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Electron Density of Delocalized Bonds As a Universal Tool for Assessing Global and Local Effects of Chemical Resonance

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Electron Density of Delocalized Bonds As a Universal Tool for Assessing Global and Local Effects of Chemical Resonance Summary of Professional Accomplishments Electron density of delocalized bonds as a universal tool for assessing global and local effects of chemical resonance Dr. Dariusz Wojciech Szczepanik Kraków 2021 1. Name Name and Surname: Dariusz Wojciech Szczepanik Degree: Doctor of Philosophy in Chemistry Current employment: Department of Theoretical Chemistry, Faculty of Chemistry, Jagiellonian University, Gronostajowa 2, 30-387 Kraków, Poland. ORCID: 0000-0002-2013-0617 ResearcherID: E-2787-2014 ScopusID: 36835515900 Personal website: http://www.eddb.pl/aboutme Presentation of the achievements: http://www.eddb.pl/hab 2. Diplomas, degrees conferred in specific areas of science or arts, including the name of the institution which conferred the degree, year of degree conferment, title of the PhD dissertation 2008 Master of Science Department of Computational Methods in Chemistry, Faculty of Chemistry UJ Thesis: „Entropic indices of the chemical bonds from information theory” Supervisor: dr. hab. Janusz Mrozek 2013 Doctor of Philosophy in Chemistry Department of Theoretical Chemistry, Faculty of Chemistry UJ Thesis: „Probabilistic models of the chemical bond in function spaces” (de- fended with honors) Supervisor: dr. hab. Janusz Mrozek 3. Information on employment in research institutes or faculties/departments or school of arts 2015 – 2020 Department of Theoretical Chemistry, Faculty of Chemistry, Jagiellonian Uni- versity. Position: Technician 2018 – 2020 Institute of Computational Chemistry and Catalysis, University of Girona, Posi- tion: EU-researcher (MSCA-IF, postdoc). 2020 – Department of Theoretical Chemistry, Faculty of Chemistry, Jagiellonian Uni- versity. Position: Adjunct 4. Description of the achievements, set out in art. 219 para 1 point 2 of the Act The basis for the scientific achievement entitled “Electron density of delocalized bonds as a universal tool for assessing global and local effects of chemical resonance” is a series of thematically linked publications composed of 9 scientific articles (H1-H9) published in 2014-2019 in peer-reviewed jour- nals from the JCR list, and 1 monographic chapter (H10) of 2021. The subject of scientific articles 2 selected for the habilitation cycle (H1-H9) concerns development of the theoretical basis of my orig- inal computational method that enables one to “extract” from the total electron density of a molecule (obtained from quantum-chemical calculations) the so-called electron density of delocalized bonds (EDDB), which allows visualization and quantification of different effects of electron delocalization in chemical species, regardless of their size, topology and the electronic state. Publication (H10), in turn, can be regarded as a review article. All publications included in the habilitation cycle are based on my own original research ideas, and in all of these works I am the first and the corresponding author. I am the sole author of H3, H4, and H8, while in the case of H9 and H10 the second co-author is Prof. Miquel Solà, at whose invitation these works were prepared. The high percentage estimate of my contribution to the remaining articles is guided by the criterion of the amount of time spent plan- ning research study, writing software and scripts automating calculations (partially carried out by PhD students), elaboration of the results, preparation and submission of the manuscript, improving them in the review process, etc. In addition, I am the sole author of the original model of the "migrat- ing π-cycles" introduced in H7 (and being a generalization of the well-known in the literature model of “the migrating Clar’s π-sextet”). 4.1. Motivation1 It is commonly accepted that electron delocalization in aromatic rings is linked with unusual thermo- dynamic stability by means of the π-electron bookkeeping rules, like “4n+2” and “4n” (depending on the system topology and multiplicity), known from the age-old chemistry textbooks. Although these qualitative criteria of aromaticity (and antiaromaticity) adequately relate topology, symmetry, and degeneracy of molecular orbitals (MO) in the [n]annulene-like systems predominated by covalent resonance forms at their singlet or the lowest-lying triplet states, their use in a more general context regarding topologically diversified poly- and macrocyclic species (e.g. expanded porphyrins predom- inated by ionic forms), non-Kekulé molecules (e.g. radicals), etc., is not well-founded. Over the last decades an overwhelming number of quantitative ‘measures’ of aromaticity has been proposed in the literature, based on energetic, structural, magnetic, and electronic properties of molecules, thus providing a far more accurate account of aromatic stabilization than the electron-bookkeeping crite- ria. The most commonly used quantitative criteria of aromaticity within each of these groups are: 1) the aromatic stabilization energy (ASE), which is an energetic measure of π-aromaticity that emanates from the theory of valence bonds, and it can be evaluated by means of thermodynamic data for iso- desmic, (hyper)homodesmotic or isomerization reactions (also, a multitude of schemes can be found in the literature that allows one to efficiently estimate the aromatic stabilization energies for a specific class of aromatic species); 2) the harmonic-oscillator model of aromaticity (HOMA), which is a π- aromaticity index based on structural properties of molecules (being a normalized measure of devia- tions of bond lengths in aromatic molecule from the corresponding optimum bond lengths in an ide- alized non-aromatic molecule as a reference) – HOMA is close to 0 for nonaromatic species, ap- proaches 1 for highly aromatic ones, while for potentially antiaromatic rings it usually assumes neg- 1 Formal definitions, descriptions and scientific arguments adduced in this section comes from review articles in the spe- cial issue Chem. Soc. Rev 44 (2015), original publications H5, H7, and H10, as well as the following papers: A. Stanger, Chem. Comm. 2009 (2009) 1939; R. Hoffmann, Am. Sci. 103 (2015) 18; M. Solà, Front. Chem. 5 (2017) 22. 3 ative values; 3) the nucleus-independent chemical shift (NICS), which quantifies the effective mag- netic shielding at the centroid (or above) of the aromatic ring in external magnetic field – the more negative (positive) value of NICS, the more aromatic (antiaromatic) is the molecular ring in question; 4) the multicenter index (MCI), which is a non-reference index of aromaticity that can be calculated from both the ab initio molecular wave function as well as the electron density; MCI has been shown to be superior to other aromaticity descriptors as the only one that passes a set of rigorous tests for aromaticity quantifiers designed by Prof. M. Solà (University of Girona, Spain). Unfortunately, each of the above-mentioned aromaticity ‘measures’ has shortcomings and lim- itations that sometimes may lead to wrong predictions. The ASE seems to outwardly be the most adequate measure of global aromaticity since it can be evaluated by means of thermodynamical data and it emerges for the direct relationship between structural consequences of electron delocalization and the stability. However, designing of isodesmic and homodesmotic reaction scenarios is very dif- ficult in practice and opens the door to a lot of arbitrariness. In contrast, an unquestionable advantage of HOMA is its computational and interpretative simplicity – it allows one to straightforwardly clas- sify any molecular ring as aromatic, non-aromatic or (potentially) antiaromatic. Unfortunately, the principal problem with HOMA is the necessity of parametrization of bond lengths for an idealized reference molecule, which obviously cannot be chosen unambiguously. Consequently, the practical use of HOMA is limited to aromatic and heteroaromatic systems since the parameters for chemical bonds with metal atoms are not available. Furthermore, parametrization of HOMA should be per- formed using exactly the same quantum-chemical method as used in calculations of equilibrium ge- ometries of the molecule under study, since routinely computed HOMA with the experimentally de- termined parameters is bound to suffer from large unsystematic errors and strong sensitivity to the choice of the basis set and the exchange-correlation functional in the density functional theory (DFT) based calculations. Aromaticity descriptors based on magnetic properties of molecules, especially NICS(0), NICS(1), and its axial component, NICS(1)zz, dissected NICS, etc., are one the most pref- erable measures of local aromaticity due to their relation to experiment; diatropic (aromatic) and paratropic (antiaromatic) ring currents indirectly manifest itself in the NMR spectra. However, the magnetic-based measures of aromaticity have also come under bitter criticism due to their complexity (NICS relies on the condensation of potentially complicated patterns of induced currents to a single number), and methodological shortcomings. Finally, the main disadvantage connected with the cal- culation of MCI is its numerical instability and computational cost which prevent using MCI to ana- lyze systems containing more than 12-14 atoms; additionally, MCI suffers from the method depend- ence – in particular, the exchange-correlation functional selection at the density functional theory (DFT) level. To summarize, most of the aromaticity descriptors suffer from serious issues such as the arbi- trariness of choice of a reference system, lack of parametrization, ring-size extensivity issue, limited applicability,
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