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

Magnetically induced ring currents in naphthalene-fused heteroporphyrinoids

The magnetically induced current density of an intriguing naphthalene-fused heteroporphyrin has been studied, using the quantum-chemical, gauge-including magnetically induced currents (GIMIC) method. The ring-current strengths and current-density pathways for the heteroporphyrin, its Pd complex, and the analogous quinoline-fused heteroporphyrin provide detailed information about their aromatic properties. The three porphyrinoids have similar current-density pathways and are almost as aromatic as free-base porphyrin. Notably, we show that the global ring current makes a branch at three specific points. Thus, the global current is composed of a total of eight pathways that include 22 pi-electrons, with no contributions from 18-electron pathways.Peer reviewe

Calculations of current densities for neutral and doubly charged persubstituted benzenes using effective core potentials

Magnetically induced current density susceptibilities and ring-current strengths have been calculated for neutral and doubly charged persubstituted benzenes C6X6 and C6X62+ with X = F, Cl, Br, I, At, SeH, SeMe, TeH, TeMe, and SbH2. The current densities have been calculated using the gauge-including magnetically induced current (GIMIC) method, which has been interfaced to the Gaussian electronic structure code rendering current density calculations using effective core potentials (ECP) feasible. Relativistic effects on the ring-current strengths have been assessed by employing ECP calculations of the current densities. Comparison of the ring-current strengths obtained in calculations on C6At6 and C6At62+ using relativistic and non-relativistic ECPs show that scalar relativistic effects have only a small influence on the ring-current strengths. Comparisons of the ring-current strengths and ring-current profiles show that the C6I62+, C6At62+, C-6(SeH)(6)(2+), C-6(SeMe)(6)(2+), C-6(TeH)(6)(2+), C-6(TeMe)(6)(2+), and C-6(SbH2)(6)(2+) dications are doubly aromatic sustaining spatially separated ring currents in the carbon ring and in the exterior of the molecule. The C6I6+ radical cation is also found to be doubly aromatic with a weaker ring current than obtained for the dication.Peer reviewe

Nuclear Magnetic Shieldings of Stacked Aromatic and Antiaromatic Molecules

Nuclear magnetic shieldings have been calculated at the density functional theory (DFT) level for stacks of benzene, hexadehydro[12]annulene, dodecadehydro[18]annulene, and hexabenzocoronene. The magnetic shieldings due to the ring currents in the adjacent molecules have been estimated by calculating nucleus independent molecular shieldings for the monomer in the atomic positions of neighbor molecules. The calculations show that the independent shielding model works reasonably well for the H-1 NMR shieldings of benzene and hexadehydro[12]annulene, whereas for the larger molecules and for the C-13 NMR shieldings the interaction between the molecules leads to shielding effects that are at least of the same size as the ring current contributions from the adjacent molecules. A better agreement is obtained when the nearest neighbors are also considered at full quantum mechanical (QM) level. The calculations suggest that the nearest solvent molecules must be included in the quantum mechanical system, at least when estimating solvent shifts at the molecular mechanics (MM) level. Current density calculations show that the stacking does not significantly affect the ring current strengths of the individual molecules, whereas the shape of the ring current for a single molecule differs from that of the stacked molecules.Peer reviewe

Computational Studies on Homogeneous Water Oxidation Catalysis for Artificial Photosynthesis

The development of new energy sources for the replacement of fossil fuels is an important task in chemistry. Artificial photosynthesis is a viable option for the generation of fuels. In it, water molecules are oxidized and the resulting protons are reduced to hydrogen, or further combined with carbon dioxide to form carbon fuels. The processes are powered by solar energy. Water oxidation is the bottleneck in the process, as the reaction requires the breaking of four O-H bonds, formation of a O-O bond, and the dissociation of the formed oxygen molecule. Research in past decades has resulted in transition metal complexes, mostly with ruthenium and iridium metal centers, which catalyze oxidation of water with moderate turnover frequencies and numbers. In order for the artificial photosynthesis to be a viable source of energy, catalysts using cheaper and more abundant first row transition metals and having better performance are needed. The the theory section the relevant inorganic and quantum chemistry and the used computational methods are presented. In the literature section, biological photosynthesis and the modular artificial photosynthetic system are presented and the most important water oxidation catalysts are highlighted. In the research section, an efficient water oxidation catalyst by the group of Sun is studied computationally. The coordination geometries and spin-state energetics of the catalyst were studied using Ru, Fe, and Os metal centers at different stages of the catalytic cycle. The Fe catalyst was a high-spin complex with weakened metal-ligand bonding due to the occupation of antibonding metal-ligand orbitals. The modification of the ligand framework with substituents was also studied. Substitution did not have a major effect in charge distributions or coordination geometries, implying that the differences in reactivities observed experimentally are due to environmental effects