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Mechanisms of Methoxide Ion Substitution and Acid- Catalyzed Z/E Isomerization of N-Methoxyimines

Description: The second order rate constants for nucleophilic substitution by methoxide of (Z)- and (E)-O-methylbenzohydroximoyl fluorides [C6H4C(F)=NOCH3] with various substituents on the phenyl ring [p-OCH3 (1h, 2h), p-CH3 (1g, 2g), p-Cl (1f, 2f), p-H (1e, 2e), (3,5)-bis-CF3 (1i, 2i)] in 90:10 DMSO:MeOH have been measured. A Hammett plot of these rate constants vs σ values gave positive ρ values of 2.95 (Z isomer) and 3.29 (E isomer). Comparison of these rates with methoxide substitution rates for Omethylbenzohydroximoyl bromide [C6H4C(Br)=NOCH3] and Omethylbenzohydroximoyl chloride [C6H4C(Cl)=NOCH3] reveal an element effect for the Z isomers of Br:Cl:F(1e) = 2.21:1.00:79.7 and for the E isomers of Cl:F(2e) = 1.00:18.3. With the p-OCH3-imidoyl halides the following element effects are found: Br:Cl:F(1h) = 2.78:1.00:73.1 for the Z isomer and Br:Cl:F(2h) = 1.97:1.00:12.1 for the E isomer. Measurement of activation parameters revealed ∆S≠ = -17 eu for 1e and ∆S≠ = -9.9 eu for 2e. Ab initio calculations (HF/6-31+G*, MP2/6-31+G*//HF/6-31+G*, B3LYP/6- 31+G*//HF/6-31+G*, HF-SCIPCM/6-31+G*//HF/6-31+G*) were performed to define the reaction surface. These calculations demonstrate a relatively large barrier for nucleophilic attack in relation to halogen loss and support the experimental findings that this reaction proceeds by an addition-elimination mechanism (AN# + DN). The imidoyl fluorides have been used to synthesize highly functionalized O-methyloximes by reaction with enolate anions derived from malononitrile, ethyl cyanoacetate, and diethyl malonate. Acid-catalyzed isomerization of compounds containing the O-methyloxime moiety have been investigated with ab initio calculations (HF/6-31+G*, MP2/6- 31+G*//HF/6-31+G*, B3LYP/6-31+G*//HF/6-31+G*). Barriers for rotation around the C-N bond following protonation have been calculated. The calculated barriers are discussed in relation to an isomerization mechanism of protonation-rotation versus a nucleophilic catalysis.
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Date: December 2001
Creator: Dolliver, Debra D.

Metal-Aluminum Oxide Interactions: Effects of Surface Hydroxylation and High Electric Field

Description: Metal and oxide interactions are of broad scientific and technological interest in areas such as heterogeneous catalysis, microelectronics, composite materials, and corrosion. In the real world, such interactions are often complicated by the presence of interfacial impurities and/or high electric fields that may change the thermodynamic and kinetic behaviors of the metal/oxide interfaces. This research includes: (1) the surface hydroxylation effects on the aluminum oxide interactions with copper adlayers, and (2) effects of high electric fields on the interface of thin aluminum oxide films and Ni3Al substrate. X-ray photoelectron spectroscopy (XPS) studies and first principles calculations have been carried out to compare copper adsorption on heavily hydroxylated a- Al2O3(0001) with dehydroxylated surfaces produced by Argon ion sputtering followed by annealing in oxygen. For a heavily hydroxylated surface with OH coverage of 0.47 monolayer (ML), sputter deposition of copper at 300 K results in a maximum Cu(I) coverage of ~0.35 ML, in agreement with theoretical predictions. Maximum Cu(I) coverage at 300 K decreases with decreasing surface hydroxylation. Exposure of a partially dehydroxylated a-Al2O3(0001) surface to either air or 2 Torr water vapor results in recovery of surface hydroxylation, which in turn increases the maximum Cu(I) coverage. The ability of surface hydroxyl groups to enhance copper binding suggests a reason for contradictory experimental results reported in the literature for copper wetting of aluminum oxide. Scanning tunneling microscopy (STM) was used to study the high electric field effects on thermally grown ultrathin Al2O3 and the interface of Al2O3 and Ni3Al substrate. Under STM induced high electric fields, dielectric breakdown of thin Al2O3 occurs at 12.3 } 1.0 MV/cm. At lower electric fields, small voids that are 2-8 A deep are initiated at the oxide/metal interface and grow wider and deeper into the metal substrate, which eventually leads to either physical collapse or dielectric ...
Date: December 2001
Creator: Niu, Chengyu

An NMR study of 2-ethyl-1-butyllithium and of 2-ethyl-1-butyllithium/lithium 2-ethyl-1-butoxide mixed aggregates

Description: A 1H, 13C, and 6Li NMR study of 2-ethyl-1-butyllithium indicated that 2-ethyl-1-butyllithium exists only as a hexameric aggregate over the entire temperature range of 25 to - 92.1 ° C in cyclopentane. Reacting 2-ethyl-1-butyllithium with 2-ethyl-1-butanol resulted in alkyllithium/lithium alkoxide mixed aggregates, apparently of the form Ra(RO)bLia+b. A multinuclear, variable temperature NMR study of samples with O:Li ratios of 0.2 and 0.4 showed, in addition to the alkyllithium, the formation of four mixed aggregates, one of them probably an octamer. Higher O:Li ratio samples showed the formation of several other mixed aggregates. Mixing 2-ethyl-1-butyllithium with independently prepared lithium 2-ethyl-1-butoxide formed the same mixed aggregates formed by in situ synthesis of lithium alkoxide. Lithium 2-ethyl-1-butoxide also exists as aggregates in cyclopentane.
Date: May 2001
Creator: Ferreira, Aluisio V. C.

Thermochemical Study of Crystalline Solutes Dissolved in Ternary Hydrogen-Bonding Solvent Mixtures

Description: The purpose of this dissertation is to investigate the thermochemical properties of nonelectrolyte solutes dissolved in ternary solvent mixtures, and to develop mathematical expressions for predicting and describing behavior in the solvent mixtures. Forty-five ternary solvent systems were studied containing an ether (Methyl tert-butyl ether, Dibutyl ether, or 1,4-Dioxane), an alcohol (1-Propanol, 2-Propanol, 1-Butanol, 2-Butanol, or 2-Methyl-1-propanol), and an alkane (Cyclohexane, Heptane, or 2,2,4-Trimethylpentane) cosolvents. The Combined NIBS (Nearly Ideal Binary Solvent)/Redlich-Kister equation was used to assess the experimental data. The average percent deviation between predicted and observed values was less than ± 2 per cent error, documenting that this model provides a fairly accurate description of the observed solubility behavior. In addition, Mobile Order theory, the Kretschmer-Wiebe model, and the Mecke-Kempter model were extended to ternary solvent mixtures containing an alcohol (or an alkoxyalcohol) and alkane cosolvents. Expressions derived from Mobile Order theory predicted the experimental mole fraction solubility of anthracene in ternary alcohol + alkane + alkane mixtures to within ± 5.8%, in ternary alcohol + alcohol + alkane mixtures to within ± 4.0%, and in ternary alcohol + alcohol + alcohol mixtures to within ± 3.6%. In comparison, expressions derived from the Kretschmer-Wiebe model and the Mecke-Kempter model predicted the anthracene solubility in ternary alcohol + alkane + alkane mixtures to within ± 8.2% and ± 8.8%, respectively. The Kretschmer-Wiebe model and the Mecke-Kempter model could not be extended easily to systems containing two or more alcohol cosolvents.
Date: May 2001
Creator: Pribyla, Karen J.