Magnetically Induced Currents in [n]Cycloparaphenylenes, n=6-11

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Computed NMR shielding values of unsaturated five-membered-heterocyclic ring compounds and their benzo-analogs as a measure of aromaticity

The gauge independent atomic orbital (GIAO) Hartree-Fock (HF) technique with a 6-31 G(d, p) basis set was used to calculate the isotropic NMR shielding values of a diatomic hydrogen probe above a set of unsaturated 5-membered heterocyclic aromatic compounds and their benzo- analogs. It has been shown that this technique produced results indicating substantial shielding of the probe over the center of aromatic rings. The current study was conducted to determine if the computed shielding of a diatomic H2 probe is related quantitatively to the extent of aromaticity. Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs of electrons or empty orbitals exhibits a stabilization due to conjugation alone. Aromaticity is both a qualitative and quantitative concept. The qualitative aspect, which is the method for identifying a molecule or species as either aromatic, non-aromatic or anti-aromatic, is soundly understood, but the quantitative aspect of aromaticity is less well defined. There are several established methods for measuring aromaticity quantitatively but they are only loosely correlated. This study’s method (?s) for calculations done over the fivemembered rings correlated well with Cyransky’s published data of ASE, ?, NICS(0), NICS(1) and HOMA calculations to yield correlation coefficients of 0.64, 0.49, 0.66, 0.88 and 0.69, respectively. This study’s method (?s) with calculations done over the heterocyclic ring portion of the benzo-analogs yielded a correlation coefficient of 0.67 when matched with Bird’s published ASE data

Magnetically induced currents and magnetic response properties of molecules

The magnetically induced currents in organic monoring and multiring molecules, in Möbius shaped molecules and in inorganic all-metal molecules have been investigated by means of the Gauge-including magnetically induced currents (GIMIC) method. With the GIMIC method, the ring-current strengths and the ring-current density distributions can be calculated. For open-shell molecules, also the spin current can be obtained. The ring-current pathways and ring-current strengths can be used to understand the magnetic resonance properties of the molecules, to indirectly identify the effect of non-bonded interactions on NMR chemical shifts, to design new molecules with tailored properties and to discuss molecular aromaticity. In the thesis, the magnetic criterion for aromaticity has been adopted. According to this, a molecule which has a net diatropic ring current might be aromatic. Similarly, a molecule which has a net paratropic current might be antiaromatic. If the net current is zero, the molecule is nonaromatic. The electronic structure of the investigated molecules has been resolved by quantum chemical methods. The magnetically induced currents have been calculated with the GIMIC method at the density-functional theory (DFT) level, as well as at the self-consistent field Hartree-Fock (SCF-HF), at the Møller-Plesset perturbation theory of the second order (MP2) and at the coupled-cluster singles and doubles (CCSD) levels of theory. For closed-shell molecules, accurate ring-current strengths can be obtained with a reasonable computational cost at the DFT level and with rather small basis sets. For open-shell molecules, it is shown that correlated methods such as MP2 and CCSD might be needed to obtain reliable charge and spin currents. The basis set convergence has to be checked for open-shell molecules by performing calculations with large enough basis sets. The results discussed in the thesis have been published in eight papers. In addition, some previously unpublished results on the ring currents in the endohedral fullerene Sc3C2@C80 and in coronene are presented. It is shown that dynamical effects should be taken into account when modelling magnetic resonance parameters of endohedral metallofullerenes such as Sc3C2@C80. The ring-current strengths in a series of nano-sized hydrocarbon rings are related to static polarizabilities and to H-1 nuclear magnetic resonance (NMR) shieldings. In a case study on the possible aromaticity of a Möbius-shaped [16]annulene we found that, according to the magnetic criterion, the molecule is nonaromatic. The applicability of the GIMIC method to assign the aromatic character of molecules was confirmed in a study on the ring currents in simple monocylic aromatic, homoaromatic, antiaromatic, and nonaromatic hydrocarbons. Case studies on nanorings, hexaphyrins and [n]cycloparaphenylenes show that explicit calculations are needed to unravel the ring-current delocalization pathways in complex multiring molecules. The open-shell implementation of GIMIC was applied in studies on the charge currents and the spin currents in single-ring and bi-ring molecules with open shells. The aromaticity predictions that are made based on the GIMIC results are compared to other aromaticity criteria such as H-1 NMR shieldings and shifts, electric polarizabilities, bond-length alternation, as well as to predictions provided by the traditional Hückel (4n+2) rule and its more recent extensions that account for Möbius twisted molecules and for molecules with open shells.Aromatiska molekyler kännetecknas av en hög stabilitet och låg reaktivitet. Organiska aromatiska molekyler är vanligen ringformade och uppbyggda av konjugerade kol-kol-bindningar. Bindningselektronerna är delvis delokaliserade över dessa molelylers kolstomme och denna delokalisering ger upphov till en ökad stabilitet. Ifall delokaliseringen av elektronerna i en ringformad molekyl leder till att molekylen är mindre stabil och mer reaktiv än motsvarande acykliska molekyl med bindningselektronerna lokaliserade till dubbel- och enkelbindningar, så är den cycliska molekylen antiaromatisk. Eftersom elektronerna i aromatiska eller antiaromatiska molekyler delvis är delokaliserade över en ringformig stomme, så kan en ringström uppstå om molekylen placeras i ett magnetfält riktat vinkelrätt mot molekylens plan. Den ringström som således uppstår kan ge information om molekylens aromatiska karaktär. En noggrann kartläggning av ringströmmens utbredning och ringströmmens styrka kan också ge kunskap om elektrondelokaliseringen i komplexa molekyler som är uppbyggda av många ringar. I detta arbete har ringströmmarna i olika molekyler beräknats med Gauge-including magnetically induced currents (GIMIC)-metoden. Molekylernas elektronstruktur och de kärnmagnetiska resonans (NMR) -skärmningarna har modellerats med hjälp av kvantkemiska beräkningsmetoder. GIMIC-metoden har använts för beräkningar av ringströmmens styrka och för att bestämma strömtäthetens utbredning. De erhållna ringströmmarna har implikationer för molekylär aromaticitet. Dessa implikationer diskuteras och jämföres med de aromaticitetsprediktioner andra molekylära egenskaper såsom H-1 NMR-skärmningar, kol-kolbindningarnas längdvariation och elektrisk polarisabilitet ger upphov till. Resultaten jämförs också med Hückels (4n+2)-regel för molekylär aromaticitet samt med de senare generaliseringarna av regeln för molekyler med öppna skal och med Möbiustopologi. I delstudierna som ligger till grund för avhandlingen har ringströmmarna beräknats i organiska monocykliska molekyler samt i komplexa molekyler som består av flera ringar. Metoden har tillämpats också för att utreda ringströmmarnas utbredning och styrka i icke-plana molekyler och i Möbiusformade molekyler. Resultaten visar att GIMIC-metoden ger ett tillförlitligt mått på den aromatiska karaktären hos enkla ringformade molekyler. I komplexa molekyler med många ringstrukturer kan man med hjälp av GIMIC kartlägga hur ringströmmarna fördelas över olika delar av molekylen. Denna information kan vara av intresse då man designar nya material med speciella elektriska och magnetiska egenskaper. I molekyler med oparade elektroner kan man också bestämma spinnströmmens utbredning. GIMIC-metoden kan hjälpa experimentalister att tolka uppmätta NMR spektrum. Ringströmmar kan nämligen ha stora effekter på det NMR kemiska skiftet speciellt för vätekärnor. Då ringströmmens styrka i olika delar av en molekyl kan bestämmas är det också lättare att identifiera andra växelverkningar i molekylen vilka kan ha stor inverkan på kemiska skift

What Do Magnetic Shieldings Tell Us About Bonding, Aromaticity and Antiaromaticity in Mono-, Bi- and Tricyclic Molecules?

Some chemical concepts such as aromaticity, antiaromaticity, and chemical bonding have been evaluated for some molecules based on isotropic magnetic shielding calculations. This includes utilising points as local shielding probes which have been used as a single point or multiple points aligned in one-, two-, or three-dimensional grids. Each of the grids was placed at a specific location around/at the molecular space of the studied molecules. Nuclear and off-nucleus NMR shielding calculations were performed at different levels of theory using different quantum chemical methods of HF, MP2, and CASSCF with a variety of basis sets. The calculations were performed on some organic and inorganic mono-, bi-, and tricyclic molecules in their ground and, in some cases, low-lying excited states. These molecules are borazine, borazanaphthalene, deltate, squarate, croconate, rhodizonate, disulfur dinitride, naphthalene, anthracene and phenanthrene. Based on analysing and scanning the changes in the magnetic shielding data of values, 1D curves, 2D contour maps and 3D isosurfaces, the targeted molecular features for the above molecules have been obtained. The chemical bonding, aromaticity, and antiaromaticity of the molecules are assessed based on the above evaluations. The results show that both borazine and the borazanaphthalene are moderately aromatic. The oxocarbon dianions vary from aromatic deltate, moderately aromatic squarate to antiaromatic croconate and rhodizonate. Also, the vertical excitation of the moderately aromatic ground state disulfur dinitride leads to strongly antiaromatic S1 and moderate antiaromatic T1 states. Naphthalene shows obvious magnetic variations among its different electronic states. In terms of decreasing aromaticity, the naphthalene states follow this order: S2 (strongly aromatic) > S0 (aromatic) > T1 (antiaromatic) > S1 (strongly antiaromatic). Both anthracene and phenanthrene display a strong magnetic behaviour. The central ring of anthracene is more magnetically shielded than the two terminal rings, whereas a contrast shielding profile is found in phenanthrene rings. For all the above molecules, the magnetic shieldings around bonds help in understanding the overall magnetic behaviour and the aromaticity level

Computational Studies of the Aromatic and Photophysical Properties of Carbaporphyrinoid

Porphyrins and porphyrin derivatives are naturally occurring molecules, whereas carbaporphyrinoids are synthesized porphyrin derivatives. They have received much attention in recent years by the scientific community due to their diverse potential applications in technological developments such as molecular electronic devices and conversion of solar energy. However, the full utilization of this class of compounds can not be realized without an in-depth understanding of their chemical and physical properties. Two of such properties are aromaticity and optical properties. In this thesis, the aromatic properties and the light absorption spectra in the ultraviolet and visible (UV/Vis) range have been studied computationally for some recently synthesized carbaporphyrins and carbachlorins using first-principle computational approaches. In the first part of the thesis, the background of carbaporphyrinoids and some examples of naturally occurring porphyrins and porphyrin derivatives are delineated. The second and third part review theoretical and computational methods that are employed in studies of the molecular aromaticity and electronic excitation spectra of molecules. The computational studies of magnetically induced current densities and electronic excitation energies are discussed in the fourth chapter. The obtained results are also presented in chapter four and the main conclusions are summarized in the last chapter. The study shows that all the carbaporphyrinoids studied sustain a magnetically induced ring current in the porphyrin macro ring. This indicates that they are aromatic according to the ring-current criterion. However, the calculated ring-current pathways differ from those predicted from the nucleus independent chemical shift (NICS) calculations and the current pathways deduced from H NMR spectroscopy studies. The vertical excitation energies which is akin to the ultraviolet-visible spectrum obtained experimentally for some of the selected carbaporphyrinoids also showed deviations from those of the experimental values. These deviations can be ascribed to solvent effects as in the calculation of the vertical excitation energies, solvent effects were not accounted for

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

Chemical and Photophysical Behaviour of π-Extended Tropyliums

Non-benzenoid annulenes have long fascinated organic chemists, due to their chemical properties, aromaticity, and photophysical behaviour. When incorporated into a polycyclic aromatic framework, such non-hexagonal rings can give rise to nonplanar structures with modified optical properties and improved solubility compared to planar, defect-free graphene. Among non-benzenoid annulenes, the tropylium cation has been of special interest, due to its unique blend of reactivity (owing to its positive charge) and stability (a result of its aromaticity). Indeed, this cation has found wide utility as a versatile ligand , stimulus-responsive dye , and an organocatalyst. Yet, reports of polycyclic aromatics featuring this heptagonal annulene are sparse. This Thesis employs tropylium and its neutral, nonaromatic homologue, cycloheptatriene (Figure 1) as key structural motifs within an extended π-framework to gain fundamental insights into the electronic and optical properties of non-benzenoid and charged polycyclic aromatics. We find that judicious engineering of strain into the framework of sterically overcrowded tropyliums can cause its aromaticity to rupture, forming an “aromatic-to-nonaromatic” equilibrium at room temperature. Moreover, modifying the conjugation length in a series of cycloheptatriene-rotors was found to vastly alter their photoluminescence properties, allowing for new modes of chemical reactivity. We also report our synthetic forays toward a highly warped redox-active warped nanographene, as well as preliminary findings on the excited-state proton transfer dynamics in a series of hydroxybenzotropyliums. 1. T. Murahashi, M. Fujimoto, M. A. Oka, Y. Hashimoto, T. Uemura, Y. Tatsumi, Y. Nakao, A. Ikeda, S. Sakaki and H. Kurosawa, Science, 2006, 313, 1104–1107 2. U. P. N. Tran, G. Oss, D. P. Pace, J. Ho and T. V. Nguyen, Chem. Sci., 2018, 9, 5145–5151. 3. D. J. M. Lyons, R. D. Crocker and T. V. Nguyen, Chem.—Eur. J., 2018, 24, 10959–10965. 4. P. K. Saha, A. Mallick, A. T. Turley, A. N. Bismillah, A. Danos, A. P. Monkman, A.- J. Avestro, D. S. Yufit and P. R. McGonigal, Nature Chem., 2023 15, 516–525. 5. A. T. Turley, P. K. Saha, A. Danos, Aisha N. Bismillah, A. P. Monkman, D. S. Yufit, B. F. E. Curchod, M. K. Etherington, and Paul R. McGonigal, Angew. Chem. Int. Ed., 2022, 61, e202202193