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.
Highlights
2-Hydroxypyridine (2-HPY) is an aromatic heterocyclic compound
Some selected geometrical parameters of 2-HPY and 2-PY molecules obtained by B3LYP [32,33], CAM-B3LYP [29], ωB97XD [34], M062X [35] and coupled cluster with singles and doubles (CCSD) [36] methods with 6-311++G** [37,38,39] and aug-cc-pvdz [30] basis sets are listed in the Supplementary Materials (Table S4)
The apparent deviation between the calculated and experimental structural parameters could be, in part, due to phase difference, i.e., the theoretical values are for gas-phase molecules, while the experimental ones are for a solid-state compound [2]
Summary
It represents an important class of compounds, which have π- and n-electrons systems It tautomerizes to form 2-pyridone (2-PY) by a proton transfer between the nitrogen and oxygen atoms [1]. Different calculation methods were used to explore the tautomeric equilibrium of 2-hydroxypyridine/2-pyridone systems in the gas-phase [5]. Parchment et al [21] obtained an energy difference of 2.9 kJ/mol between the two gas-phase tautomers using a large basis set (TZV2P) and electron correlation at the QCISD(T) level. A semi-empirical (CNDO/2) calculation predicted a barrier height of 296 kJ/mol for the single molecular 1,3-proton tautomerization [24]. This activation energy approximates to 206 kJ/mol using a Hartree–Fock (HF)/3-21G level of theory [25]. We employed the natural bond orbital technique to support our conjectures
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