput. Chem. 5, 466-470(1984). [Pg.134]

None of the complexes pairing an amine with a hydrogen halide were computed to be of the pure ion pair variety with a minimal basis set ° . However, there were some that were close. More specifically, since the equilibrium position of the proton need not shift precipitously from one atom to the other, but rather can move gradually as the acidity and basicity increase, a proton-transfer parameter was devised to indicate the degree of transfer of the proton from the acid to the base. The quantity p was defined as... [Pg.333]

The analysis could be extended to other processes, as acid-base equilibria and reactions related to intermolecular proton transfer, or involving a change of electronic state, but what has been said is sufficient to convey the essential message, i.e. the determination of the energetic balance of a reaction, and of the equilibrium between reagents and products, albeit conceptually simple, requires a serious consideration of the computational tools one has to select. [Pg.12]

The efficient photodecarboxylation of the keto acids (77) has been studied. The reactions involve the formation of the carbanions (78). Aqueous solutions of fenofibric acid (79) at pH 7.4 show the formation of two intermediates when subjected to laser excitation. The study has indicated that the triplet state of the acid in water is of a jtji type. Photoionization is an important process in the aqueous medium. New photoreactive phenylalanine analogues (80) and (81) have been prepared. These were incorporated into position 5 of the pentapeptide, thymopentin. The resultant derivatives were photolabile and underwent decomposition on irradiation at 365 nm. Computational methods have been used to analyse the photoreactivity of the tryptophan derivative (82). The calculations were directed towards an understanding of the quenching of the fluorescence. The results indicate that hydrogen transfer alone does not quench the fluorescence, but that an aborted decarboxylation path is involved. Proton transfer... [Pg.11]


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