Browsing by Author "Maciel, Gary E., advisor"
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Item Open Access NMR investigation of the behavior of chlorpyrifos and methyl parathion sorbed on clays, and quantitative carbon-13 NMR analysis of sequence distributions in poly(ethylene-co-1-hexene)(Colorado State University. Libraries, 2008) Seger, Mark R., author; Maciel, Gary E., advisorChapters 1 and 2 (and Appendix). Decomposition of chlorpyrifos and methyl parathion on kaolinite and various cation-exchanged montmorillonites (at room temperature, in the dark) was monitored by 31P NMR. Decomposition products included the results of hydrolysis reactions, isomerization reactions and oxidation reactions; mineralization also appears to occur in some cases. Assignments of 31P peaks was based mostly on literature values of chemical shifts of similar structures and 31P NMR experiments on DMSO-d6 extracts of the pesticide/clay samples. When initially sorbed onto the clay, both pesticides appear by solid-state 31P NMR to exhibit significant motion on the molecular level, resulting in almost liquid-like spectra. Over a period of days or weeks, the signal due to unreacted pesticide diminishes and was replaced by new 31P NMR signals arising from various decomposition products. The rate of pesticide decomposition was found to vary greatly, depending on the cation present in montmorillonite. The fastest initial decomposition (disappearance of unreacted pesticide) occurred with the Cu2+-exchanged montmorillonites. Higher hydration levels of Al-exchanged montmorillonite were found to reduce the decomposition rate of methyl parathion; similarly, chlorpyrifos decomposed more quickly when sorbed on Zn-montmorillonite with lower hydration levels. Chapter 3. Different 13C NMR methods of determining triad distributions in two poly(ethylene-co-1-hexene) copolymers are examined using high signal-to-noise 126 MHz 13C spectra of the copolymers dissolved in deuterated 1,2,4-trichlorobenzene at 398K. This examination includes three integration techniques, the experimental impact of decoupler sidebands and significantly non-equal 13C nOe values. A least-squares regression analysis technique for solving for triad mole fractions is tested and appears to be more reliable than two published algebraic expressions. The resultant triad mole fractions are compared to sequence distribution parameters expected by Bernoullian and first-order Markovian statistical models. On the basis of 13C NMR-determined average reactivity ratios, the copolymer designated sample H (5.3 mol % 1-hexene) appears to be a Bernoullian copolymer resulting from a single-site catalytic system. The copolymer designated sample L (3.6 mol % 1-hexene overall) is better described as a mixture of polyethylene and a Bernoullian copolymer with 6.4 mol % 1-hexene content.Item Open Access Solid-state NMR (¹³C, ²⁷Al and ²⁹Si) study of the reaction between AlMe₃ and the silica gel surface(Colorado State University. Libraries, 2007) Li, Jianhua, author; Maciel, Gary E., advisorThe reaction between the silica gel surface and trimethylaluminum (AlMe3) has been studied in this thesis research. We have examined the AlMe3/silica reaction in the following stages: the initial AlMe3-reacted silica surface after it had been treated with AlMe3; the AlMe3-treated surface after it was washed with dry diethyl ether; the ether-washed surface after it was treated, in steps, with limited amounts of H2O; and finally the H2O-reacted surface after an excess-H2O workup. Solid-state NMR (13C, 27 Al and 29Si) have been used to elucidate the structures of moieties generated on the silica gel surface at each of the stages listed above. Solid-state 13C NMR showed that Al(Me)n is the major type of moieties generated on the surface in the initial AlMe3/silica reaction and Si-OMe is the second most important moiety generated. After the sample has been washed with dry diethyl ether, strong ether signals were observed by 13C NMR, which implies that diethyl ether is strongly attached to the surface, even after evacuation. There are no significant changes for the other surface moieties after the diethyl ether treatment. In the series of limited-amount H2O treatments that followed, the AlMen signal intensity decreased as more H2O was added to the surface. In the sample resulting from the final (excess H2O) work-up, AlMen and Si-OMe moieties are completely gone and peaks corresponding to Si-Me and Si(Me)2 are the only signals left in the 13C NMR spectrum. In the 29Si NMR spectra, the signal intensity of the (SiO)3Si(OH) (Q3) peak typical of silica dropped after the AlMe3 treatment. Q3 signal intensity was replaced with a broad peak centered at about-104 ppm, as expected for a conversion in which most of the Si-OH groups on the silica surface have reacted with AlMe3 and turned into Si-O-Al moieties. The formation of Si-Me, Si(Me)2 and Si(Me)3 moieties were also observed in the 29Si spectra. 29Si spectra didn't show significant changes in the sample-treatment stages that follow the initial AlMe3/SiO2 reaction. In the 27 Al spectra of AlMe3-treated silica samples, 4-, 5- and 6-coordinate Al moieties were observed. In the initial reacted sample, 5-coordinate Al moieties are the major initial products from the reaction. After the samples were washed with diethyl ether, the 5-coordinate Al moieties are still the major moieties. With limited amounts of H2O introduced onto the surface, the AlMen moieties reacted with H2O, as shown by the 13C spectra; in 27 Al NMR spectra, signal intensity of 5- coordinate Al moieties decreased, while that of 4- and 6-coordinate Al moieties increased, which implies that the 5-coordinate Al moieties turned into 4- and 6- coordinate Al moieties as a result of reaction with H2O. On the final work-up surface, the 4- and 6-coordinate Al moieties are the major Al structures remaining on the surface. This is the first observation of this kind of change of Al atom coordination on a AlMe3-reacted silica surface. The structures of surface Al moieties are much more complicated than those proposed in previous publications on AlMe3/silica reactions. In the initial reaction between AlMe3 and silica gel, we also made quantitative measurements aimed at tracking the route of methyl groups in the whole system. The methane generated during the reaction was trapped in a N2(I)-cooled trap and the volume of trapped methane was measured as a gas with the water-displacement method. Unreacted Al-Me groups in the supernatant liquid were measured by the liquid-sample 13C NMR spin-counting method. The amount of methyl groups attached on the silica surface were measured by the solid-state 13C NMR spin-counting method. The total amount of methyl groups tracked in the AlMe3/silica/toluene system is about 108% of the amount of methyl groups present in the initial AlMe3 and is about 90% for the AlMe3/silica/cyclohexane system. Relaxation studies were carried out on both the initial AlMe3-reacted and ether-washed AlMe3/silica samples using 13C CP/MAS NMR. The methyl-group proton T1 values were measured by the saturation-recovery technique and the cross polarization relaxation time (TCH) and rotating-frame proton spin-lattice relaxation time (T1p) were measured using variable-contact-time experiments. The AlMen moieties in the initial AlMe3-reacted sample showed very long (5 s ~ 7 s) proton T1 values, which implies that the AlMen moieties may be in a very restrained environment. This result supports the existence of 5-coordination Al structures indicated from 27Al results; in these structures methyl groups are bridged/shared between adjacent AlMen moieties. After the initial AlMe3-reacted silica sample was washed with diethyl ether, methyl-group proton T1 values were reduced by half, which may be due to replacing the methyl bridges with electron-rich centers consisting of the O atoms of the ether molecules introduced by the washing. This interpretation also explains why we have strong ether signals in 13C NMR spectra of the ether-washed sample and in the H2O-treated samples that followed. Overall, the moieties generated in the AlMe3/silica reaction have been characterized by solid-state NMR methods in this thesis work. And, methods were developed which quantitatively characterize the fate of all the Al-Me groups added into the reaction system.Item Open Access Towards the characterization of silicon surfaces: solid state nuclear magnetic resonance studies(Colorado State University. Libraries, 2011) Caylor, Rebecca Anne, author; Maciel, Gary E., advisor; Bernstein, Elliot, committee member; Van Orden, Alan, committee member; Watson, Ted, committee member; Prieto, Amy, committee memberOne of the developing areas in silicon chemistry is in small silicon particles, primarily the nanoparticles regime. When on the 'nano' scale, silicon possesses very different properties and characteristics from bulk silicon. These properties include novel optical and electronic properties that are size dependent. Semiconductor nanoparticles possess a unique bright photoluminescence when in the nanoparticle regime. The photoluminescence in the nanoparticle regime answers the problem of inefficient emissions, which have previously been a problem in bulk silicon, for use in solar cells. Nanoparticle silicon (np-Si) is also biocompatible, allowing for the use in various biological applications including biological tracers, biosensors, delivery of medicine, as well as many others. Although np-Si is widely used, its surface structure still remains largely debated. The surface structure of np-Si is of critical importance as it affects the reactivity of the sample as well as the properties the samples possess. Relative to other silicon samples, np-Si lends itself to be studied by solid state NMR due to its higher surface area, although other types of silicon samples have been studied to some degree in this dissertation project. The surface structure and adjacent interior of np-Si, obtained as commercially available silicon nanopowder, were studied in this project using multinuclear, solid-state NMR spectroscopy. The results are consistent with an overall picture in which the bulk of the np-Si interior consists of highly ordered ('crystalline') silicon atoms, each bound tetrahedrally to four other silicon atoms. From a combination of 1H and 29Si magic-angle-spinning (MAS) NMR results and quantum mechanical 29Si chemical shift calculations, silicon atoms on the surface of 'as-received' np-Si were found to exist in a variety of chemical structures, including primarily structures of the types (Si-O-)n(Si-)3-nSi-H (with n = 1 - 3) and (Si-O-)2Si(H)OH, where Si stands for a surface silicon atom and Si represents another silicon atom that is attached to Si by either a Si-Si bond or a Si-O-Si linkage. The relative populations of each of these structures can be modified by chemical treatment, including with O2 gas at elevated temperature. A deliberately oxidized sample displays an increased population of (Si-O-)3Si-H, as well as (Si-O-)3SiOH sites. Considerable heterogeneity of some types of surface structures was observed. A comparison of 29Si and 1H MAS experiments provide strong evidence for a modest population of silanol (Si-OH) moieties, along with the dominant Si-H sites, on the surface of 'unmodified' np-Si; the former moieties are enhanced by deliberate oxidation of the sample. Dipolar-dephasing experiments provide further evidence of Si-H sites on the surface.