Performance of 3D-space-based atoms-in-molecules methods for electronic delocalization aromaticity indices

Several definitions of an atom in a molecule (AIM) in three-dimensional (3D) space, including both fuzzy and disjoint domains, are used to calculate electron sharing indices (ESI) and related electronic aromaticity measures, namely, Iringand multicenter indices (MCI), for a wide set of cyclic planar aromatic and nonaromatic molecules of different ring size. The results obtained using the recent iterative Hirshfeld scheme are compared with those derived from the classical Hirshfeld method and from Bader's quantum theory of atoms in molecules. For bonded atoms, all methods yield ESI values in very good agreement, especially for C-C interactions. In the case of nonbonded interactions, there are relevant deviations, particularly between fuzzy and QTAIM schemes. These discrepancies directly translate into significant differences in the values and the trends of the aromaticity indices. In particular, the chemically expected trends are more consistently found when using disjoint domains. Careful examination of the underlying effects reveals the different reasons why the aromaticity indices investigated give the expected results for binary divisions of 3D spaceM.S. is grateful for the nancial help furnished by the Spanish MICINN Project No. CTQ2008-03077/BQU and by the Catalan DIUE through project No. 2009SGR63

Why 1,2‑quinone derivatives are more stable than their 2,3‑analogues?

In this work, we have studied the relative stability of 1,2- and 2,3-quinones. While 1,2-quinones have a closed-shell singlet ground state, the ground state for the studied 2,3-isomers is open-shell singlet, except for 2,3-naphthaquinone that has a closed-shell singlet ground state. In all cases, 1,2-quinones are more stable than their 2,3-counterparts. We analyzed the reasons for the higher stability of the 1,2-isomers through energy decomposition analysis in the framework of Kohn–Sham molecular orbital theory. The results showed that we have to trace the origin of 1,2-quinones’ enhanced stability to the more efficient bonding in the π-electron system due to more favorable overlap between the SOMOπ of the ·C4n−2H2n–CH·· and ··CH–CO–CO· fragments in the 1,2-arrangement. Furthermore, whereas 1,2-quinones present a constant trend with their elongation for all analyzed properties (geometric, energetic, and electronic), 2,3-quinone derivatives present a substantial breaking in monotonicity.European Union in the framework of European Social Fund through the Warsaw University of Technology Development Programme. O.A. S., H. S. and T.M. K

Computational study of Aromaticity in Porphyrinoid Systems and Photosensitizers from Chemical Bonding Descriptors

291 p.La presente tesis está dividida en dos bloques. El primer bloque se centra en una de las propiedadesafectadas por el error de deslocalización, la aromaticidad, presente en algunas aproximaciones delfuncional de la densidad. Se estudia el carácter aromático de sistemas una familia de porfirínas simples,una serie de anulenos y un anillo de seis porfirínas. También se discute el método computacionalapropiado para caracterizar la estructura electrónica de moléculas aromáticas medianas y grandes.Siguiendo la misma línea, en el segundo bloque de la tesis se han examinado diferentes familias defotosensibilizadores y catalizadores para diseñar un protocolo riguroso para el estudio de la estructuraelectrónica, la simulación de espectros UV-Vis y para el cálculo de potenciales redox. Losfotosensibilizadores son moléculas captadoras de luz que presentan excitaciones de transferencia decarga. La simulación de estas excitaciones está afectada también por las deficiencias de lasaproximaciones al funcional de la densidad

Electron delocalization and aromaticity in low-lying excited states of archetypal organic compounds

Aromaticity is a property usually linked to the ground state of stable molecules. Although it is well-known that certain excited states are unquestionably aromatic, the aromaticity of excited states remains rather unexplored. To move one step forward in the comprehension of aromaticity in excited states, in this work we analyze the electron delocalization and aromaticity of a series of low-lying excited states of cyclobutadiene, benzene, and cyclooctatetraene with different multiplicities at the CASSCF level by means of electron delocalization measures. While our results are in agreement with Baird's rule for the aromaticity of the lowest-lying triplet excited state in annulenes having 4n pi-electrons, they do not support Soncini and Fowler's generalization of Baird's rule pointing out that the lowest-lying quintet state of benzene and septet state of cyclooctatetraene are not aromatic

Exploring the validity of the Glidewell-Lloyd extension of Clar's pi-sextet rule: assessment from polycyclic conjugated hydrocarbons

The Clar pi-sextet rule was formulated as a tool to qualitatively assign the local aromatic character of six-membered rings in benzenoid species. This simple rule has been widely validated both experimentally and theoretically. In 1984, Glidewell and Lloyd reported an extension of this rule to polycyclic conjugated hydrocarbons having rings with any even number of carbon atoms in their structure. In this work, we assess the validity of the Glidewell-Lloyd extension in 69 polycyclic conjugated hydrocarbons composed of different combinations of four-, six-, and eight-membered rings. Our results support the validity of this extension with some exceptions that are discussed. Finally, a minor modification to the rule is proposed

Quantum Chemical Studies of Ring Currents of Aromatic Molecules

Aromaticity - the delocalization of electrons along a closed atomic circuit - has its manifestations in the energetic, structural, electronic, and spectroscopic properties of molecules and in how they react with each other. This phenomenon is central in chemistry, and the history of chemists using the concept as an “intuition pump” to understand and design new molecules goes back to the 1800s when Kekulé first time came up with the snake-eating-its-tail model of benzene. Those days predate the discovery of electron and quantum mechanics, and the concept has evolved since. While the physics of chemistry is understood, the utility of intuitive concepts still remains. Science as of today is still a human business, and to most of us chemists the fluctuations of fermionic field, or their computational representation as tensors, don’t give much food for thought. In this PhD thesis, I present my research in which quantum chemical methods were used to study different types of aromatic compounds. The focus is on assessing their aromaticity by probing the ring currents of molecules - the net flow of electrons around when it’s placed in a magnetic field. Calculation of this magnetically induced current density and the bond currents yield an accurate measure for electron delocalization. The studied systems present different types of aromaticities and aromatic molecules: through-bond aromaticity in the substituent ring of benzene derivatives, the intricacies of current pathways in naphtalene-fused porphyrinoids and in copper coordination complexes, and the magnetic-field orientation dependence of aromaticity in gaudiene, a spherical aromatic, but not spherically aromatic compound. The presented results disprove old conclusions for some compounds and enrich the understanding of others. In addition, the thesis gives a brief overview of computational quantum chemistry, and a slightly deeper one on aromaticity, presenting the ups and downs of different methods used to assess it computationally, and taking the reader on a tour to the zoo of different types of aromatic compounds.Kemiassa aromaattisuuden käsite ei tyypillisesti viittaa tuoksuihin, vaikka sen juuret ulottuvatkin näiden eriskummallisten molekyylien aromiin. Kemistit kutsuvat aromaattisuudeksi ilmiötä, jossa molekyylin elektronit eivät tyydy kohtaloonsa kahden atomiytimen välimaastossa, vaan leviävät yli atomikehikon muodostaen suljetun virtapiirin. Tuon syklisen delokalisaation myötä aromaattisilla molekyyleillä on erityisiä ominaisuuksia. Vastaavasti kuten elektroniikan virtapiirissä hieman vääränlainen resistori voi saada aikaan ennalta-arvaamattoman oikosulun, myös atomitasolla nämä näennäisen pienet muutokset voivat kytkeä aromaattisuuden pois päältä ja muuttaa molekyylin ominaisuuksia suuresti - kokonaisuus on enemmän kuin osiensa summa. Aromaattisia molekyylejä on kaikkialla, arkkityyppinä niistä on kuusikulmion muotoinen bentseeni. Evoluutio on valjastanut nämä aromaattiset molekyylit osaksi olevaisuuttamme: geeniperimämme on kirjoitettu DNA:n vakailla aromaattisilla emäspareilla. Solujemme toiminta pyörii aromaattisuutensa keinoin elektroneja välittävillä koentsyymeillä, ja aromaattisuudella on mitä keskeisin rooli porfyriinimolekyylien kyvyssä niin sitoa happea kuljettavat rauta-atomit verisoluissamme kuin vastaanottaa auringon säteilemä energia kasvien viherhiukkasissa. Luonto on löytänyt aromaattisille molekyyleille paljon käyttöä, ja näistä prosesseista kummunneet kemistit jatkavat sen työtä. Aromaattisuuden käsite on tärkeä väline tässä työkalupakissa. Toinen tärkeä työkalu on kvanttifysiikka - sen avulla olemme noin sadan vuoden ajan kyenneet kunnolla ymmärtämään molekyylejä, sekä sittemmin tietokoneiden kehittymisen myötä onnistuneet laskemaan niiden ominaisuuksia tarkasti. Väitöstutkimuksessani sovelsin laskennallisen kvanttikemian menetelmiä aromaattisuuden määrittämiseksi. Väitöskirja rakentuu neljän tieteellisen julkaisun ympärille, joissa tarkastelemme erilaisten molekyylien aromaattisuutta laskemalla rengasvirtoja - ulkoisen magneettikentän aiheuttamaa elektronien virtausta atomiydinten muodostaman virtapiirin ympäri. Tulosten avulla kykenimme ymmärtämään tiettyjen molekyylien aromaattisuutta paremmin sekä kumoamaan joitakin väärinkäsityksiä. Tehdyn perustutkimuksen löydökset todentavat rengasvirtojen antavan tarkan ja fysikaalisesti perustellun kuvan aromaattisuudesta - erityisesti jos sitä vertaa 1800-luvun hajunvaraiseen toimintaan - ja rengasvirtojen olevan tärkeä menetelmä aromaattisuuden ymmärtämisessä

The Concept of Multicenter Bonds in Chemistry and Materials Science

Chemical bonds are components of a universal and compact language of chemistry that was empirically developed before the modern concepts of quantum physics. This language explains how molecules and solids keep together. In particular, Lewis’s shared electron-pair bonding model may be considered the most successful and generally accepted theory of chemical bonding due to its simplicity and predictive power. However, there is an entire world of chemical species where the classical Lewis bonding language fails to describe the bonding pattern adequately. Those cases include but are not limited to compounds with a significant electron delocalization (where electron density spread on a region that spans more than 2 atoms) such as so-called aromatic and anti-aromatic compounds. In this dissertation, we are showing that there are some essential “words” missing in the “vocabulary” of classical Lewis’s chemical bonds language. To cover most of the chemical species, the electron-pair bonding model can be extended with the inclusion of multicenter bonds where the number of centers can reach the number of atoms in the described system. This dissertation includes six research projects, that investigate and expand the applicability of the concept of multicenter bonds in chemistry and materials science. We showed that such a developed chemical bonding model has great predictive power and can explain the structure, stability, and several physical properties of various unusual clusters and solids. Since the chemical bonding pattern can be related to reactivity, structure, and physical properties, we believe, that the concept of multicenter bonds could be developed in the future up to the level where we will be able to design novel materials with ever-wanted physical and chemical properties

Quest for the Most Aromatic Pathway in Charged Expanded Porphyrins

Despite the central role of aromaticity in the chemistry of expanded porphyrins, the evaluation of aromaticity remains difficult for these extended macrocycles. The presence of multiple conjugation pathways and different planar and nonplanar π-conjugation topologies makes the quantification of global and local aromaticity even more challenging. In neutral expanded porphyrins, the predominance of the aromatic conjugation pathway passing through the imine-type nitrogens and circumventing the amino NH groups is established. However, for charged macrocycles, the question about the main conjugation circuit remains open. Accordingly, different conjugation pathways in a set of neutral, anionic, and cationic expanded porphyrins were investigated by means of several aromaticity indices rooted in the structural, magnetic, and electronic criteria. Overall, our results reveal the predominance of the conjugation pathway that passes through all nitrogen atoms to describe the aromaticity of deprotonated expanded porphyrins, while the outer pathway through the perimeter carbon atoms becomes the most aromatic in protonated macrocycles. In nonplanar and charged macrocycles, a discrepancy between electronic and magnetic descriptors is observed. Nevertheless, our work demonstrates AVmin remains the best tool to determine the main conjugation pathway of expanded porphyrins.M.A. and I.C.R. wish to acknowledge the VUB for a Strategic Research Program awarded to ALGC. The resources and services used in this work were provided by the Flemish Supercomputer Center (VSC), funded by the Research Foundation - Flanders (FWO), and the Flemish Government. I.C.R. acknowledges co-funding from the European Union′s Horizon 2020 research and innovation Maria Skłodowska-Curie Actions, under grant agreement number 945380. It has been also supported by grants from the Spanish government MICINN (PGC2018-098212-B-C21, PID2019-104772GB, PID2019-105488GB-I00, and PCI2019-103657), Diputación Foral de Gipuzkoa (2019-CIEN-000092-01), Gobierno Vasco (IT1346-19, IT1254-19, and PIBA19-0004), and the DIPC (DIPC_INV_003132). Open Access funding provided by University of Basque Country

Aromatic Pathways in Porphyrinoids by Magnetically Induced Ring Currents

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