الفهرس 
Chapter I : IntroductionOnly 14 pages are availabe for public view
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Abstract
Hole interaction is a major area of interest within the arena of noncovalent interactions owing to its prevalent roles in molecular recognition and crystal materials. Notwithstanding, the nature and origin of the σ-hole and π-hole interactions within the electrostatic hole containing complexes are a source of debate. The current thesis is accordingly devoted to thoroughly provide a premier insight into the basic features and fundamental linchpins of σ hole and π-hole as the most prevalent types of hole interactions using a plethora of quantum mechanical calculations.
For σ-hole interactions, the ability of carbon-containing molecules (W-C-F3, where W is a withdrawing atom or group) to interact with Lewis bases (B), Lewis acids (A), and σ hole containing molecules (X2) with W-C∙∙∙B/A/X2 angle of 180° was elucidated. −σ Hole and +σ-hole terminologies were also employed to describe the interactions of W-C-F3 with nucleophilic and electrophilic sites, respectively. According to the results, the most favorable interaction energies were denoted when the F-C-X3 interact with X2 molecules compared to those with B and A molecules.
Besides, two unconventional Type III halogen∙∙∙halogen interactions, namely: σ-hole∙∙∙σ-hole and di-σ-hole interactions, will be introduced in a series of halogenated complexes. In Type III, the A halogen∙∙∙halogen angles are typically equal to 180°, and the occurrence of σ hole on halogen atoms is mandatory. Substantial binding energies were denoted for all the studied halomolecules, interpreted as the sum of (i) attractive electrostatic forces between the positive σ-hole of one halogen atom and the negative belt of the other halogen atom, (ii) repulsive electrostatic forces between the positive σ-holes of the two halogen atoms, (iii) repulsive electrostatic forces between the negative belts of the two halogen atoms, (iv) the van der Waal interactions between the two halogen atoms, and (v) polarization contribution of one halogen atom by the other halogen atom. Fluorine-based interactions were recognized as traditional halogen bonds, due to the absence of σ-hole on the fluorine atom. Symmetry adapted perturbation theory-based energy decomposition analysis (SAPT-EDA) revealed that dispersion energy plays a crucial role in the σ-hole∙∙∙σ-hole and di-σ-hole interactions. However, the traditional halogen bond in the studied fluorine-containing complexes was stabilized by electrostatic forces. Through exploring the Cambridge Structure Database (CSD), several crystal structures were identified to reveal the experimental reliability of σ hole∙∙∙σ-hole interactions.
Moreover, the versatility of the X-T-X3 compounds (where T = C, Si, and Ge, and X = F, Cl, and Br) to participate in tetrel- and halogen-based interactions, was settled out at MP2/aug-cc-pVTZ level of theory, within the tetrel∙∙∙tetrel, tetrel∙∙∙halogen, type III halogen∙∙∙halogen and type II halogen∙∙∙halogen configurations. The energetic findings significantly unveiled the favorability of the tetrel∙∙∙tetrel directional configuration with considerable negative binding energies over tetrel∙∙∙halogen, type III halogen∙∙∙halogen, and type II halogen∙∙∙halogen analogs. Quantum theory of atoms in molecules (QTAIM) and noncovalent interaction (NCI) analyses were accomplished to disclose the nature of the tetrel- and halogen-bonding interactions within designed configurations, giving good correlations between the total electron densities and binding energies. Further insight into the binding energy physical meanings was invoked through using symmetry-adapted perturbation theory-based energy decomposition analysis (SAPT-EDA), featuring the dispersion term as the most prominent force beyond the examined interactions. The theoretical results were supported by versatile crystal structures which were characterized by the same type of interactions.
As an essential issue, the potentiality of sp2-hybridized chalcogen atoms in di-substituted carbon-containing molecules (i.e., Y=C=Y, where Y = O, S, and Se) to engage in various types of chalcogen∙∙∙chalcogen interactions were thoroughly assessed. A plethora of quantum mechanical calculations, including molecular electrostatic potential (MEP), maximum/minimum electrostatic potential (Vs,max/Vs,min), point-of-charge (PoC), quantum theory of atoms in molecules (QTAIM), noncovalent interaction (NCI), and symmetry-adapted perturbation theory-based energy decomposition analysis (SAPT-EDA) calculations, were executed. The energetic findings revealed a preferential tendency of the studied chalcogen-bearing molecules to engage in type I, II, III, and IV chalcogen∙∙∙chalcogen interactions, albeit within disparate energies. Among like∙∙∙like complexes, type IV interactions showed the most favorable negative binding energies, whereas type III interactions exhibited the scrawniest binding energies. Unexpectedly, oxygen-containing complexes within type IV interactions showed an alien pattern of binding energies that decreased along with the chalcogen atomic size level up. QTAIM analysis provided a solo BCP, via chalcogen∙∙∙chalcogen interactions, with no clues for any secondary ones. SAPT-EDA outlined the domination of the explored interactions by the dispersion forces and indicated to the pivotal shares of the electrostatic forces, except type III -hole∙∙∙-hole and di -hole interactions.
With regard to π-hole interaction, a comparative investigation of trivalent triel-containing molecules to participate in ±π-hole interactions with Lewis bases, Lewis acids, σ-hole-containing molecules, and lp-hole-containing molecules was comparatively investigated using a plethora of quantum mechanical calculations. According to the results, it was found that the −π-hole interactions were more favorable than the +π-hole ones, with larger negative interaction energies and shorter intermolecular distance. +π-hole interactions with lp-hole-containing molecules were observed with larger substantial interaction energies than Lewis acids, and σ-hole-containing molecules varied from –0.65 to –5.18 kcal/mol. Quantum theory of atoms in molecules and noncovalent interaction index analyses revealed the noncovalent nature for ±π-hole interactions. As well, symmetry-adapted perturbation theory-based energy decomposition analysis affirmed that electrostatic and dispersion forces controlled the −π-hole interactions, whereas the +π-hole analogs were dominated by dispersion forces only.
Furthermore, the π-hole-based interactions were elucidated for the 2nd row elements of groups III-VI in the periodic table with benzene (BZN) and hexafluorobenzene (HFB) as electron-rich and electron-deficient π-systems, respectively. Besides, the π-hole∙∙∙Lewis base interactions were investigated and compared to the π-hole∙∙∙π-system candidates. Various quantum mechanical calculations, including geometrical optimization, molecular electrostatic potential (MEP) analyses, and point-of-charge (PoC) calculations, will be executed on AlF3, SiF2O, PFO2, and SO3 systems as triel, tetrel, pnicogen, and chalcogen π-hole bond donors. Using a wide range of quantum mechanical calculations, it was manifested that the investigated π-hole∙∙∙π-system/Lewis base interactions exhibited preferential negative binding energies with values up to –30.38 kcal/mol for AlF3∙∙∙NH3 complex. π-hole∙∙∙Lewis base complexes participated in stronger π-hole bond than the π-hole∙∙∙π-system analogs. In comparison, the reversed pattern was noticed for the π-hole∙∙∙electron-deficient π-system. Quantum theory of atoms in molecules (QTAIM) analysis announced the closed-shell nature of all complexes under study except π-hole∙∙∙NH3 interactions that recognized with a salient covalent nature. Symmetry-adapted perturbation theory-based energy decomposition analysis (SAPT-EDA) announced that the π-hole∙∙∙π-system interactions were governed by the dispersion forces, whereas the π-hole∙∙∙Lewis base interactions were dominated by the electrostatic terms.
An innovative elucidation of the potentiality of the X2CY molecules (X = F, Cl; Y = O, S) to engage in -hole∙∙∙-hole, di -hole, -hole∙∙∙π-hole, and π-hole∙∙∙π-hole interactions was assessed. In that spirit, the potential energy surface (PES) scan will be devoted to thoroughly characterize the features of the (X2CY)2 complexes within a series of configurations, including halogen∙∙∙halogen, halogen∙∙∙chalcogen, chalcogen∙∙∙chalcogen, halogen∙∙∙tetrel, chalcogen∙∙∙tetrel, and tetrel∙∙∙tetrel. In most instances, the strength of the inspected interactions declined according to the order: tetrel∙∙∙tetrel (staggered) > tetrel∙∙∙tetrel (eclipsed) > chalcogen∙∙∙tetrel > halogen∙∙∙tetrel > chalcogen∙∙∙chalcogen > halogen∙∙∙chalcogen > halogen∙∙∙halogen configurations. Benchmarking of the binding energies emphasized an approximate similarity between the resulted energetic features that were evaluated at the MP2/aug-cc-pVTZ and CCSD/CBS levels of theory. Quantum theory of atoms in molecules (QTAIM) critically unveiled the closed-shell nature of the halogen-, chalcogen-, and tetrel-bonding interactions within the adopted configurations. Symmetry-adapted perturbation theory-based energy decomposition analysis (SAPT-EDA) demonstrated the domination of the -hole interactions by the dispersion forces (Edisp). In addition to the dominant Edisp, preferential contributions of the electrostatic (Eelst) and induction (Eind) forces were detected for the π-hole bonded complexes within all the scouted configurations.
All calculations were executed using High-Performance Computer (HPC) located at CompChem Lab, Minia University, and supported by the Science and Technology Development Fund, STDF, Egypt, Grants No. 5480 & 7972.