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

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 × 10(12), 3.3 × 10(9), 2.7 × 10(11), 2.3 × 10(12), and 2.3 × 10(12) 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 × 10(5), 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.

Ring Opening and Closure Reactions of β‐Propiolactone in the Superacids HF/MF5 (M=Sb, As)

“…The energy gain of the system by ring opening appears quite small with only 0.7 kJ/mol, but the energy barrier of the intramolecular proton transfer is very high (243 kJ/mol). For proton transfers several theoretical studies have been performed to investigate the influence of solvents on these calculations [24–27] . These studies concluded that in particular protic solvents are able to decrease calculated intrinsic barriers compared to the gas phase and facilitate proton transfer [24–27] .…”

“…For proton transfers several theoretical studies have been performed to investigate the influence of solvents on these calculations [24–27] . These studies concluded that in particular protic solvents are able to decrease calculated intrinsic barriers compared to the gas phase and facilitate proton transfer [24–27] . Yu et al.…”

The Substituent Effect on the Energy Profiles of Inter and Intramolecular Proton Transfers between Phosphine Oxide‐Phosphinous Acid Tautomers

“…It has been proven that the energy profiles of different phenomena in the solvent are close to the results obtained from the real biological system and in many computational studies, solvents have been employed to obtain the more reliable results. Therefore, because of the importance of the phosphine oxide-Phosphinous acid tautomeric process, to continue our previous studies in this area [20][21][22], it could be useful to study all possible mechanistic pathways for this process and determine their thermodynamic and kinetic behaviors (in the gas phase and solvent), especially in the presence of different substituents to study their effect on the kinetics and thermodynamics of the tautomeric process.…”

DFT Study of the effects of counter ions on bonding, molecular and spectral properties of pentaflourophenyl xenonium diflouride cation

“…One of them is the effect of counter ions on the stabilities and properties of organoxenon cations. Therefore, to pursue our recent studies on computational chemistry of chemical structures [50][51][52] and study of noble gas compounds, 53 we have decided to focus on the bonding, spectral and molecular properties of new organoxenon compounds.…”