title: The Thermodynamic and Kinetic Properties of 2-Hydroxypyridine/2-Pyridone Tautomerization: A Theoretical and Computational Revisit.
abstract: The gas-phase thermal tautomerization reaction between 2-hydroxypyridine (2-HPY) and 2-pyridone (2-PY) was investigated by applying 6-311++G** and aug-cc-pvdz basis sets incorporated into some density functional theory (DFT) and coupled cluster with singles and doubles (CCSD) methods. The geometrical structures, dipole moments, HOMO-LUMO energy gaps, total hyperpolarizability, kinetics and thermodynamics functions were monitored against the effects of the corrections imposed on these functionals. The small experimental energy difference between the two tautomers of 3.23 kJ/mol; was a real test of the accuracy of the applied levels of theory. M062X and CCSD methods predicted the preference of 2-HPY over 2-PY by 5-9 kJ/mol; while B3LYP functional favoured 2-PY by 1-3 kJ/mol. The CAM-B3LYP and ωB97XD functionals yielded mixed results depending on the basis set used. The source of preference of 2-HPY is the minimal steric hindrance and electrostatic repulsion that subdued the huge hyperconjugation in 2-PY. A 1,3-proton shift intramolecular gas-phase tautomerization yielded a high average activation of 137.152 kJ/mol; while the intermolecular mixed dimer interconversion gave an average barrier height of 30.844 kJ/mol. These findings are boosted by a natural bond orbital (NBO) technique. The low total hyperpolarizabilities of both tautomers mark out their poor nonlinear optical (NLO) behaviour. The enhancement of the total hyperpolarizability of 2-HPY over that of 2-PY is interpreted by the bond length alternation.
authors.affiliation: Chemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia. shegazi@hotmail.com.
authors.forename: Safiyah A
authors.initials: SA
authors.lastname: Hejazi
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. aalyoubi@kau.edu.sa.
authors.forename: Abdulrahman O
authors.initials: AO
authors.lastname: Alyoubi
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. rhilal@kau.edu.sa.|Chemistry Department, Faculty of Science, Cairo University, Cairo 11258, Egypt. rhilal@kau.edu.sa.
authors.forename: Rifaat H
authors.initials: RH
authors.lastname: Hilal
mesh_terms: D007536:Isomerism; D007700:Kinetics; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D011728:Pyridones / Q000737:chemistry*; D011789:Quantum Theory; D013816:Thermodynamics
chemical_list: D011728:Pyridones; C017679:2-hydroxypyridine
keywords: 2-hydroxypyridine; 2-pyridone; NBO; NLO; tautomerization