Structural and electronic properties of an azamacrocycle, C26H18N6

We compute the structure of an azamacrocycle, C26H18N6. Two approximatively planar elliptical structures with C2 or CI symmetry are found to be nearly degenerate. The roughly circular conformation observed in metal complexes turns out to be ~ 0.6 eV higher in energy. We suggest that this difference is mainly due to electrostatic interactions. We discuss the results on various levels of theory (Hartree-Fock, local density and gradient corrected density functional calculations).Comment: to appear in J Phys Chem

Substrate activation in the urate oxidase reaction

The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file.Title from title screen of research.pdf file (viewed on September 2, 2008).Thesis (M.S.) University of Missouri-Columbia 2008.The enzyme urate oxidase (UO) is involved in nitrogen metabolism of many organisms. Urate oxidation is carried out by UO without the aid of a metal ion or cofactor. A catalytic diad has previously been proposed to be involved in formation of dianionic urate during the UO reaction. Proton inventory, Raman spectroscopy, and kinetic isotope effects (KIE) methods have been utilized to further investigate how UO accomplishes activation of urate for oxidation. A proton inventory was conducted to determine the number of protons in flight during each round of catalysis. A "bowl shaped" proton inventory at pH 8.0 is indicative of more than one proton being in flight during catalysis. This observation supports the proposed Thr-Lys catalytic diad being involved in dianion formation. Raman spectroscopy has been used along with the competitive inhibitor 8-nitroxanthine to show evidence for dianion formation upon the binding of substrate to UO. To further study how UO activates the substrate, new methods for measuring kinetic isotope effects (KIE) are being developed. The use of continuous flow mass spectrometry to measure carbon isotope effects (IE) at positions C4 and C5 of urate will give insight into the transition state structure of the reaction. Continuous flow MS has the advantage of measuring the naturally abundant isotopic ratios of solid samples. Another new technique is the use of isotope depleted samples to measure carbon IE's of multi-carbon molecules. These new techniques have the potential to be employed in the study of many enzymatic reactions.Includes bibliographical reference

Excursion Across East-Central Vermont, Barre, to Strafford via Bradford

Guidebook: Fifty-third annual meeting October 13-15, 1961: Trip A-

Limitation of entanglement due to spatial qubit separation

We consider spatially separated qubits coupled to a thermal bosonic field that causes pure dephasing. Our focus is on the entanglement of two Bell states which for vanishing separation are known as robust and fragile entangled states. The reduced two-qubit dynamics is solved exactly and explicitly. Our results allow us to gain information about the robustness of two-qubit decoherence-free subspaces with respect to physical parameters such as temperature, qubit-bath coupling strength and spatial separation of the qubits. Moreover, we clarify the relation between single-qubit coherence and two-qubit entanglement and identify parameter regimes in which the terms robust and fragile are no longer appropriate.Comment: 7 pages, 3 figures; revised version, accepted for publication in Europhys. Let

Ground-state properties of rutile: electron-correlation effects

Electron-correlation effects on cohesive energy, lattice constant and bulk compressibility of rutile are calculated using an ab-initio scheme. A competition between the two groups of partially covalent Ti-O bonds is the reason that the correlation energy does not change linearly with deviations from the equilibrium geometry, but is dominated by quadratic terms instead. As a consequence, the Hartree-Fock lattice constants are close to the experimental ones, while the compressibility is strongly renormalized by electronic correlations.Comment: 1 figure to appear in Phys. Rev.

Structure prediction based on ab initio simulated annealing for boron nitride

Possible crystalline modifications of chemical compounds at low temperatures correspond to local minima of the energy landscape. Determining these minima via simulated annealing is one method for the prediction of crystal structures, where the number of atoms per unit cell is the only information used. It is demonstrated that this method can be applied to covalent systems, at the example of boron nitride, using ab initio energies in all stages of the optimization, i.e. both during the global search and the subsequent local optimization. Ten low lying structure candidates are presented, including both layered structures and 3d-network structures such as the wurtzite and zinc blende types, as well as a structure corresponding to the beta-BeO type

Comparison of ERBS orbit determination accuracy using batch least-squares and sequential methods

The Flight Dynamics Div. (FDD) at NASA-Goddard commissioned a study to develop the Real Time Orbit Determination/Enhanced (RTOD/E) system as a prototype system for sequential orbit determination of spacecraft on a DOS based personal computer (PC). An overview is presented of RTOD/E capabilities and the results are presented of a study to compare the orbit determination accuracy for a Tracking and Data Relay Satellite System (TDRSS) user spacecraft obtained using RTOS/E on a PC with the accuracy of an established batch least squares system, the Goddard Trajectory Determination System (GTDS), operating on a mainframe computer. RTOD/E was used to perform sequential orbit determination for the Earth Radiation Budget Satellite (ERBS), and the Goddard Trajectory Determination System (GTDS) was used to perform the batch least squares orbit determination. The estimated ERBS ephemerides were obtained for the Aug. 16 to 22, 1989, timeframe, during which intensive TDRSS tracking data for ERBS were available. Independent assessments were made to examine the consistencies of results obtained by the batch and sequential methods. Comparisons were made between the forward filtered RTOD/E orbit solutions and definitive GTDS orbit solutions for ERBS; the solution differences were less than 40 meters after the filter had reached steady state