Partially Covalent Two-Electron/Multicentric Bonding between Semiquinone Radicals

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An investigation of molecular properties using magnetic shielding calculations

Isotropic shielding calculations were performed across finely spaced two- and three-dimensional grids positioned through and around a wide range of molecules. These magnetic shielding calculations were used to investigate aromaticity, antiaromaticity and a variety of chemical bonding features. This technique was found to be incredibly sensitive and able to distinguish between bonds of different order as well as bonds of the same order but in different environments. The shielding along the whole bonding region, as well as 1 Å above the bond and cross-sections through the bond, can be used to provide detailed information about the nature of the chemical bonding and the conjugation with the rest of the system. Regions of deshielding have been found around unsaturated nuclei and these areas can be used to determine relative aromaticities as well as degrees of conjugation. The same is true of shielding features found at 1 Å above the molecular plane. Unsaturated heavy atoms also display these deshielded surroundings, but they can be harder to observe. Antiaromatic systems exhibit a dumbbell shaped region of deshielding at the ring centre as well as significantly bent bonding regions which have been found to be a result, primarily, of the antiaromaticity rather than ring strain. H-bonding can also be studied with this technique and it has been found that the shielding on the atoms involved is most informative. In the case of substituted malonaldehydes, the oxygen shieldings were used to determine relative aromaticities in the pseudo rings and, therefore, H-bond strength. The sensitivity and information-rich nature of this technique has proven far superior to existing methods, such as the commonly used nucleus-independent chemical shift (NICS) technique, and therefore has great scope for future applications

Importance of Electrostatically Driven Non-Covalent Interactions in Asymmetric Catalysis

Computational chemistry has become a powerful tool for understanding the principles of physical organic chemistry and rationalizing and even predicting the outcome of catalytic and non-catalytic organic reactions. Non-covalent interactions are prevalent in organic systems and accurately capturing their impact is vital for the reliable description of myriad chemical phenomena. These interactions impact everything from molecular conformations and stability to the outcome of stereoselective organic reactions and the function of biological macromolecules. Driven by the emergence of density functional theory (DFT) methods that can account for dispersion-driven noncovalent interactions, there has been a renaissance in terms of computational chemistry shaping modern organic chemistry. DFT Studies of the origins of stereoselectivity in asymmetric organocatalytic reactions can not only provide key information on the mode of asymmetric induction, but can also guide future rational catalyst design. We start with an overview of weak intermolecular interactions and aromatic interactions. Special emphasis is given to the methods that one can use to study these ephemeral interactions. We next provide a brief account how computational chemistry has aided our understanding of chiral phosphoric acid (CPA) catalyzed reactions. Thereafter, three case studies showcasing the importance of non-covalent interactions in chiral NHC catalysis, CPA catalysis, and chiral nucleophilic catalysis has been elaborated. Each of these studies highlights the importance of electrostatically-driven non-covalent interactions in controlling reactivity and selectivity. Moreover, unprecedented activation modes are identified and new predictive selectivity models developed that can be used to rationalize the outcome of future reactions. Studying these reactions using state of art DFT methods, we aimed not only to contribute to the understanding of their selectivity and the importance of noncovalent interactions in catalysis, but also to bring a sound understanding that will enable the design of new reactions and better catalysts. Overall, this dissertation highlights the underappreciated role of electrostatic interactions in controlling reactivity and selectivity in asymmetric catalysis

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

Density Functional Theory

Density Functional Theory (DFT) is a powerful technique for calculating and comprehending the molecular and electrical structure of atoms, molecules, clusters, and solids. Its use is based not only on the capacity to calculate the molecular characteristics of the species of interest but also on the provision of interesting concepts that aid in a better understanding of the chemical reactivity of the systems under study. This book presents examples of recent advances, new perspectives, and applications of DFT for the understanding of chemical reactivity through descriptors forming the basis of Conceptual DFT as well as the application of the theory and its related computational procedures in the determination of the molecular properties of different systems of academic, social, and industrial interest

Computational Prediction and Rational Design of Novel Clusters, Nanoparticles, and Solid State Materials

The creation of new materials is absolutely essential for developing new technologies. However, experimental efforts toward the material discovery are usually based on trial-and-error approach and thus require a huge amount of time and money. Alternatively, computational predictions can now provide a more systematic, rapid, inexpensive, and reliable method for the design of novel materials with properties suitable for new technologies. This dissertation describes the technique of theoretical predictions and presents the results on the successfully predicted and already produced (in some cases) unusual molecules, clusters, nanoparticles, and solids. The major part of scientific efforts in this dissertation was devoted to rationalizing of size- and composition-dependent properties of the materials based on understanding of their electronic structure and chemical bonding. It was shown that understanding relations between bonding and geometric structure, bonding and stability, and bonding and reactivity is an important step toward rational design of new, yet unknown materials with unusual properties. Our findings led to the discovery of the first simplest inorganic double helix structures, which can be used in the design of novel molecular devices. A significant part of this work also deals with the pseudo John-Teller effect, which potentially can be a powerful tool for rationalizing and predicting molecular and solid state structures, their deformations, transformations, and properties. Therefore, the works on the pseudo Jahn-Teller effect presented in this dissertation can be considered the steps toward further generalization and elevation of the pseudo Jahn-Teller effect to a higher level of understanding of the origin of molecular and solid state properties

Non-empirical molecular orbital calculations related to aromatic and heteroaromatic systems

A large program of ab initio MO studies of conjugated molecules has been completed. A major point of interest was the molecular geometry in a number of planar and non-planar cases. The method was first applied to small molecules of known geometry, and then to large systems including cyclo-octatetraene, the longer annulenes, and various 7- and 9- membered ring heterocycles

Intramolecular Hydrogen Bonding 2021

This book describes the results of both theoretical and experimental research on many topical issues in intramolecular hydrogen bonding. Its great advantage is that the presented research results have been obtained using many different techniques. Therefore, it is an excellent review of these methods, while showing their applicability to the current scientific issues regarding intramolecular hydrogen bonds. The experimental techniques used include X-ray diffraction, infrared and Raman spectroscopy (IR), nuclear magnetic resonance spectroscopy (NMR), nuclear quadrupole resonance spectroscopy (NQR), incoherent inelastic neutron scattering (IINS), and differential scanning calorimetry (DSC). The solvatochromic and luminescent studies are also described. On the other hand, theoretical research is based on ab initio calculations and the Car–Parrinello Molecular Dynamics (CPMD). In the latter case, a description of nuclear quantum effects (NQE) is also possible. This book also demonstrates the use of theoretical methods such as Quantum Theory of Atoms in Molecules (QTAIM), Interacting Quantum Atoms (IQA), Natural Bond Orbital (NBO), Non-Covalent Interactions (NCI) index, Molecular Tailoring Approach (MTA), and many others