<p>In the history of Chemistry, materials chemists have developed their ideas mainly by doing experiments in laboratories. The underlying motivation for this laboratory work has generally been pure curiosity or the ambition to find a solution to a specific problem. Minor changes in the composition or structure of a material can cause major changes in its properties. The development of powerful computers has now opened up the possibility to calculate properties of new materials using quantum mechanical methods.</p><p>The Chemistry of different boron-related materials has been evaluated in this thesis by Density Functional Theory (DFT). Cubic boron nitride (c-BN) is a most interesting material for the microelectronics and tool industry. During thin film deposition of c-BN, several problems arise which most often result in unwanted BN isomorphs. Chemical processes at the (110) and (111) surface of c-BN have been investigated in order to shed light upon some of these complex processes. Typically adsorption energies and surface reconstruction were found to differ significantly between the two surfaces. </p><p>Other materials investigated are layered transition-metal diborides (MeB₂). Incorporation of transition-metal atoms into elemental boron in its most fundamental structure, ά-boron, has also been investigated. The calculations on MeB₂ focused on the stability of the planar compared to the puckered structure of MeB₂. Stability was investigated by calculating Density of States (DOS) and bond populations. Deviations in the cell parameters from their ideal values were also considered. </p><p>A separate project concerned reactivity of the TiB₂(001) surface. Molecular and dissociated adsorption energies and adsorption geometries were calculated for H₂, H₂O and O₂. It was concluded that the titanium surface was more reactive than the boron surface and that the adsorption energies were comparable to or stronger than other well known surface-active compounds like TiO₂.</p>

The correlation between NMR chemical shifts of interstitial atoms and electronic structures of boron- and carbon-centered hexazirconium halide clusters was investigated by density functional theory (DFT) calculation. The influences of bridging halide and terminal ligand variations on electronic structure were examined respectively. Inverse proportionality was found between the chemical shifts and the calculated energy gaps between two Kohn-Sham orbitals of t1u symmetry, which arose from the bonding and antibonding interaction between the zirconium cage bonding orbitals and the interstitial 2p orbitals. Chemical shielding properties of the interstitial atoms were calculated with Gauge Including Atomic Orbital (GIAO) method. Stepwise ligand substitution of terminal chlorides on [(Zr6CCl12)Cl6]4-cluster by tri(n-butyl)-phosphine oxide (Bu3PO) was conducted with the aid of TlPF6. Composition of the reaction mixtures was analyzed by use of both 13C and 31P NMR. A preliminary scheme for synthesis and separation of [(Zr6CCl12)Cl6-x(Bu3PO)x]x-4 (x = 3 ?? 5) mixture based on solubility difference was reevaluated. Three 1,10-phenanthroline based bidentate ligands, namely, 2,9-Bis(diphenyl-phosphinyl)-1,10-phenanthroline, 2,9-Bis(diethoxyphosphoryl)-1,10-phenanthroline, and 2,9-Bis(di-n-butoxyphosphoryl)-1,10-phenantholine, were synthesized for bridge-chelating the hexazirconium clusters. Coordination chemistry of these ligands with the [Zr6BCl12] and [Zr6CCl12] clusters was subject to preliminary investigation.

Theoretical studies of point defect interactions and structural stability of compounds have been performed using density functional theory. The defect-related properties, such as activation energy of diffusion, electronic and magnetic structure of selected materials have been studied. The major part of the present work is devoted to a very important material for semiconductor industry, GaAs. The formation energies of intrinsic point defects and the solution energies of 3d transitions in GaAs have been calculated from first principles. Based on the calculated energies, we analysed the site preference of defects in the crystal. The tendency of defects to form clusters has been investigated for the intrinsic defects as well as for impurities in GaAs. The magnetic moment of 3d impurities has been calculated as a function of the chemical environment. The possibility of increasing the Curie temperature in (Ga,Mn)As by co-doping it with Cr impurities has been examined on the basis of calculated total energy difference between the disordered local moment and the ferromagnetically ordered spin configurations. We found that, in order to reach the highest critical temperature, GaAs should be separately doped with either Cr or Mn impurities. Also, we have shown that diffusion barrier of interstitial Mn depends on the charge state of this impurity in (Ga, Mn)As. The formation of defect complexes between interstitial and substitutional Mn atoms, and their influence on the value of diffusion barrier for interstitial Mn, has been studied. The pair interactions energies between interstitial oxygen atoms in hcp Zr, Hf and Ti have been calculated using first principles. Based on the calculated energies, the oxygen ordering structures in IVB transition metal solid solutions have been explained. A prediction of nitrogen ordering in Hf-N solid solution has been made. The thermodynamic description of intermetallic compounds in the Zr-Sn binary system has been obtained. The conclusion has been made that Zr substitution on the Sn sites takes place in the Zr4Sn phase, which accounts for the unusual stoichiometry of this Cr3Si structure type compound. The influence of pressure on the phase stability in the Fe-Si system has been investigated. We have found instability of the hcp Fe0.9Si0.1 random alloy with respect to the decomposition onto the Si-poor hcp Fe alloy and the B2 FeSi under high pressure. The tendency of this decomposition becomes stronger with increasing the applied pressure. / QC 20100624

Point defects are an inherent part of crystalline materials and they influence important physical and chemical properties, such as diffusion, hardness, catalytic activity and phase stability. Increased understanding of point defects enables us to tailor the defect-related properties to the application at hand. Modeling and simulation have a prominent role in acquiring this knowledge. In this thesis thermodynamic and kinetic properties of point defects in metals and ceramics are studied using first-principles calculations based on density functional theory. Phenomenological models are used to translate the atomic level properties, obtained from the first-principles calculations, into functional materials properties. The next paragraph presents the particular problems under study. The formation and migration of vacancies and simple vacancy clusters in copper are investigated by calculating the energies associated with these processes. The structure, stability and electronic properties of the low-oxygen oxides of titanium, TiOx with 1/3 &lt; x &lt; 3/2, are studied and the importance of structural vacancies is demonstrated. We develop an integrated first-principles and Calphad approach to calculate phase diagrams in the titanium-carbon-nitrogen system, with particular focus on vacancy-induced ordering of the substoichiometric carbonitride phase, TiCxNy (x+y &lt; 1). The possibility of forming higher oxides of plutonium than plutonium dioxide is explored by calculating the enthalpies for nonstoichiometric defect-containing compounds and the analysis shows that such oxidation is only produced by strong oxidants. For ceria (CeO2) doped with trivalent ions from the lanthanide series we probe the connection between the choice of a dopant and the improvement of ionic conductivity by studying the oxygen-vacancy formation and migration properties. The significance of minimizing the dopant-vacancy interactions is highlighted. We investigate the redox thermodynamics of CeO2-MO2 solid solutions with M being Ti, Zr, Hf, Th, Si, Ge, Sn or Pb and show that reduction is facilitated by small solutes. The results in this thesis are relevant for the performance of solid electrolytes, which are an integral part of solid oxide fuel cells, oxygen storage materials in automotive three-way catalysts, nuclear waste materials and cutting tool materials. / QC 20100622

Hybrid density functional theory (DFT) B3LYP method is applied to study the four diverse enzyme systems: zinc-containing peptidases (thermolysin and stromelysin), methyl-coenzyme M reductase, ribonucleotide reductases (classes I and III), and superoxide dismutases (Cu,Zn- and Ni-dependent enzymes). Powerfull tools of modern quantum chemistry are used to address the questions of biological pathways at their molecular level, proposing a novel mechanism for methane production by methyl-coenzyme M reductase and providing additional insights into hydrolysis by zinc peptidases, substrate conversion by ribonucleotide reductases, and biological superoxide dismutation. Catalysis by these enzymes, with the exception of zinc peptidases, involves radical chemistry.

The serendipitous discovery of the potent cytotoxic properties of cisplatin brought about a revolution in the treatment of certain types of cancer, but almost fifty years later, there still remain unknown areas in the chemistry of cisplatin. There are questions regarding which form of the drug reaches its DNA target, or why certain DNA sequences are more preferred than others for reaction with cisplatin. The work presented here aims to address some of these problems, using quantum chemical calculations to complement and interpret available experimental data. Cisplatin's activation reactions are explored by Density Functional Theory (DFT) on two model systems, one solely using a self-consistent reaction field (SCRF) for modeling bulk water, and one including an additional partial solvation shell of water molecules. It is concluded that adding explicit solvation provides a better picture than using SCRF solvation alone. The energy surface supports the view that the active form of cisplatin is the monoaquated form. The activation reactions of the cisplatin-derived drug, JM118, are investigated using DFT and SCRF calculations using three solvation model systems. The results show a slower rate of hydrolysis for the first reaction, and a faster rate for the second, suggesting diaquated JM118 as the main DNA binding form of the drug. Diaquated cisplatin's first and second reaction with guanine and adenine are studied using DFT and SCRF solvation. Cisplatin's propensity toward guanine in the first substitution is explained by larger stabilisation energy for the initially formed complex and by favoured kinetics. For the second substitution, higher stability in complexation with guanine over adenine is ascribed as the main factor favouring guanine over adenine substitution. This provides the first explanation for the predominance of 1,2-d(GpG) over 1,2-d(ApG) adducts, and the direction specificity of the 1,2-d(ApG) adducts.

In the history of Chemistry, materials chemists have developed their ideas mainly by doing experiments in laboratories. The underlying motivation for this laboratory work has generally been pure curiosity or the ambition to find a solution to a specific problem. Minor changes in the composition or structure of a material can cause major changes in its properties. The development of powerful computers has now opened up the possibility to calculate properties of new materials using quantum mechanical methods. The Chemistry of different boron-related materials has been evaluated in this thesis by Density Functional Theory (DFT). Cubic boron nitride (c-BN) is a most interesting material for the microelectronics and tool industry. During thin film deposition of c-BN, several problems arise which most often result in unwanted BN isomorphs. Chemical processes at the (110) and (111) surface of c-BN have been investigated in order to shed light upon some of these complex processes. Typically adsorption energies and surface reconstruction were found to differ significantly between the two surfaces. Other materials investigated are layered transition-metal diborides (MeB2). Incorporation of transition-metal atoms into elemental boron in its most fundamental structure, ά-boron, has also been investigated. The calculations on MeB2 focused on the stability of the planar compared to the puckered structure of MeB2. Stability was investigated by calculating Density of States (DOS) and bond populations. Deviations in the cell parameters from their ideal values were also considered. A separate project concerned reactivity of the TiB2(001) surface. Molecular and dissociated adsorption energies and adsorption geometries were calculated for H2, H2O and O2. It was concluded that the titanium surface was more reactive than the boron surface and that the adsorption energies were comparable to or stronger than other well known surface-active compounds like TiO2.

In this thesis I have studied some problems of condensed matter at high pressures and temperatures by means of numerical simulations based on Density Functional Theory (DFT). The stability of MgCO3 and CaCO3 carbonates at the Earth's mantle conditions may play an important role in the global carbon cycle through the subduction of the oceanic crust. By performing ab initio electronic structure calculations, we observed a new high-pressure phase transition within the Pmcn structure of CaCO3. This transformation is characterized by the change of the sp-hybridization state of carbon atom and indicates a change to a new crystal-chemical regime. By performing ab initio Molecular Dynamics simulations we show the new phase to be stable at 250 GPa and 1000K. Thus, the formation of sp3 hybridized bonds in carbonates can explain the stability of MaCO3 and CaCO3 at pressures corresponding to the Earth's lower mantle conditions. We have also calculated phase transition sequence in CaCO3, SrCO3 and BaCO3, and have found that, despite the fact that these carbonates are isostructural and undergo the same type of aragonite to post-aragonite transition, their phase transformation sequences are different at high pressures. The continuous improvement of the high-pressure technique led to the discovery of new composite structures at high pressures and complex phases of many elements in the periodic table have been determined as composite host-guest incommensurate structures. We propose a procedure to accurately describe the structural parameters of an incommensurate phase using ab initio methods by approximating it with a set of analogous commensurate supercells and exploiting the fact that the total energy of the system is a function of structural parameters. By applying this method to the Sc-II phase, we have determined the incommensurate ratio, lattice parameters and Wyckoff positions of Sc-II in excellent agreement with the available experimental data. Moreover, we predict the occurrence of an incommensurate high-pressure phase in Ca from first-principle calculations within this approach. The implementation of DFT in modern electronic structure calculation methods proved to be very successful in predicting the physical properties of a solid at low temperature. One can rigorously describe the thermodynamics of a crystal via the collective excitation of the ionic lattice, and the ab initio calculations give an accurate phonon spectra in the quasi-harmonic approximation. Recently an elegant method to calculate phonon spectra at finite temperature in a self-consistent way by using first principles methods has been developed. Within the framework of self-consistent ab initio lattice dynamics approach (SCAILD) it is possible to reproduce the observed stable phonon spectra of high-temperature bcc phase of Ti, Zr and Hf with a good accuracy. We show that this method gives also a good description of the thermodynamics of hcp and bcc phases of Ti, Zr and Hf at high temperatures, and we provide a procedure for the correct estimation of the hcp to bcc phase transition temperature.

The analysis of nature of bonding in non-classical structures is always an intriguing area of research. Typical examples of such systems are polyhedral boranes that exhibit fascinating cluster bonding where the traditional 2-center-2-electron (2c-2e) bond model fails. This thesis involves the investigation of such polyhedral borane structures and their reactivity by employing both qualitative and quantitative tools of electronic structure theory. There is an intense current interest in the macropolyhedral boranes for their applications pharmaceuticals and materials chemistry. The mno rule had been formulated to account for the electronic requirements for the macropolyhedral structures. Though useful in explaining and designing structures, electron counting rules provide a yes or no answer; not all the molelcules having stipulated number of electrons are equally stable. We have used the concept of orbital compatibility to explain the relative energies of different macropolyhedral structural patterns such as closo-closo, closo-nido and nido-nido. One of the major problems in polyhedral boron cage chemistry has been the lack of general synthetic routes for the construction of large cage systems . With this view, we explored the mechanism of the reaction of macropolyhedron B20H16 with MeCN and similar ligands, which provide an understanding of the skeletal rearrangement that occur in macropolyhedral boranes. This can help in the design and synthesis of new macropolyhedral boranes. The early examples of metallaboranes were found to adopt structures which are analogous to that of boranes and carboranes. Hypercloso metallaboranes have closo structure with less number of electrons than required by Wades rule. We have carried out a detailed DFT analysis to explore the structure and electronic relationship of 9-12 vertex closo and hypercloso structures of both borane and metallaboranes. Calculations show that in vertex hypercloso metallaborane needs only n skeleton electron pairs rather than n+1 as suggested by Wade’s rules. Stabilization of supraicosahedral boranes with more than 12 vertices by substituting BH groups by transition metal fragments is also explored with DFT calculations. Calculations show that as the number and the size of the metal atom increases the stability of supraicosahedral and condensed supraicosahedral borane structures also increases. These studies will open up new possibilities for the development of polyhedral clusters of extraordinary size.

This thesis presents the synthesis and investigations of physical and chemical properties of multiferroic materials experimentally as well as theoretically. Multiferroics are materials in which at least two of the three ferroic orders, ferroelectricity, ferromagnetism and ferroelasticity occur in the same phase. Multiferroics, have the potential to be used as a four state as well as cross switchable memory devices. The thesis is organized into seven Chapters. Chapter 1 gives a brief overview of the different facets of multiferroics, explaining the origin of Multiferroicity and magnetoelectric coupling, their possible technological applications and the challenges involved. Chapter 2-4 concerns the experimental aspects and chapter 5-7 concerns the theoretical aspects. Chapter 2 deals with experimental investigations on nanoscale charge-ordered rare earth manganites. It shows with decreasing particle size the ferromagnetic interaction increases and the charge-ordering vanishes down to the lowest sizes. Chapter 3 describes magneto-dielectric, magnetic and ferroelectric properties of hexagonal LuMnO3. It also describes the Raman spectroscopy of this compound through the magnetic and ferroelectric transition temperatures. Chapter 4 deals with the anisotropic multiferroic properties in single crystals of hexagonal ErMnO3. In chapter 5 a brief introduction of density functional theory (DFT) is given. Chapter 6 deals with the magneto-structural changes, spin-phonon couplings and crystal field splittings for the different magnetic orderings LuMnO3. Chapter 7 elucidates the role of Lu d0-ness for the ferroelectricity observed of this compound.