Abstract
The gas-phase thermal tautomerization reaction between imidazole-4-acetic (I) and imidazole-5-acetic (II) acids was monitored using the traditional hybrid functional (B3LYP) and the long-range corrected functionals (CAM-B3LYP and ωB97XD) with 6-311++G** and aug-cc-pvdz basis sets. The roles of the long-range and dispersion corrections on their geometrical parameters, thermodynamic functions, kinetics, dipole moments, Highest Occupied Molecular Orbital–Lowest Unoccupied Molecular Orbital (HOMO–LUMO) energy gaps and total hyperpolarizability were investigated. All tested levels of theory predicted the preference of I over II by 0.750–0.877 kcal/mol. The origin of predilection of I is assigned to the H-bonding interaction (nN8→σ*O14–H15). This interaction stabilized I by 15.07 kcal/mol. The gas-phase interconversion between the two tautomers assumed a 1,2-proton shift mechanism, with two transition states (TS), TS1 and TS2, having energy barriers of 47.67–49.92 and 49.55–52.69 kcal/mol, respectively, and an sp3-type intermediate. A water-assisted 1,3-proton shift route brought the barrier height down to less than 20 kcal/mol in gas-phase and less than 12 kcal/mol in solution. The relatively high values of total hyperpolarizability of I compared to II were interpreted and discussed.
Highlights
Imidazole is a heterocyclic polar organic compound
∆G values at 298.15 K for all applied levels of theory are dictated mostly by ∆H (61%–67%) with moderately less contribution from T∆S (33%–39%). These percentage differences are attributed to long-range and dispersion corrections on functionals [15,16], rather than by basis set effects
The barrier height decreased further to ca. 11.713 kcal/mol. These results indicate that the gas- or solution-phase IØII tautomerization of imidazoleacetic acid are feasible at room temperature when assisted by water molecules
Summary
Imidazole is a heterocyclic polar organic compound. Its 5-membered ring is planar. Imidazoleacetic acid is one of the most important derivatives of imidazole It is one possible paths of histamine metabolism by being an intermediate in the physiological oxidation of histamine [2]. Imidazole-5-acetic acid derivatives have anti-inflammatory activities [7], an antagonistic effect on the peptide hormone, angiotensin II, in addition to some hypotensive activities. They are useful as hypotensive agents [8]. Do not facilitate chelatiomne[t1a1Al ]cn.aotitohners very important application of imidazole-4-acetate anion is its affinity for transition forming complexes via bonding through the carboxylic oxygen and the un-protonated. The crystal structure of imidaLzeovleel-4o-facetic Nac8id–Ch1y–dNro4chloriCde1–isNg8i–vCen for coNm8p–aCr2is–oCn3purpoCse3s–.N4–C1
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