title: Gas-Phase Thermal Tautomerization of Imidazole-Acetic Acid: Theoretical and Computational Investigations.
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.
authors.affiliation: Chemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia. saziz@kau.edu.sa.
authors.forename: Saadullah G
authors.initials: SG
authors.lastname: Aziz
authors.affiliation: Chemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia. oabdelkarim@kau.edu.sa.|Chemistry Department, Faculty of Science, University of Khartoum, P.O. Box 321, Khartoum 11111, Sudan. oabdelkarim@kau.edu.sa.
authors.forename: Osman I
authors.initials: OI
authors.lastname: Osman
authors.affiliation: Chemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia. skamel@kau.edu.sa.|Chemistry Department, Faculty of Science, Beni-Suef University, Beni-Suef 62511, Egypt. skamel@kau.edu.sa.
authors.forename: Shaaban A
authors.initials: SA
authors.lastname: Elroby
authors.affiliation: Chemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia. rhilal@kau.edu.sa.|Chemistry Department, Faculty of Science, Cairo University, Giza 12613, Egypt. rhilal@kau.edu.sa.
authors.forename: Rifaat H
authors.initials: RH
authors.lastname: Hilal
mesh_terms: D000085:Acetates / Q000737:chemistry*; D006860:Hydrogen Bonding; D007093:Imidazoles / Q000737:chemistry*; D008956:Models, Chemical; D008958:Models, Molecular; D008962:Models, Theoretical; D013816:Thermodynamics
chemical_list: D000085:Acetates; D007093:Imidazoles; C029899:imidazole
keywords: dispersion; imidazole-4-acetic acid; long-range; tautomerization