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paper20134

Barrierless Inter and Intramolecular Proton Transfer; A DFT Study of Tautomerism in Microsolvated and Protonated Systems

 

Hossein Tavakol*

Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran
 
J. Phys. Chem. A 2013, 117, 6809−6816
 
ABSTRACT: DFT calculations were employed to study tautomers of 2,2-di(pyrimidin-2-
yl)acetic acid (DPA) in the gas phase and in the presence of water, methanol, DMSO
(microsolvation models), and proton (H+). In the gas phase, all three tautomers were in
equilibrium with each other (T2 was slightly more favorable); but, in microsolvation with
protic solvent, T3 was a major isomer. Moreover, unexpectedly enough, T1 (common
form of DPA) was the least stable tautomer in the gas phase, while, in the water (as
solvent), T1 was the most stable one. The calculated energy values indicated a very special
barrierless tautomeric system between T2 and T3. In the gas phase, ΔG# for T2 → T3
was only 0.30 kcal/mol, while it was 56.4 and 44.8 kcal/mol for T2 → T1 and T3 → T1,
respectively. Rate constants for conversion of T2 into T3 were high in all cases, and their
values in the gas phase and in the presence of water, methanol, DMSO, and proton were 3.6 × 1012, 3.3 × 109, 2.7 × 1011, 2.3 ×
1012, and 2.3 × 1012 s−1, respectively. All types of microsolvation slightly decreased the rate constant of this conversion. The rate
constant for conversion of T2 into T1 (1,3-H shift) in the gas phase and in the presence of water, methanol, DMSO, and proton
was 3.0 × 10−29, 2.2 × 105, 1.1 × 10−34, 5.2 × 10−40, and 4.2 × 10−26 s−1, respectively. These values showed that only
microsolvation with water strongly increased rate constants of tautomerism for T2 → T1 and this type of tautomerism was
impossible in the gas phase or other microsolvation models. Although using water as the solvent enhanced rate constants of all
types of tautomeric interconversions, its enhancement was less than that of microsolvation with water.

 

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