published_or_final_version / Chemistry / Master / Master of Philosophy

This thesis describes the development of a new type of sensor for nitric oxide (NO) that can be used in a variety of combustion diagnostics and control applications. The sensor utilizes the absorption of ultraviolet (UV) radiation by the NO molecule to determine the concentration via optical absorption spectroscopy. UV radiation at 226.8 nm is generated by sum frequency mixing the outputs from a 395-nm external cavity diode laser (ECDL) and a 532-nm diode-pumped, intracavity frequency doubled Nd:YAG laser in a beta-barium borate (BBO) crystal. This radiation is used to probe the (v'=0, v"=0) band of the Α*Σ+ - Χ*π electronic transition of NO. The ECDL is tuned so that the UV radiation is in resonance with a specific energy level transition, and it is then scanned across the transition to produce a fully resolved absorption spectrum. Preliminary experiments were performed in a room-temperature gas cell in the laboratory to determine the accuracy of the sensor. Results from these experiments indicated excellent agreement between theoretical and experimental absorption line shapes as well as NO concentrations. Further experiments were performed at two actual combustion facilities to demonstrate the operation of the sensors in realistic combustion environments. Tests on a gas turbine auxiliary power unit (APU) at Honeywell Engines and Systems and on a well-stirred reactor (WSR) at Wright-Patterson Air Force Base produced excellent results despite the harsh temperatures and vibrations present. Overall, the sensitivity was estimated to be 0.8 parts per million (ppm) of NO (at 1000 K) for a 1 meter path length and the measurement uncertainty was estimated to be ±10%.

Despite previous extensive study of the widely-employed ruthenium-catalysed olefin metathesis reaction, the finer mechanistic details have not been elucidated. An area that is noticeably lacking is spectroscopic exploration of the relevant complexes. In this work, organometallic ruthenium complexes of importance to olefin metathesis have been investigated using Ru K-edge X-ray absorption spectroscopy. The lowest energy feature in the Ru K-edge spectrum has been unambiguously assigned as due to Ru 4d←1s transitions. These electric-dipole-forbidden transitions are extremely sensitive to geometry. For centrosymmetric complexes, the pre-edge feature has very low intensity because it is limited by the weak electric quadrupole mechanism. By contrast, non-centrosymmetric complexes exhibit a substantial increase in pre-edge intensity because Ru 5p-4d mixing introduces electric-dipole-allowed character to the Ru 4d←1s transitions. The energy of the edge feature in the Ru K-edge spectrum corresponds to ionisation of 1s electrons and is a good indicator of the charge on the metal centre. Unexpectedly, we found that the first-generation (L = PCy₃) Grubbs precatalyst (1) has a higher 1s ionisation energy than the second-generation (L = H₂IMes) complex (2). This effect provides a compelling rationale for the unexplained differences in phosphine dissociation kinetics for complexes 1 and 2: the phosphine dissociation rate of 2 is slower than 1 because the metal centre is more electron-deficient in 2. Density functional theory calculations confirm the charge differences and offer some insight into the nature of bonding in these complexes, particularly with regard to the N-heterocyclic carbene and trialkylphosphine ligands. On the basis of these results, we propose that, for this system, the NHC ligand is a weaker σ-charge donor than the phosphine ligand, and that the NHC accepts significant π-electron density from the metal; both interactions function to reduce the electron density on the ruthenium centre. An ultimate goal is to investigate reactive species in the olefin metathesis mechanism; accordingly, we have made considerable progress toward collecting XAS data for a metallacyclobutane species, and we are pursuing methods to trap the four-coordinate intermediate in the metathesis cycle.

Selenium has been shown to have multiple biochemical effects ranging from nutrient deficiency at low levels to toxicity at high levels. This duality of concern has led to a demand for increased numbers of highly accurate and precise determinations of selenium in biological materials. A convenient procedure was developed for determining selenoamino acids by HPLC-THG-AAS, based on the derivatization of these analytes with Sanger's reagent. Selenomethionine, selenocystine and selenocysteine (after blocking the free selenol group with phenylmercurio cation) were converted to their N-2,4-dinitrophenyl derivatives, and separated on a Nucleosil 5-NO$ sb2$ column with methanolic mobile phase containing acetic acid and triethylamine. Furthermore, an improved HPLC-AAS interface design was modified and optimized for the detection of selenium in HPLC column eluate. The new design was (i) compatible with aqueous mobile phases containing volatile buffers and (ii) provided equivalent molar response to analytes containing Se($-$II), Se(+IV) and Se(+VI). A method for simultaneously determination of selenate, selenite, selenocystine, selenomethionine and selenoethionine was developed by using the HPLC-AAS system with aqueous acetic acid containing ammonium acetate as eluate solution on the cyanopropyl column. The equivalent low ng limits of detection (1-2 ng as Se) for different oxidation states of selenium analytes were obtained using several different mobile phases and/or columns. A phenol extraction procedure for selenate, selenite, selenocystine, selenomethionine and selenoethionine was evaluated for the determination of these selenium analytes in natural waters and wheat samples. The current HPLC-AAS system provides an inexpensive alternative to conventional techniques for the determination of selenium analytes in environmental samples.

X-ray absorption spectroscopy (XAS) is a powerful probe of electronic and spatial structure that has been at the heart of many advances in physics, biology, chemistry, engineering and the earth sciences. Unfortunately, the existing experimental techniques are subject to fundamental limitations that complicate the interpretation of x-ray absorption spectra in many important cases. These limitations have motivated an effort to develop an alternative measure of the absorption cross-section that is not subject to the same set of limitations. In this thesis, a technique known as inverse partial fluorescence yield (IPFY) is described which addresses this problem. IPFY differs from existing approaches in a significant way — by using an energy-discriminating photon detector, one gains access to fluorescence information from both resonant and non-resonant x-ray emission processes. We will show that the non-resonant emission is fundamentally related to the total absorption cross-section of a material through an inverse relation. This will be proven by extension of the general theory of fluorescence yield for the case of a thick, homogeneous specimen. We will also demonstrate the utility of IPFY with measurements of NiO, NdGaO₃, LNSCO, and stainless steel 304 at soft and intermediate x-ray energies. These experiments will highlight some essential features of IPFY spectroscopy and demonstrate how it can be an invaluable tool when the other experimental techniques fail to provide reliable spectra. We will also demonstrate how one can exploit the geometry dependence of IPFY to quantitatively determine the composition of a sample and the total x-ray absorption coefficient. Additionally, we will consider the special cases of multilayers and powder specimens, where the theory of fluorescence yield requires approximations and is not as well-behaved as in thick, homogenous specimens. Ultimately, these experiments and theoretical developments will be used to support the claim that IPFY is a bulk sensitive measure of the total x-ray absorption coefficient. Moreover, we will show that IPFY is not affected by saturation effects, is insensitive to surface contamination layers and provides reliable spectra even for strongly insulating materials. These properties make IPFY a suitable spectroscopic technique for studying XAS in a wide range of materials.

Despite previous extensive study of the widely-employed ruthenium-catalysed olefin metathesis reaction, the finer mechanistic details have not been elucidated. An area that is noticeably lacking is spectroscopic exploration of the relevant complexes. In this work, organometallic ruthenium complexes of importance to olefin metathesis have been investigated using Ru K-edge X-ray absorption spectroscopy. The lowest energy feature in the Ru K-edge spectrum has been unambiguously assigned as due to Ru 4d←1s transitions. These electric-dipole-forbidden transitions are extremely sensitive to geometry. For centrosymmetric complexes, the pre-edge feature has very low intensity because it is limited by the weak electric quadrupole mechanism. By contrast, non-centrosymmetric complexes exhibit a substantial increase in pre-edge intensity because Ru 5p-4d mixing introduces electric-dipole-allowed character to the Ru 4d←1s transitions. The energy of the edge feature in the Ru K-edge spectrum corresponds to ionisation of 1s electrons and is a good indicator of the charge on the metal centre. Unexpectedly, we found that the first-generation (L = PCy₃) Grubbs precatalyst (1) has a higher 1s ionisation energy than the second-generation (L = H₂IMes) complex (2). This effect provides a compelling rationale for the unexplained differences in phosphine dissociation kinetics for complexes 1 and 2: the phosphine dissociation rate of 2 is slower than 1 because the metal centre is more electron-deficient in 2. Density functional theory calculations confirm the charge differences and offer some insight into the nature of bonding in these complexes, particularly with regard to the N-heterocyclic carbene and trialkylphosphine ligands. On the basis of these results, we propose that, for this system, the NHC ligand is a weaker σ-charge donor than the phosphine ligand, and that the NHC accepts significant π-electron density from the metal; both interactions function to reduce the electron density on the ruthenium centre. An ultimate goal is to investigate reactive species in the olefin metathesis mechanism; accordingly, we have made considerable progress toward collecting XAS data for a metallacyclobutane species, and we are pursuing methods to trap the four-coordinate intermediate in the metathesis cycle.

Thesis -- Departmental honors in Chemistry. / Bibliography: ℓ. 26-28.

Thesis (M. Phil.)--University of Hong Kong, 1998. / Includes bibliographical references.

Made available in DSpace on 2014-10-09T12:28:22Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:56:28Z (GMT). No. of bitstreams: 0 / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Made available in DSpace on 2014-10-09T12:27:40Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:05:26Z (GMT). No. of bitstreams: 0 / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP