Theoretical and Computational Investigations on Gas-Phase Thermal Tautomerization of Imidazole-Acetic Acid

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Theoretical and Computational Investigations on Gas-Phase Thermal Tautomerization of Imidazole-Acetic Acid

February 17, 2021 Chemistry and Biochemistry 0

Using standard hybrid functional (B3LYP) and long-range corrected functional sets (CAM-B3LYP and ⁇ B97XD) with 6-311++G** and aug-cc-pvdz basis sets, the gas-phase thermal tautomerization reaction between imidazole-4-acetic (I) and imidazole-5-acetic (II) acids was monitored. On their geometrical parameters, thermodynamic functions, kinetics, dipole moments, Highest Occupied Molecular Orbital-Lowest Unoccupied Molecular Orbital (HOMO-LUMO) energy gaps and complete hyperpolarizability, the roles of the long-range and dispersion corrections were studied. The preference of I over II by 0.750–0.877 kcal/mol was predicted by all tested theory levels. The H-bonding relationship is assigned to the root of the predilection of I ( ). I stabilized this interaction to 15.07 kcal/mol. A 1.2-proton shift mechanism was assumed by gas-phase interconversion between the two tautomers, with two transition states (TS), TS1 and TS2, with energy barriers of 47.67-49.92 and 49.55-52.69 kcal/mol, respectively, and an intermediate sp3-type. The barrier height was reduced to less than 20 kcal/mol in gas-phase and less than 12 kcal/mol in solution by a water-assisted 1,3-proton shift path. Compared to II, the relatively high values of total hyperpolarizability of I were interpreted and debated. As the conventional hybrid functional overestimates it, the long-range correction is very important for complete hyperpolarizability DFT evaluation. Finally, the estimation of total values is independent of the base set.

Author (s) Details

Prof. Saadullah G. Aziz
Chemistry Department, Faculty of Science, King Abdulaziz University, P.O.Box 80203, Jeddah 21589, Saudi Arabia.

Prof. Osman I. Osman
Chemistry Department, Faculty of Science, King Abdulaziz University, P.O.Box 80203, Jeddah 21589, Saudi Arabia and Chemistry Department, Faculty of Science, University of Khartoum, P.O.Box 321, Khartoum 11111, Sudan.

Prof. Shaaban A. Elroby
Chemistry Department, Faculty of Science, King Abdulaziz University, P.O.Box 80203, Jeddah 21589, Saudi Arabia and Chemistry Department, Faculty of Science, Beni-Suef University, Beni-Suef 62511, Egypt.

Prof. Rifaat H. Hilal
Chemistry Department, Faculty of Science, King Abdulaziz University, P.O.Box 80203, Jeddah 21589, Saudi Arabia and Chemistry Department, Faculty of Science, Cairo University, Giza 12613, Egypt.

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