Synthesis and structural characterizations of Cu(I) and Ag(I) clusters containing i-MNT (1,1-dicyanoethylene-2,2-dithiolate) ligands : from a hexagon and octahedra to tetrahedra and cubes /
Case-based conceptual database design is the storage
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| Format: | Thesis Book |
| Language: | English |
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[Place of publication not identified] :
[publisher not identified] ;
1994.
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| Online Access: | http://proxy.library.tamu.edu/login?url=http://proquest.umi.com/pqdweb?did=741965531&sid=1&Fmt=2&clientId=2945&RQT=309&VName=PQD |
| Summary: | Case-based conceptual database design is the storage and reuse of existing conceptual database schemata. Following the formalization of a methodology using the Enhanced Entity-Relationship (EER) Model of Elmasri and Navathe (1989] for conceptual design, principles and techniques of case-based reasoning were applied for the automation of the methodology. To verify feasibility, a prototype, CAse-Based SYstem for Database Design (CABSYDD), was developed. The knowledge together with their conceptual design. Part of the Standard Industrial Classification, as defined by United States President's office of Management and Budget, was stored as the directory of the case base. This classification forms the structure of the case-base directory and is used by CABSYDD for traversal in locating an appropriate case. To design a new database, CABSYDD locates an existing is found, CABSYDD involves a sub-system, SYstem f or Database Design (SYDD) , to perform conceptual view definition by applying a formal design methodology with the EER Model. In either case, CABSYDD learns from experience by storing and classifying the new design. Empirical evaluation of CABSYDD resulted in three major findings. First, CABSYDD's experts to be accurate. output was validated by Second, out of the three effectiveness measures (user satisfaction, the task itself, and view definition method) , it was found that the case- based approach is more effective than the design from first principles approach in terms of view def inition method. Third, given the choice between the two, users preferred the case-based approach, Metal clusters with eight atoms in a cubane core were virtually unknown until the synthesis and structural characterization of the CU8(i-MNT)6' cluster, 1, by Fackler and Knox et aL over 25 years ago. The structure consists of a slightly distorted cube of copper atoms embedded within a cluster of 12 sulfur atoms which lie on the vertices of a distorted icosahedron. Each sulfur atom is coordinated to two copper atoms on an edge of the cube, and each copper atom is trigonally coordinated by three sulfur atoms of different ligands. The structural features of this cluster appear to be biologically relevant to the yeast copper metallothioneins and their transcription factors, protein CUP2 (also known as ACEI). Upon oxidation of the homocubane, Cu8(i-MNT)6', with powdered sulfur, a novel sulfur-rich product, CU6(S,i-MNT)6', 2, has been isolated and structurally characterized. Six copper atoms are located at the vertices of a nearly regular hexagon with a mean distance between the two copper planes of 0. 345A. Removal of six of the disulfide sulfur atoms from this cluster by PPh3 produces a new tetrahedral cluster structurally characterized as Cu4(i-MNT) 4 4-, 4. Interestingly, the CU4 cluster will transform into the CU8 cubane and a Cu' species, 3, in solution. The synthetic details and the spectroscopic results (UV-Vis and 13 C-NMR) of the cluster transformation will be described in Chapter 11. In Chapter III, the syntheses and structural characterization of three new Agi clusters with the I-MNT ligand are reported. The Ag4(i-MNT)4 4- cluster, 9, is isostructural with CU4(i- MNT)4-, 4. The reaction of two or three equivalents of Ag(PPh3)2 'with A96(i-MNT)6' in CH3CN solution produces two novel clusters, Ag9(i-MNT)6(PPh3)4', 11, and Ag9('- MNT)6(pph3)6'-, 13. The common structural feature of these clusters consists of an octahedral AG6S12 core with two or liquid N2 temperature. The origin of the emission bands is likely to be either LMCT or metal influenced ligand-centered transitions. 252 CF-PD mass spectrometry has been found useful to characterize several CuI and Ag' dithiolate clusters where other mass spectroscopic tools such as FAB and field desorption have failed. This technique provides a rapid screening of the products of a cluster synthesis in advance of more detailed structural studies. Since CuI and Ag' ions are isoelectronic within the valence electron shell, their metal-thiolate clusters should show similar structural principles. However, the chemistry and structural features reported in this dissertation demonstrate that this is not always true. |
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| Item Description: | Vita. "Major Subject: Chemistry". In title, symbols and numerals are used. |
| Physical Description: | xiii, 161 leaves : illustrations ; 28 cm. Issued also on microfiche from University Microfilms Inc. |
| Bibliography: | Includes bibliographical references. |