Thermoelectric energy (TE) conversion can be used to create electricity from temperature gradients. Hence power can be generated from waste heat using TE materials, e.g. from the exhaust in automotives. This power in turn may lead to a reduction of gas consumption by reducing the alternator load on the engine. Because of the increasing demand and limited availability of energy sources, there is strong and renewed interest in advancing thermoelectric materials. Past research shows that the best TE materials are narrow band gap semiconductors composed of heavy elements, exhibiting a large Seebeck coefficient, S, combined with high electrical conductivity, σ, and low thermal conductivity, κ. Various research projects have been attempted during the past four years of my Ph.D. studies. These include the synthesis, crystal structure studies, electronic structure calculations and thermoelectric properties of transition metal oxides and thallium main group chalcogenides. Because of the good thermal stability, lack of sensitivity to the air, and non-toxicity, transition metal oxides are potential candidates for commercial thermoelectric applications. During the investigation of oxides for thermoelectric application, several interesting features of different transition metal oxides have been discovered: 1. A new quaternary layered transition-metal oxide, Na2Cu2TeO6, has been synthesized under air using stoichiometric mixtures of Na2CO3, CuO and TeO2. Na2Cu2TeO6 crystallizes in a new structure type, monoclinic space group C2/m with a = 5.7059(6) Å, b = 8.6751(9) Å, c = 5.9380(6) Å,  = 113.740(2)°, V = 269.05(5) Å3 and Z = 2, as determined by single crystal X-ray diffraction. The structure is composed of[Cu2TeO6] layers with the Na atoms located in the octahedral voids between the layers. Na2Cu2TeO6 is a green nonmetallic compound, in agreement with the electronic structure calculation and electrical resistance measurement. 2. An n-type narrow band gap semiconductor, LaMo8O14, exhibiting the high Seebeck coefficient of -94 μVK-1 at room temperature has been investigated. 3. Pb0.69Mo4O6 with a new modulated structure and stoichiometry was determined from single-crystal X-ray diffraction data. The compound crystallizes in the tetragonal super space group, P4/mbm(00g)00ss, with a = 9.6112(3) Å, c = 2.8411(1) Å, q = 0.25c*, which is different from the previously reported structure. As for the research of thermoelectric properties of thallium main group chalcogenides, three new ternary thallium selenides, Tl2.35Sb8.65Se14, Tl1.97Sb8.03Se13 and Tl2.04Bi7.96Se13, have been discovered. All three compounds crystallize in the same space group P21/m with different cell parameters, and in part different Wyckoff sites, hence different structure types. The three selenides with similar structures are composed of distorted edge-sharing (Sb,Bi)Se6 octahedra, while the distorted Tl/(Sb, Bi) sites are coordinated by 8 - 9 Se atoms. Electronic structure calculations and physical property measurements reveal they are semiconductors with high Seebeck coefficient but low electrical conductivity, and therefore not good thermoelectrics. On the other hand, our transport property measurements on the unoptimized Tl2SnTe3 sample show interesting thermoelectric properties of this known compound. Advanced thermoelectrics are dominated by antimonides and tellurides so far. The structures of the tellurides are mostly composed of NaCl-related motifs, hence do not contain any Te–Te bonds. All of the antimonide structures containing Sb–Sb bonds of various lengths are much more complex. The Sb atom substructures are Sb24– pairs in β-Zn4Sb3, linear Sb37– units in Yb14MnSb11, planar Sb44– rectangles in the skutterudites, e.g., LaFe3CoSb12, and Sb8 cubes interconnected via short Sb–Sb bonds to a three-dimensional network in Mo3Sb5Te2. The results of electronic structure calculations suggested that these interactions have a significant impact on the band gap size as well as on the effective mass around the Fermi level, which represent vital criteria for advanced thermoelectrics. The crystal structure and electronic structure investigation for the unique T net planar Sb–Sb interactions in Hf5Sb9 will be also presented, although Hf5Sb9 is metallic compound with poor thermoelectric performances.

Diamond-like semiconductors (DLSs) are a class of semiconductor materials having structures similar to that of either cubic or hexagonal diamond. These normal valence compounds are of interest for their wide variety of technologically useful properties that can be tuned for specific applications. Until recently, DLS research has been focused on binary and ternary compositions due to their relative ease of synthesis. However, quaternary DLSs have gained considerable popularity due to their increased compositional flexibility and their potential as multifunctional materials. Despite their growing reputation, the vast number of possible combinations and conceivable solid solutions, DLSs remain fairly unexplored.<br>This work focuses on quaternary DLSs of the formula Ag2-II-IV-S4 in order to advance the knowledge of structure-property relationships for this entire class of materials. Toward this goal, a more complete understanding of the crystal structures of these materials is necessary. This task is often problematic due to the presence of isoelectronic, or nearly isoelectonic elements, that can complicate X-ray structure refinements. In this work, Ag2CdGeS4 is used as a case study to demonstrate that this problem can be resolved with careful consideration of bonding environments as well as the use of high-resolution X-ray sources. For the novel DLS Ag2ZnSiS4, the relationship between the structure and optical properties is probed with the combination of single crystal X-ray diffraction, optical diffuse reflectance spectroscopy and electronic structure calculations using the software package Wien2k. Finally, the current set of predictive tools employed to forcast diamond-like structures are reviewed, including the adherence of these guidelines to the novel compound Ag2FeSiS4 as well all over 60 ternary and quaternary diamond-like materials currently reported in the literature. Furthermore, the most common radii sets used for the prediction of bond distance and cell parameters in these materials are compared to the observed bond distances in quaternary diamond-like nonoxide materials. / Bayer School of Natural and Environmental Sciences / Chemistry and Biochemistry / PhD; / Dissertation;

Thermoelectric energy (TE) conversion can be used to create electricity from temperature gradients. Hence power can be generated from waste heat using TE materials, e.g. from the exhaust in automotives. This power in turn may lead to a reduction of gas consumption by reducing the alternator load on the engine. Because of the increasing demand and limited availability of energy sources, there is strong and renewed interest in advancing thermoelectric materials. Past research shows that the best TE materials are narrow band gap semiconductors composed of heavy elements, exhibiting a large Seebeck coefficient, S, combined with high electrical conductivity, σ, and low thermal conductivity, κ. Various research projects have been attempted during the past four years of my Ph.D. studies. These include the synthesis, crystal structure studies, electronic structure calculations and thermoelectric properties of transition metal oxides and thallium main group chalcogenides. Because of the good thermal stability, lack of sensitivity to the air, and non-toxicity, transition metal oxides are potential candidates for commercial thermoelectric applications. During the investigation of oxides for thermoelectric application, several interesting features of different transition metal oxides have been discovered: 1. A new quaternary layered transition-metal oxide, Na2Cu2TeO6, has been synthesized under air using stoichiometric mixtures of Na2CO3, CuO and TeO2. Na2Cu2TeO6 crystallizes in a new structure type, monoclinic space group C2/m with a = 5.7059(6) Å, b = 8.6751(9) Å, c = 5.9380(6) Å,  = 113.740(2)°, V = 269.05(5) Å3 and Z = 2, as determined by single crystal X-ray diffraction. The structure is composed of[Cu2TeO6] layers with the Na atoms located in the octahedral voids between the layers. Na2Cu2TeO6 is a green nonmetallic compound, in agreement with the electronic structure calculation and electrical resistance measurement. 2. An n-type narrow band gap semiconductor, LaMo8O14, exhibiting the high Seebeck coefficient of -94 μVK-1 at room temperature has been investigated. 3. Pb0.69Mo4O6 with a new modulated structure and stoichiometry was determined from single-crystal X-ray diffraction data. The compound crystallizes in the tetragonal super space group, P4/mbm(00g)00ss, with a = 9.6112(3) Å, c = 2.8411(1) Å, q = 0.25c*, which is different from the previously reported structure. As for the research of thermoelectric properties of thallium main group chalcogenides, three new ternary thallium selenides, Tl2.35Sb8.65Se14, Tl1.97Sb8.03Se13 and Tl2.04Bi7.96Se13, have been discovered. All three compounds crystallize in the same space group P21/m with different cell parameters, and in part different Wyckoff sites, hence different structure types. The three selenides with similar structures are composed of distorted edge-sharing (Sb,Bi)Se6 octahedra, while the distorted Tl/(Sb, Bi) sites are coordinated by 8 - 9 Se atoms. Electronic structure calculations and physical property measurements reveal they are semiconductors with high Seebeck coefficient but low electrical conductivity, and therefore not good thermoelectrics. On the other hand, our transport property measurements on the unoptimized Tl2SnTe3 sample show interesting thermoelectric properties of this known compound. Advanced thermoelectrics are dominated by antimonides and tellurides so far. The structures of the tellurides are mostly composed of NaCl-related motifs, hence do not contain any Te–Te bonds. All of the antimonide structures containing Sb–Sb bonds of various lengths are much more complex. The Sb atom substructures are Sb24– pairs in β-Zn4Sb3, linear Sb37– units in Yb14MnSb11, planar Sb44– rectangles in the skutterudites, e.g., LaFe3CoSb12, and Sb8 cubes interconnected via short Sb–Sb bonds to a three-dimensional network in Mo3Sb5Te2. The results of electronic structure calculations suggested that these interactions have a significant impact on the band gap size as well as on the effective mass around the Fermi level, which represent vital criteria for advanced thermoelectrics. The crystal structure and electronic structure investigation for the unique T net planar Sb–Sb interactions in Hf5Sb9 will be also presented, although Hf5Sb9 is metallic compound with poor thermoelectric performances.

No description available.

This thesis examines the synthesis of α-fluoro-β-amino acids, and the influence of the constituent fluorine on the conformation and biological properties of β-peptide derivatives. Chapter One discusses the unique properties of the C-F bond, and applications of fluorine substitution in organic and medicinal chemistry. This is followed by a review of fluorinated analogues of α-amino acids, and how their incorporation into α-peptides has resulted in profound modifications, such as enhanced thermal and chemical stability, increased affinity for lipid bilayers, stronger self-association and improved biological activity. Experimental and theoretical data indicate two conformational effects associated with fluoroamides: the F-C-C(O)-N(H) moiety in α-fluoroamides adopts an antiperiplanar conformation, and in N-β-fluoroethylamides a gauche conformation between the vicinal C-F and C-N(CO) bonds is favoured. Chapter Two details the synthesis of a series of fluorinated β-peptides (2.13-2.24) designed to investigate the use of these stereoelectronic effects to control the conformation of β-peptide bonds. X-ray crystal structures were obtained for seven of these compounds and revealed the compounds had the expected conformations: when fluorine was positioned β to a nitrogen a gauche conformation was observed, and when fluorine was positioned α to a C=O group the structure adopted an antiperiplanar conformation. Thus, the strategic placement of fluorine can control the conformation of β-peptide bonds, and hence could be used to direct the secondary structures of β-peptides. The chapter is prefaced by an introduction to β-amino acids and the secondary structures of β-peptides. Chapter Three outlines the stereoselective synthesis of a series of α-fluorinated-β-amino acids. The synthesis of α-fluoro-β3-amino acids was achieved via direct fluorination of β3-amino acids with LDA and NFSI. The fluorination of N-Boc-protected β3-homophenylalanine, β3-homoleucine, β3-homovaline and β3-homoalanine all proceeded with good diastereomeric excesses (> 85 % de). However, the fluorination of N-Boc-protected β3-homophenylglycine occurred with a lower diastereomeric excess of 66%. Replacement of the Boc amine protecting group of β3-homophenylglycine with Cbz and Bz groups did not alter the stereoselectivity of the fluorination reaction, and substitution with an acetyl amine protecting group reduced the diastereomeric excess to 26%. The stereoselective synthesis of an α-fluoro-β2-homophenylalanine from 3-phenylpropanoic acid is also detailed. Conversion of the acid to the Evan's oxazolidinone followed by enantioselective fluorination and alkylation in high diastereomeric excess, and subsequent amination gave the α-fluorinated β2-amino acid. Chapter Four describes the enzyme assays carried out to assess the inhibitory activity of α-fluoro-β-amino acids, and the analogous non-fluorinated β-amino acids, against α-chymotrypsin. Both fluorinated and non-fluorinated β-amino acid derivatives were found to be competitive inhibitors of α-chymotrypsin, with Ki values in the low millimolar range. The fluorinated β2-homophenylalanine and β3-homophenylglycine derivatives (2.35, 3.26a, 3.43a and 3.44) were found to be more active against α-chymotrypsin than their non-fluorinated analogues (5.27, 3.24, 3.40 and 3.41), whereas the fluorinated β3-homophenylalanine methyl ester (2S,3S)-2.49 was inactive against α-chymotrypsin although the corresponding non-fluorinated derivative (S)-3.28 was a potent inhibitor. In Chapter Five a series of N-succinyl-β-amino acids-p-nitroanilides (5.8-5.13), containing both fluorinated and non-fluorinated β-amino acids, were designed and synthesised as possible substrates of α-chymotrypsin. β-Peptides are stable towards proteolytic hydrolysis, but the introduction of fluorine at the α-position in a β-amino acid was proposed to increase the activity of the adjacent amide bond, and thus make the β-peptide more susceptible to protease cleavage. However, the incorporation of fluorine had no influence on the proteolytic stability of compounds 5.8-5.13 as they were all found to be stable towards hydrolysis by α-chymotrypsin. Compounds 5.8, 5.9 and 5.13 were established as reversible competitive inhibitors of α-chymotrypsin Chapter Six is an experimental chapter and outlines the synthesis, purification and characterisation of the compounds prepared in this thesis.

No description available.

The structural architecture found in low-dimensional materials can lead to a number of interesting physical properties including anisotropic conductivity, magnetic frustration and non-linear optical properties. There is no standard synthesis concept described thus far to apply when searching for new low-dimensional compounds, and therefore control on the design of the new materials is of great importance.This thesis describes the synthesis, crystal structure and characterization of some new transition metal oxohalide compounds containing p-elements having a stereochemically active lone-pair. First row transition metal cations have been used in combination with SeIV, SbIII and TeIV ions as lone-pair elements and Cl- and Br- as halide ions. The lone-pairs do not participate in covalent bonding and are responsible for an asymmetric one-sided coordination. Lone-pair elements in combination with halide ions have shown to be powerful structural spacers that can confine transition metal building blocks into low-dimensional arrangements. The halide ions and lone-pairs reside in non-bonded crystal volumes where they interact through weak van der Waals forces. The transition metal atoms are most often arranged to form sheets, chains or small clusters; most commonly layered compounds are formed.To further explore the chemical system and to separate the transition metal entities even more the possibility to include tetrahedral building blocks such as phosphate-, silicate-, sulphate- and vanadate building blocks into this class of compounds has been investigated. Tetrahedral building blocks are well known for their ability of segmenting structural arrangements by corner sharing, which often leads to the formation of open framework structures. The inclusion of tetrahedral building blocks led to the discovery of interesting structural features such as complex hydrogen bonding, formation of unusual solid solutions or faulted stacking of layers.Compounds for which phase pure material could be synthesized have been characterized in terms of their magnetic properties. Most compounds were found to have antiferromagnetic spin interactions and indications of magnetic frustration could be observed in some of them. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 6: Manuscript. Paper 9: Manuscript. Paper 10: Manuscript.</p>

No description available.

Nucleic acids are responsible for the storage of genetic information and directly participate in gene replication, transcription and expression, and thereby the control of nucleic acids leads to the regulation of genetic information flow and gene expression. Meanwhile, many non-coding RNAs are in-volved in signal transduction directly. Moreover, nucleic acid-based therapeutic strategies have been lead to drug candidates and are effective tools in drug discovery and disease study at the molecular level as well as the genetic level. Consequently, the 3D crystal structure study and related functional research on natural and unnatural nucleic acids have become very popular area, expanding their potential appli-cation in medicinal and biological chemistry. Since oxygen, sulfur, selenium and tellurium are in the same elemental family (VIA) in the peri-odic table, we anticipate that oxygen atoms in nucleic acids can most likely be replaced with the other chalcogen atoms without causing significant perturbations. Owing to the special K edge and unique properties of selenium, our lab has completed the chemical and enzymatic synthesis of unnatural nucle- ic acids with selenium substitutions at various positions. The selenium functionality in nucleic acid is es-sential for nucleic acids’ structural determination at the atomic level. Additionally this novel elemental feature (atomic size and electronic nature) provides nucleic acids with unique properties. In addition, the selenium derivatization can facilitate crystal growth. Other chalcogen elements are applicable as well to modify nucleic acid, generating some special biofunctions, like the application of phosphorthioate oligonucleotide in gene therapy. This dissertation will outline the chalcogen elements (especially selenium) modifications of nucleic acids, including syntheses strategies, structure studies and potential therapeutic applications. Our research work here tries to show that (1) Selenium functionality is able to facilitate the crystal structure determination, by both helping solve phase problem and accel-erating crystal growth; (2) Selenium functionality can generate special capability to nucleic acids, like improved base pair fidelity, novel atomic interactions and feasibility to be biological chemistry probe; (3) Selenium derivatized oligonucleotides are extraordinary good candidates for gene therapy discovery, considering its stability under nuclease environment. In general, these atom-specific replacements gen-erate a new paradigm of nucleic acids. INDEX WORDS: Nucleic acid, Selenium, X-ray crystal structure, Biofunction, Therapeutics

京都大学 / 新制・課程博士 / 博士(理学) / 甲第23728号 / 理博第4818号 / 新制||理||1689(附属図書館) / 京都大学大学院理学研究科化学専攻 / (主査)教授 北川 宏, 教授 吉村 一良, 教授 竹腰 清乃理 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM