Intermolecular Covalent Interactions:A Quantitative Molecular Orbital Perspective

This thesis reports detailed quantum chemical investigations on the nature and strength of intermolecular interactions in DmZ•••A– complexes, mediated via atoms Z of groups 15–17 in the periodic table, based on quantitative Kohn-Sham molecular orbital theory. In the first stage, accurate ab initio benchmark and density functional theory (DFT) validation studies have been done. For each type of bond, pnictogen bond (PnB) and chalcogen bond (ChB), and, for comparison, halogen bond (XB) and hydrogen bonds (HB), accurate trends in bond length and strength are computed, based on a consistent set of data from our validated relativistic DFT approach. The main purpose is to provide a unified picture of chalcogen bonds and pnictogen bonds, together with hydrogen bonds and halogen bonds. The analyses herein reveal that the intramolecular interactions have a strong covalent component and are certainly not dominantly electrostatic in nature, as it is incorrectly suggested by the sigma-hole model whose weaknesses are consistently exposed. The findings in this thesis work thus suggest that the commonly accepted designation "Non-Covalent Interactions (NCI)" for the pertinent intermolecular interactions does not properly cover their nature and it is proposed to replace this designation with the more appropriate "Intermolecular Covalent Interactions (ICI)"

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

The Halogen Bond

The halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. In this fairly extensive review, after a brief history of the interaction, we will provide the reader with a snapshot of where the research on the halogen bond is now, and, perhaps, where it is going. The specific advantages brought up by a design based on the use of the halogen bond will be demonstrated in quite different fields spanning from material sciences to biomolecular recognition and drug design

Feature Papers in Compounds

This book represents a collection of contributions in the field of the synthesis and characterization of chemical compounds, natural products, chemical reactivity, and computational chemistry. Among its contents, the reader will find high-quality, peer-reviewed research and review articles that were published in the open access journal Compounds by members of the Editorial Board and the authors invited by the Editorial Office and Editor-in-Chief

Orbital interactions

It is widely accepted that the sharing of electrons constitutes a bond. Conversely, molecular interactions that do not involve electron transfer, such as van der Waals forces and electrostatics are defined as “non-bonding” or “non-covalent” interactions. More recently computational and experimental observations have shown situations where the division between “bonding” and “non-bonding” interactions is blurred. One such class of interactions are known as σ-hole interactions. Chapter 1 provides a literature review of investigations into the nature of σ-hole interactions, highlighting the individual contributing factors. Chapter 2 provides a detailed analysis into the nature of chalcogen-bonding interactions. Synthetic molecular balances are employed for experimental measurements of conformational free energies in different solvents, facilitating a detailed examination of the energetics and associated solvent and substituent effects on chalcogen-bonding interactions. The chalcogen-bonding interactions examined were found to have surprisingly little solvent dependence. The independence of the conformational free energies on solvent polarity, polarisability and H-bond characteristics showed that electrostatic, solvophobic or dispersion forces were not dominant factors in accounting for the experimentally observed trends. A molecular orbital analysis provided a quantitative relationship between the experimental free energies and the molecular orbital energies, which was consistent with chalcogen-bonding interactions being dominated by an n→σ* orbital delocalisation. Chapters 3 and 4 both use the molecular orbital modelling approach established in Chapter 2 to investigate the potential partial covalency in H-bonding and carbonyl···carbonyl interactions. H-bonding is generally considered to be an electrostatically dominated interaction. However, computational results have suggested a partial covalent character in H-bonding. The molecular orbital analysis revealed an n→σ* electron delocalisation in all H-bonding systems evaluated. However, no quantitative correlation could be found with experimental free energies. Similarly, the nature of carbonyl···carbonyl interactions has been subject to debate, with electrostatic or an n→π* electron delocalisation having been proposed as the dominant factors. The molecular orbital analysis employed here showed that n→π* delocalisation was exceptionally geometry dependent. Studies of literature systems reveal that n→π* delocalisation contributes to overall stability of a range of systems, with a quantitative link between molecular orbital energy and conformational free energies

van der Waals epitaxy and electronic transport in topological insulators

Topological insulators (TI) have been demonstrated as a unique electronic phase of matter, possessing topological surface states (TSS) with promising applications in spin-based logic and memory, heterostructures in 2D electronics and exotic physical phenomena such as Majorana quantum computing, axion electrodynamics and topological magnetoelectronics. Since the early stages of discovery, the field of applied research in TIs has evolved. However, demonstration of scalable applications remain challenging due to practical hurdles such as rapid prototyping of new TI compounds, and efficient probing of TSS for device applications. This research work endeavors to take a two-pronged approach: to address the challenges of reliable material growth and to explore TI transport physics. While indirect spectroscopy methods have indisputably shown the presence of TSS, transport in TI devices remains challenging, in part due to parasitic conduction channels. As an alternative to staple binaries, ternary and quaternary compounds (Bi₁₋[subscript y]Sb[subscript y])₂(Te₁₋[subscript x]Se[subscript x])₃ are being explored to reduce unintentional bulk-doping and gain better access to the Dirac point. The sulfur-based ternary Bi₂Te₂S has received little attention, even as its potential as a promising TI is theoretically predicted. We demonstrate first-time van der Waals epitaxial (vdWE) growth of crystalline Bi₂Te₂₋[subscript x]S₁₊[subscript x] (BTS) nanosheets on SiO₂, hBN and mica. We also perform detailed magnetotransport measurements on BTS devices, establishing BTS as a candidate TI with readily accessible TSS and providing a sound picture of multiple transport channels in TI devices. A versatile process for large-area custom-feature TI growth and fabrication is also demonstrated using BTS as the candidate TI, achieved through selective-area modification of surface free-energy on mica. TI features grow epitaxially in large single-crystal trigonal domains, exhibiting armchair or zigzag edges highly oriented with the substrate lattice. Unusual nonlinear thickness dependence on lateral dimensions and denuded zones are observed, explained by semi-empirical two-species surface migration modeling with robust estimates of growth parameters. TSS contribute up to 60% of device conductance at room-temperature, indicating excellent electronic quality. The process is constructed from highly adaptable microfabrication technology, and in conjunction with multi-species modeling, it can be customized for TI and other vdW materials device fabrication processes ranging from rapid prototyping to scalable manufacturing.Electrical and Computer Engineerin

Structural, Optical and Transport Properties of Copper Chalcogenide Nanocrystal Superlattices

This cumulative thesis is based on three publications. It investigates the self-assembly of nanocrystal (NC) superlattices, charge transport in NC assembly, and application of these superlattices in optoelectronic and vapor sensing. The materials of choice are copper chalcogenide NCs such as binary copper sulfide Cu1.1S NCs, binary copper selenide Cu2Se NCs and ternary Cu2-xSeyS1-y NCs and the organic semiconductors metal (Cu or Co) centered -4,4′,4″,4″,4‴-tetraaminophthalocyanine (Cu/CoTAPc). Macroscopic superlattices of NCs are prepared by Langmuir-type self-assembly at the air/liquid interface followed by simultaneous ligand exchange with an organic semiconductor. To enhance interparticle coupling, we cross-link the nanocrystals with the organic π-system Cu-4,4′,4″,4″,4‴-tetraaminophthalocyanine and observe a significant increase in electrical conductivity. Ultraviolet-visible-near-infrared (UV-vis-NIR) and Raman spectroscopy are used to track the chemical changes on the nanocrystals’ surface before and after ligand exchange and develop a detailed picture of the various components which dominate the surface chemistry of this material. Grazing-incidence small-angle X-ray scattering (GISAXS) serve to study the importance of electronic conjugation in the organic π-system vs interparticle spacing for efficient charge transport. Transport measurements reveal that Cu4APc provides efficient electronic coupling for neighboring Cu1.1S NCs. The electrical properties of monolayers of this hybrid ensemble are consistent with a two-dimensional semiconductor and exhibit two abrupt changes at discrete temperatures (120 and 210 K), which may be interpreted as phase changes. This material provides the opportunity to apply the hybrid ensemble as a chemiresistor in organic vapor sensing. The vapor sensing experiments exhibits a strong selectivity between polar and nonpolar analytes, which we discuss in light of the role of the organic π-system and its metal center. Next, we choose ternary alloyed Cu-based chalcogenide NCs Cu2SeyS1–y and checked the effect of ligand exchange with the organic π-system Cobalt β-tetraaminophthalocyanine (CoTAPc) along with its binary counterpart Cu2Se NCs. We analysed changes in the structural, optical as well as electric properties of thin films of these hybrid materials. Strong ligand interaction with the surface of the NCs is revealed by UV/vis absorption and Raman spectroscopy. GISAXS studies show a significant contraction in the interparticle distance upon ligand exchange. For copper-deficient Cu2-xSe, this contraction has a negligible effect on electric transport, while for copper-deficient Cu2-xSeyS1-y, the conductivity increases by eight orders of magnitude and 8 results in metal-like temperature-dependent transport. We discuss these differences in the light of varying contributions of electronic vs. ionic transport in the two materials and highlight their effect on the stability of the transport properties under ambient conditions. With photocurrent measurements, we demonstrate high optical responsivities of 200-400 A/W for CoTAPc-capped Cu2SeyS1–y and emphasize the beneficial role of the organic π-system in this respect, which acts as an electronic linker and an optical sensitizer at the same time. Finally, we report on the in-situ monitoring of the formation of conductive superlattices of Cu1.1S nanodiscs via cross-linking with semiconducting Co-4,4′,4″,4″,4‴-tetraaminophthalocyanine (CoTAPc) molecules at the liquid/air interface by real-time grazing incidence small angle X-ray scattering (GISAXS). We determine the structure, symmetry and lattice parameters of the superlattices, formed during solvent evaporation and ligand exchange on the self-assembled nanodiscs. Cu1.1S nanodiscs self-assemble into two-dimensional hexagonal superlattice with a minor in-plane contraction (~ 0.2 nm) in the lattice parameter. A continuous contraction of the superlattice has been observed during ligand exchange, preserving the initial hexagonal symmetry. We estimate a resultant decrement of about 5% in the in-plane lattice parameters. The contraction is attributed to the continuous replacement of the native oleylamine surface ligands with rigid CoTAPc. The successful cross-linking of the nanodiscs is manifested in terms of the high electrical conductivity observed in the superlattices. This finding provides a convenient platform to understand the correlation between the structure and transport of the coupled superstructures of organic and inorganic nanocrystals of anisotropic shape