A long-standing research interest of the Leighton group has been the utilization of strained silanes in the rapid and efficient synthesis of polyketides. Recently, we have been interested in how the effects of strain might manifest itself in the conductance and functionality of silicon-based molecular junctions. As electronic components continue to miniaturize to the point where transistor size and structure begin to resemble small molecules, understanding the principles that guide charge transport in single molecule junctions will be crucial. Herein, we describe our studies on a series of single molecule junctions formed by strained silicon wires. We demonstrate that high conductance pathways are accessed for the cis diastereomers of conformationally locked 1,2-disilaacenaphthenes via a bipodal binding motif which provides a stable electrical contact between the Si—Si bond and the gold electrodes. We then elucidate the mechanism of voltage-induced breakdown in silicon-based single molecule junctions. We show that the naphthalene bridge provides a parallel conductance pathway to the silicon backbone, altering bond rupture behavior of the Si—Si bond. We further investigate the bond rupture mechanism through DFT and molecular dynamics calculations and conclude that breakdown occurs by the excitation of vibrational modes in the molecular junction by tunneling electrons, leading to homolytic Si—Si bond rupture.

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Thesis (M.S.)--University of Florida, 2004. / Title from title page of source document. Document formatted into pages; contains 50 pages. Includes vita. Includes bibliographical references.

No description available.

published_or_final_version / abstract / Chemistry / Doctoral / Doctor of Philosophy

The first two chapters of this thesis are devoted to exploring the reactivity of electron rich molybdenum and tungsten trimethylphosphine complexes with hydrosilanes. These complexes, Mo(PMe3)6 and W(PMe3)4(n2-CH2PMe2)H, have been shown to be highly reactive species that undergo a number of bond cleavage reactions. In the presence of the hydrosilanes PhxSiH4-x (x = 0 - 4), Mo(PMe3)6 and W(PMe3)4(n2-CH2PMe2)H effect Si-H and Si-C bond cleavage, along with Si-Si bond formation; however, the products derived from these reactions are drastically different for Mo(PMe3)6 and W(PMe3)4(n2-CH2PMe2)H and are highly dependent on the substitution of the silane. Mo(PMe3)6 reacts with SiH4, PhSiH3, and Ph2SiH2 to afford novel silyl, hypervalent silyl, silane, and disilane complexes, as respectively illustrated by Mo(PMe3)4H2(SiH3)2, Mo(PMe3)4H(k2-H2-H2SiPh2H), Mo(PMe3)3H4(s-HSiHPh2), and Mo(PMe3)3H2(k2-H2-H2Si2Ph4). Mo(PMe3)4H(k2-H2-H2SiPh2H) is the first example of a complex with a hypervalent [H2SiPh2H] ligand, and Mo(PMe3)3H2(k2-H2-H2Si2Ph4) represents the first structurally characterized disilane complex. In addition to being structurally unique, these complexes also possess interesting reactivity. For example, Mo(PMe3)4(SiH3)2H2 undergoes isotope exchange with SiD4, and NMR spectroscopic analysis of the SiHxD4-x isotopologues released indicates that the reaction occurs via a sigma bond metathesis pathway. In contrast, W(PMe3)4(n2-CH2PMe2)H affords a range of products that includes metallacycle, disilyl, silane, and bridging silylene complexes. The disilyl compounds, W(PMe3)4H3(SiH2SiHPh2) and W(PMe3)3H4(SiH2Ph)(SiH2SiHPh2), exhibit the ability of W(PMe3)4(n2-CH2PMe2)H to cause both redistribution and Si-Si bond formation. A mechanism involving silylene intermediates is proposed for the generation of these complexes, and this mechanism is supported computationally. Additional support for the presence of intermediates comes from the isolation of a unique complex with a bridging silylene ligand, "WSiW". The bridging silylene bonding motif is unprecedented. The reactivity of the simplest hydrosilane, SiH4, was also examined with IrCl(CO)(PPh3)2 (i.e. Vaska's compound). Previous reports on this reaction have assigned the product as trans-IrH(SiH3)(Cl)(CO)(PPh3)2, in which the hydride and silyl ligands are mutually trans. It is noteworthy, therefore, that we have now obtained a crystal structure of the product of this reaction in which the hydride and silyl ligands are cis, namely cis-IrH(SiH3)(Cl)(CO)(PPh3)2. Calculated energies of the isomeric species also suggest that the product of this reaction was originally misassigned. These results, and the analogous reactions with germane (GeH4), are described in Chapter 4. Chapter 4 also discusses some reactions of transition metal phosphine complexes, including Ru(PMe3)4H2, Mo(PMe3)6, W(PMe3)4(n2-CH2PMe2)H, and Mo(PMe3)4(n2-CH2PMe2)H, with industrially relevant substrates. Ru(PMe3)4H2 effects the water gas shift reaction of CO and H2O to form CO2 and H2. Furthermore, Ru(PMe3)4H2 reacts with CO2, CS2, and H2S to respectively form formate, thiocarbonate, and hydrosulfido complexes. The reactivity of Mo(PMe3)6 and W(PMe3)4(n2-CH2PMe2)H towards molecules relevant to the hydrodeoxygenation industry, including dihydrofuran and benzofuran, was studied. The products of these reactions exhibit hydrogenation of unsaturated bonds and C-O bond cleavage, both of which are essential to the hydrodeoxygenation process. Mo(PMe3)4(n2-CH2PMe2)H reacts with PhI to form an alkylidyne species, [Mo(PMe3)4(CPMe2Ph)I]I, which was structurally characterized by X-ray diffraction. W(PMe3)4(n2-CH2PMe2)H forms a k2-adduct when treated with 2-seleno-2-methylbenzimidazole, namely W(PMe3)4(sebenzimMe)H. Chapter 3 discusses the development of two new ruthenaboratrane complexes, [k4-B(mimBut)3]Ru(CO)(PR3) (R = Ph, Me). The structures of these complexes are described, and their d6 metal configuration is supported by both Fenske-Hall and Natural Bond Orbital calculations. Some reactivity of these complexes was also explored. For example, [k4-B(mimBut)3]Ru(CO)(PMe3) appears to add MeI across the Ru-B bond. Finally, as an extension of the work that we have done on tungsten trimethylphosphine complexes, the structure of W(PMe3)3H6 in solution was investigated, and the results are presented in Chapter 5. T1 measurements of the hydride ligands and deuterium isotope effect shifts both confirm that this complex exists as a classical hydride in solution, which is in accord with the classical hydride formulation in the solid state that was previously determined by X-ray diffraction.

Thesis (Ph. D.)--University of Hong Kong, 2006. / Title proper from title frame. Also available in printed format.

No description available.

The development and implementation of a high resolution, direct absorption, rapidscanning infrared diode laser spectrometer incorporating a supersonic jet expansion source is described. High sensitivity is achieved by directly modulating absorption signals at frequencies in excess of 50 kHz, enabling their separation from lower frequency mechanical and diode 1/f noise. This is accomplished by rapidly scanning the diode laser across a small frequency window (~0.5-1.5cm^-1) synchronously with a pulsed supersonic expansion in a time period of 1 or 2 ms. Absorptions appear as small attenuations in the overall variation of the laser mode power across the scan window. This background profile is removed by recording the laser power without gas pulsing and subtracting. Relative frequency calibration is effected by simultaneously recording the spectrum of a reference gas and the interference fringes of an etalon. Absorption signals are recorded by means of a fast 12-bit analog-to-digital converter operating at 1 MHz. This is housed within a dedicated PC microprocessor which performs spectrometer control, data coaddition, signal processing and spectrum calibration functions. The spectrometer has been used to measure the infrared spectra of two weakly bound complexes, CO-OCS and Ne-SiH₄. The infrared absorption spectrum of CO-OCS was measured in the 5μm region of the OCS ν₃ asymmetric stretch. In addition microwave spectra of CO-OCS and two isotopomers ¹³CO-OCS and CO-OC³⁴S have been recorded using a pulsed nozzle microwave Fourier transform spectrometer. The lines have been fitted to a Watson S reduction Hamiltonian yielding rotational, quartic and (for the ground states) sextic centrifugal distortion constants. A T-shaped structure is determined and this is rationalised by a simple potential model incorporating a distributed multipole analysis of the electrostatic charge distribution, distributed dispersion contribution and a cylindrical hard-core repulsion. The infrared spectrum of Ne-SiH4 was recorded in the vicinity of the SiH₄ ν₃ triply degenerate stretching vibration centred at 2189.19 cm^-1. Ne-SiH₄ is only the second atomspherical top complex to be successfully recorded and analysed. The complex exhibits an intermolecular potential with considerably smaller anisotropy than its argon analogue Ar-SiH₄. Consequently the SiH₄ unit is almost free to rotate within the complex, resulting in novel Coriolis interaction between the angular momentum of the SiH₄ unit and that of the overall complex. Individual bands are fitted to Coriolis interaction Hamiltonians, and the band centres for all the transitions of the complex are fitted to an anisotropic intermolecular potential. Finally, applications of the spectrometer to the study of air sensitive compounds and species generated by electric discharge sources are considered.