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Photodissociation spectroscopy and dynamics of free radicals, clusters, and ions

Description: The photodissociation spectroscopy and dynamics of free radicals and ions is studied to characterize the dissociative electronic states in these species. To accomplish this, a special method of radical production, based on the photodetachment of the corresponding negative ion, has been combined with the technique of fast beam photofragment translational spectroscopy. The photofragment yield as a function of photon energy is obtained, mapping out the dissociative and predissociative electronic states. Branching ratios to various product channels, the translational energy distributions of the fragments, and bond dissociation energies are then determined at selected photon energies. The detailed picture of photodissociation dynamics is provided with the aid of ab initio calculations and a statistical model to interpret the observed data. Important reaction intermediates in combustion reactions have been studied: CCO, C{sub 2}H{sub 5}O, and linear C{sub n} (n = 4--6).
Date: December 16, 1999
Creator: Hyeon, Choi
Partner: UNT Libraries Government Documents Department

Photoisomerization and photodissociation dynamics of reactive free radicals

Description: The photofragmentation pathways of chemically reactive free radicals have been examined using the technique of fast beam photofragment translational spectroscopy. Measurements of the photodissociation cross-sections, product branching ratios, product state energy distributions, and angular distributions provide insight into the excited state potential energy surfaces and nonadiabatic processes involved in the dissociation mechanisms. Photodissociation spectroscopy and dynamics of the predissociative {tilde A}{sup 2}A{sub 1} and {tilde B}{sup 2}A{sub 2} states of CH{sub 3}S have been investigated. At all photon energies, CH{sub 3} + S({sup 3}P{sub j}), was the main reaction channel. The translational energy distributions reveal resolved structure corresponding to vibrational excitation of the CH{sub 3} umbrella mode and the S({sup 3}P{sub j}) fine-structure distribution from which the nature of the coupled repulsive surfaces is inferred. Dissociation rates are deduced from the photofragment angular distributions, which depend intimately on the degree of vibrational excitation in the C-S stretch. Nitrogen combustion radicals, NCN, CNN and HNCN have also been studied. For all three radicals, the elimination of molecular nitrogen is the primary reaction channel. Excitation to linear excited triplet and singlet electronic states of the NCN radical generates resolved vibrational structure of the N{sub 2} photofragment. The relatively low fragment rotational excitation suggests dissociation via a symmetric C{sub 2V} transition state. Resolved vibrational structure of the N{sub 2} photofragment is also observed in the photodissociation of the HNCN radical. The fragment vibrational and rotational distributions broaden with increased excitation energy. Simple dissociation models suggest that the HNCN radical isomerizes to a cyclic intermediate (c-HCNN) which then dissociates via a tight cyclic transition state. In contrast to the radicals mentioned above, resolved vibrational structure was not observed for the ICNN radical due to extensive fragment rotational excitation, suggesting that intermediate bent states are strongly coupled along the dissociation pathway. The measurements performed in ...
Date: August 24, 2000
Creator: Bise, Ryan T.
Partner: UNT Libraries Government Documents Department

Steric and electronic effects of 1,3-disubstituted cyclopentadienyl ligands on metallocene derivatives of Cerium, Titanium, Manganese, and Iron

Description: Sterically demanding 1,3-disubstituted cyclopentadienyl ligands were used to modify the physical properties of the corresponding metallocenes. Sterically demanding ligands provided kinetic stabilization for trivalent cerium compounds. Tris(di-t-butylcyclopentadienyl)cerium was prepared and anion competition between halides and cyclopentadienyl groups which had complicated synthesis of the tris(cyclopentadienyl)compound was qualitatively examined. Bis(di-t-butylcyclopentadienyl)cerium methyl was prepared and its rate of decomposition, by ligand redistribution, to tris(di-t-butylcyclopentadienyl)cerium was shown to be slower than the corresponding rate for less sterically demanding ligands. Asymmetrically substituted ligands provided a symmetry label for examination of chemical exchange processes. Tris[trimethylsilyl(t-butyl)cyclopentadienyl]cerium was prepared and the rate of interconversion between the C1 and C3 isomers was examined. The enthalpy difference between the two distereomers is 7.0 kJ/mol. The sterically demanding cyclopentadienyl ligands ansa-di-t-butylcyclopentadiene (Me2Si[(Me3C)2C5H3]2), ansa-bis(trimethylsilyl)cyclopentadiene (Me2Si[(Me3Si)2C5H3]2) and tetra-t-butylfulvalene and metallocene derivatives of the ligands were prepared and their structures were examined by single crystal X-ray crystallography. The effect that substituents on the cyclopentadienyl ring have on the pi-electron system of the ligand was examined through interaction between ligand and metal orbitals. A series of 1,3-disubstituted manganocenes was prepared and their electronic states were determined by solid-state magnetic susceptibility, electron paramagnetic resonance, X-ray crystallography, and variable temperature UV-vis spectroscopy. Spin-equilibria in [(Me3C)2C5H3]2Mn and [(Me3C)(Me3Si)C5H3]2Mn were examined and indicate an enthalpy difference of 15 kJ/mol between the high-spin and low-spin forms. Cyclopentadienyl groups resistant to intramolecular oxidative addition allowed isolation of compounds susceptible to intramolecular decomposition. A kinetically stable, base-free titanocene was prepared using di-t-butylcyclopentadienyl ligands and the reactivity of the compound toward small molecules was investigated. The titanocene reacts reversibly with hydrogen to form the titanocene dihydride and the equilibrium in solution between titanocene dihydride, and titanocene and hydrogen, was examined.
Date: May 19, 2000
Creator: Sofield, C.D.
Partner: UNT Libraries Government Documents Department

Redox shuttle additives for overcharge protection in lithium batteries

Description: Seven new redox shuttle additives with shuttle current onset potentials above 4.2 V vs Li/Li+ are reported, along with diffusion coefficients for the neutral additives. The dependence of the limiting shuttle current on the respective diffusion coefficients of the oxidized and reduced forms of an additive is clarified. Overcharge protection in liquid electrolyte Li/LiMn{sub 2}O{sub 4} cells is demonstrated.
Date: November 11, 1999
Creator: Richardson, Thomas J. & Ross Jr., P.N.
Partner: UNT Libraries Government Documents Department

Electrochemical studies of the Mg2Si thin films prepared with pulsed laser deposition

Description: Electrochemically active thin films of Mg2Si (film thickness of 137 nm) have been prepared with the pulsed laser deposition technique. The film showed stable cycle behavior at 0.1 * 1.0 V vs Li with capacity greater than 800 mAh/g for more than 100 cycles. Though film morphology becomes remarkably rougher with cycling, this film showed continuous high stability in cycling. The capacity retention might be attributed to limited structural volume change in 2-dimensional film, easier lithium diffusion to film surface and enhanced conductivity supported from stainless steel substrate. The goal of this film study is to help clarify capacity failure of powder intermetallics alloy anodes.
Date: September 15, 2001
Creator: Song, Seung-Wan; Striebel, Kathryn & Cairns, Elton
Partner: UNT Libraries Government Documents Department

An experimental and density functional theory study of the interactions of CH4 with H-ZSM-5

Description: The interactions of methane with Bronsted acid sites in H-ZSM-5 were investigated both experimentally and theoretically. Diffuse reflectance infrared spectroscopy was used to acquire spectra for methane adsorbed on H-ZSM-5 at room temperature and at 77 K. Upon adsorption, the v1 and v3 vibrational bands of methane shift by -15 and -23 cm-1, respectively, and the vibrational band for OH groups associated with Bronsted acid sites shifts by -93 cm-1. Quantum chemical calculations conducted at the DFT level of theory with a 6-31g**++ basis set show that the observed shifts for methane are attributable to the effects of the electrostatic field created by the atoms of the zeolite. To represent the influence of the zeolite on the adsorbed methane correctly, it is essential to take into account the effects of the Madelung field, as well as the local effects of the acid center. The calculated shift in the vibrational frequency of the bridging OH group lies within the range observed experimentally. However, the quantitative agreement of the calculated and observed shift is not as good as that seen for the bands of CH4.
Date: August 25, 2001
Creator: Khaliullin, Rustam Z.; Bell, Alexis T. & Kazansky, Vladimir B.
Partner: UNT Libraries Government Documents Department

Impedance behavior of the LiMn2O4/LiPF6-DMC-EC interface during cycling

Description: Room temperature impedance measurements of the LiMn2O4/LiPF6-EC-DMC interface have been used to identify a previously unreported step in the formation of the SEI layer on this cathode. The low frequency impedance and potential of pure dense LiMn2O4 films was found to depend logarithmically on time in the end-of-discharge (EOD) state. The rate of the impedance rise decreased with Mn3+ content. The increased impedance was removed by oxidation of the film to 4.5 V vs. Li/Li+. The observations are consistent with a reversible disproportionation of part of the LiMn2O4 into Li2Mn2O4 and l-Mn2O4. Analyses of cyclic voltammograms and impedance spectra at intervals during constant current cycling of the LiMn2O4 films suggest that Li2Mn2O4 on the surface also plays a major role in the capacity fade.
Date: December 19, 2000
Creator: Striebel, Kathryn A.; Sakai, Eiji & Cairns, Elton J.
Partner: UNT Libraries Government Documents Department

New limit on the electron electric dipole moment

Description: We present the result of our most recent search for T-violation in 205Tl, which is interpreted in terms of an electric dipole moment of the electron de. We find de = (6.9 plus/minus 7.4) times 10{sup -28} e cm. The present apparatus is a major upgrade of the atomic beam magnetic resonance device used to set the previous limit on de.
Date: August 8, 2001
Creator: Regan, B.C.; Commins, Eugene D.; Schmidt, Christian J. & DeMille, David
Partner: UNT Libraries Government Documents Department

Local and average crystal structure and displacements of La{sup 11}B{sub 6} and EuB{sub 6} as a function of temperature

Description: Measurements of both the average crystal structure from Rietveld refinement of neutron powder diffraction (NPD) data and the local structure from La L{sub III}-edge x-ray-absorption fine-structure (XAFS) are presented for a La{sup 11}B{sub 6} sample as a function of temperature ({approx}10-320 K). These data are compared to XAFS results on a EuB{sub 6} sample. The single-site La and B positional distribution widths and the La-B and La-La bond length distribution widths and their temperature dependence are compared. This comparison allows an estimate of the La and B site displacements, and we find that these sublattices are only slightly correlated with each other. Moreover, while the temperature dependence of the displacement parameters of the average sites from diffraction fit an Einstein model well, the temperature dependence of the La-B bond length distribution width requires at least two vibrational frequencies, corresponding to the La and B frequencies of the individual sites. XAFS data on EuB{sub 6} indicate that the situation is the same in the Eu compound. In addition, comparisons between data taken below and above the ferromagnetic transition temperature for EuB{sub 6} place stringent limits on the lattice involvement in the associated metal-insulator transition and the ensuing large magnetoresistance effect. This lack of lattice involvement in the magnetoresistance transition is in sharp contrast to the strong lattice involvement observed in the colossal magnetoresistance lanthanum manganese perovskites.
Date: January 30, 2001
Creator: Booth, C.H.; Sarrao, J.L.; Hundley, M.F.; Cornelius, A.L.; Kwei, G.H.; Bianchi, A. et al.
Partner: UNT Libraries Government Documents Department

Interdisciplinary applications of Pauling's metallic orbital and unsynchronized resonance to problems of modern physical chemistry: Conductivity, magnetism, molecular stability, superconductivity, catalysis, photoconductivity, and chemical reactions

Description: No abstract prepared.
Date: December 8, 2000
Creator: Pavao, Antonio C.; Taft, Carlton A.; Guimaraes, Tereza C.F.; Leao, Marcelo B.C.; Mohallem, Jose R. & Lester Jr., William A.
Partner: UNT Libraries Government Documents Department