symmetry S S Article Evaluation of (Z)-5-(Azulen-1-ylmethylene)-2- thioxothiazolidin-4-ones Properties Using Quantum Mechanical Calculations Alina-Alexandra Vasile (Corbei) 1 , Eleonora-Mihaela Ungureanu 2, *, Gabriela Stanciu 3 , Mihaela Cristea 4 and Amalia Stefaniu 5, *   Citation: Vasile (Corbei), A.-A.; Ungureanu, E.-M.; Stanciu, G.; Cristea, M.; Stefaniu, A. Evaluation of (Z)-5-(Azulen-1-ylmethylene)-2- thioxothiazolidin-4-ones Properties Using Quantum Mechanical Calculations. Symmetry 2021, 13, 1462. https://doi.org/10.3390/ sym13081462 Academic Editors: Ramon Carbó-Dorca and Miroslav Miletín Received: 30 April 2021 Accepted: 2 August 2021 Published: 10 August 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Inorganic Chemistry, Physical Chemistry and Electrochemistry, University “Politehnica” of Bucharest, 011061 Bucharest, Romania; vasilecalinaalexandra@yahoo.com 2 Faculty of Applied Chemistry and Materials Science, University “Politehnica” of Bucharest, Gheorghe Polizu 1-7, Sector 1, 011061 Bucharest, Romania 3 Department of Chemistry and Chemical Engineering, “Ovidius” University Constanta, 900527 Constanta, Romania; gstanciu@univ-ovidius.ro 4 Institute of Organic Chemistry “C. D. Nenitzescu” of the Romanian Academy, 71141 Bucharest, Romania; mihaela.cristea@ccocdn.ro 5 National Institute of Chemical—Pharmaceutical Research and Development—Bucharest, 031299 Bucharest, Romania * Correspondence: em_ungureanu2000@yahoo.com (E.-M.U.); astefaniu@gmail.com (A.S.) Abstract: Derivatives of (Z)-5-(azulen-1-ylmethylene)-2-thioxothiazolidin-4-one are reported as heavy metal (HM) ligands in heterogeneous systems based on chemically modified electrodes. Their ability to coordinate HMs ions has recently been shown to be very selective. In this context, an additional computer-assisted study of their structure was performed using density functional theory (DFT) to achieve a complex structural analysis. Specific molecular descriptors and properties related to their reactivity and electrochemical behaviour were calculated. The correlation between certain quantum parameters associated with the general chemical reactivity and the complexing properties of the modified electrodes based on these ligands was carried out to facilitate the design of molecular sensors. Good linear correlations between DFT-calculated HOMO/LUMO energies and experimental redox potentials were found. A good agreement between the chemical shifts predicted by the DFT method and those determined experimentally from NMR data for these ligands demonstrated the accuracy of the calculations to assess the structural data. Such a computational approach can be used to evaluate other properties, such as electrochemical properties for similar azulene derivatives. Keywords: (Z)-5-(azulen-1-ylmethylene)-2-thioxothiazolidin-4-one derivatives; quantum mechanical calculations; molecular and QSAR properties; reactivity parameters; electrochemical properties 1. Introduction Previous studies correlate the electrochemical properties for various organic com- pounds with structural parameters using density functional theory (DFT) calculations to achieve the rational design of new materials with improved electrochemical proper- ties [1–8]. They are based on the link between the energy levels corresponding to the highest occupied molecular orbital (HOMO) or the lowest unoccupied molecular orbital (LUMO) and the electrochemical oxidation and reduction potentials, respectively [9–11]. Strong linear correlations of DFT-calculated HOMO/LUMO energies using B3LYP/6- 31G(d) functional [12,13] and experimental redox potentials were found for polycyclic aromatic hydrocarbons by D. Méndez-Hernández and co-workers [14], highlighting the idea that quick, accurate and low-cost predictions using the B3LYP/6-31G(d) functional represent a reliable approach to apply on other molecules to evaluate their electrochemical properties. A comparative computational study using different density functionals on Symmetry 2021, 13, 1462. https://doi.org/10.3390/sym13081462 https://www.mdpi.com/journal/symmetry