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A Spectral Approach to the Relativistic Inverse Stellar Structure Problem
No full textWe present a new method for solving the inverse stellar structure problem, which determines an expression for the high density range of the neutron star equation of state (EOS) based on observations of total masses, M, and radii, R of these stars. This approach determines spectral representations of the EOS that are very accurate and require only a few spectral parameters to converge. This method can determine the underlying high density EOS from just a few mass-radii observations {Mi, Ri}. While accurate mass-radii data are not available yet, we tested the accuracy of this method to determine the EOS from a set of {Mi, Ri} values computed from two tabulated theoretical EOS. When applied to the PAL6 [ 6] and MS 1 [7] tabulated EOS, this method converged to the original EOS to within a few percent using {Mi, Ri} data from only two stellar models
Electronic Structures and Reactivity Patterns of Dipalladium(11,11) and Diplatinum(11,11) Complexes
Get PDFThis work explores the electronic structures and reactivity patterns of diplatinum(II, II) and dipalladium(II, II) complexes. Complexes containing metal-metal bonds play important roles in both inorganic and organometallic chemistry. Among the many examples of these complexes, dimers of square planar RhI, IrI, and PtII centers comprise a special class that feature attractive d8–d8 interactions. The unique electronic structure characteristic of these complexes gives rise to chemical, photochemical, and photophysical properties that have engaged researchers for the past 35 years.
One of the best-known examples of these compounds is tetrakis(µ-pyrophosphito)platinum(II), Pt(pop). Herein, the photophysical properties of Pt(pop) are compared with its fluoroborated analogue, Pt(pop-BF2). This complex possesses eight BF2 groups that replace the hydrogen atoms located between each “pop” ligand. When compared with Pt(pop), Pt(pop-BF2) has a much greater singlet lifetime (1.56 ns) and singlet quantum yield (0.27). The enhancement is the result of a drastically slower 1A2u → 3A2u intersystem crossing rate. In particular, the thermal barrier to intersystem crossing is significantly higher in Pt(pop-BF2) (2230 cm-1 vs. 1190 cm-1). We believe this is primarily the result of the increased rigidity of the complex afforded by the BF2 groups. The rigidity increases the energy of symmetry-lowering vibrational modes, which are necessary to promote spin-orbit mixing of the 1dσ*pσ and 3dσ*pσ states.
Despite the many examples of M-M bonded d8–d8 complexes of RhI, IrI, and PtII in the literature, until recently there were no PdII complexes fitting this description. Our investigations of clamshell-shaped PdII dimers [(2-phenylpyridine)Pd(µ-X)]2 and [(2-p-tolylpyridine)Pd(µ-X)]2 (X = OAc or TFA) revealed short Pd–Pd distances (~ 2.85 Å). The molecules adopt this unusual geometry in part because of a d8–d8 bonding interaction between the two Pd centers. Density functional theory (DFT) and ab initio (AI) analyses confirm the presence of a Pd–Pd bonding interaction in [(2-phenylpyridine)Pd(µ-X)]2 and show that the HOMO is a dz2 σ*Pd–Pd antibonding orbital, while the LUMO and proximal unoccupied orbitals are mainly located on the 2-phenylpyridine rings. Computational analyses of other PdII–PdII dimers that have short Pd–Pd distances yield an orbital ordering similar to that of [(2-phenylpyridine)Pd(µ-X)]2, but quite different from that found for d8–d8 dimers of Rh, Ir, and Pt. This difference in orbital ordering arises because of the unusually large energy gap between the 4d and 5p orbitals in Pd, and may explain why Pd d8–d8 dimers do not exhibit the distinctive photophysical properties of related Rh, Ir, and Pt species.
Our work on PdII–PdII electronic structures led us to employ these complexes as electrocatalysts in the regioselective chlorination of C–H bonds. Previous work on d8–d8 complexes has established that when treated with halogens (Cl2, Br2, or I2), the complex undergoes two-center oxidative addition to form an axially coordinated X–d7–d7–X species. Similar products are observed following electrochemical oxidation in the presence of a halide (Cl–, Br–, or I–). Recently, related PdII–PdII complexes were found to selectively chlorinate benzo[h]quinoline through reductive elimination from a Cl–PdIII–PdIII–Cl species. This led us to probe the viability of the analogous electrochemical route. Cyclic voltammetry, spectroelectrochemistry, and bulk electrolysis measurements confirm that electrochemical oxidation of PdII–PdII yields the identical Cl–PdIII–PdIII–Cl intermediate, which is capable of reductive chlorination of C–H bonds. Additional evidence for formation of axially coordinated bromide and acetate species is also presented. Over 10 turnovers of 10-chlorobenzo[h]quinoline were achieved at 80% isolated yield. Further research into the area may lead to a potentially versatile, useful, and green route for C–H bond functionalization reactions.</p
Mechanistic Insights into Alkane C-H Activation and Functionalization by Metal Oxide Surfaces and Organometallic Complexes
Get PDFAlkanes are the major components of natural gas and petroleum; however, there are only few practical processes that can functionalize them into more valuable products such as alkene or alcohols. The reason for this difficulty is because alkanes possess strong and inert C-H bonds. The development of such a process that can convert alkanes to other more valuable functionalized hydrocarbons in a catalytic fashion would produce enormous economic benefits. The key to achieve this goal is to develop a proper catalyst. The catalysts can be organometallic complexes or metal oxide surfaces that catalyze alkane C-H activation and functionalization in homogeneous or heterogeneous conditions.
In this thesis, we apply quantum mechanics to study the known alkane functionalization reactions to provide more insight into those catalytic processes, and we further utilize our computational results to design new reaction pathways for alkane functionalization. Each chapter presented herein constitutes an independent publication focusing on different aspects of the problem.
Chapter 1: Single-Site Vanadyl Activation, Functionalization, and Reoxidation Reaction Mechanism for Propane Oxidative Dehydrogenation on the Cubic V4O10 Cluster: Vanadium oxide is a powerful heterogeneous catalyst that can convert oxidative dehydrogenation (ODH) of propane. Despite numerous studies, either computational or experimental, on this topic, no complete catalytic cycle is provided. In this paper, we examined the detailed mechanism for propane reacting with a V4O10 cluster to model the catalytic oxidative dehydrogenation (ODH) of propane on the V2O5(001) surface. We reported the mechanism of the complete catalytic cycle, including the regeneration of the reduced catalyst using gaseous O2, in which only a single vanadyl site is involved. This mechanism is applicable to propane ODH on the supported vanadium oxide catalysts where only monovanadate (O=V-(O)4-) species is present.
Chapter 2: The Magnetic and Electronic Structure of Vanadyl Pyrophosphate from Density Functional Theory: We have studied the magnetic structure of the high-symmetry vanadyl pyrophosphate, focusing on the spin exchange couplings, applying density functional theory with exact exchange and the full three-dimensional periodicity to this system for the first time. Based on the local density of states and the response of spin couplings to varying the cell parameter a, we found that two major types of spin exchange couplings originate from different mechanisms: one from a super-exchange interaction and the other from a direct exchange interaction. Based on the variations in V–O bond length as a function of strain along a, we found that the V–O bonds of V–(OPO)2–V are covalent and rigid, whereas the bonds of V–(O)2–V are fragile and dative.
Chapter 3: The Para-Substituent Effect and pH-Dependence of the Organometallic Baeyer-Villiger Oxidation of Rhenium-Carbon Bonds: Organometallic Baeyer-Villiger represents another means of oxidizing M-R to M-OR. In this work, we conducted a series of calculations with the goal of providing more insights into the reaction. We find that during this organometallic BV oxidation, the migrating phenyl plays the role of a nucleophile and the leaving group OH is nucleophile. Moreover, we also find that for R = Ph the reaction rate is much faster than that for R = Me, which is later confirmed by experiments.
Chapter 4: Carbon-Oxygen Bond-Forming Mechanisms in Rhenium Oxo-Alkyl Complexes: Intramolecular 1,2-migration of hydrocarbyl across metal-oxo bonds is one of the few means of oxy-functionalizing M-R to M-OR bonds. This strategy works for R = Ph, but fails for R = Me and Et. In this work, we study these systems with the goal of understanding the reason. We find that when R = Me and Et the α-hydrogen is very acidic and easy to abstract even with weak base, such as the counter ion of the complex, leading to unwanted by-products. We find that these side reactions can be avoided by two means: (1) use counter ions with weaker basicity to increase proton abstraction barriers, and (2) use R = iPr, which has a higher migratory aptitude, to accelerate the 1,2-migration rate.
Chapter 5: A Homolytic Oxy-Functionalization Mechanism: Intermolecular Hydrocarbyl Migration from M-R to Vanadyl Oxo: Oxy-functionalization Mδ+-Rδ- to M-OR bonds is one of the key challenges in the development of hydrocarbon hydroxylation catalysts. This can be achieved by limited means: (1) organometallic Baeyer-Villiger oxidation, and (2) intramolecular 1,2-migration of hydrocarbyl across metal-oxo bonds. In this work, we have examined C-O bond formation in the reaction of OVCl3 with Ph2Hg to generate phenol using quantum mechanics. Surprisingly, we find this reaction is through an unprecedented bimolecular, one-electron oxidation of the V-Ph bond by a second V=O moiety, not through the experimentally proposed intramolecular phenyl 1,2-migration across V=O bonds. Our calculations on the oxidation of Rh-CH3 and Ir-CH3 complexes by OVCl3 further suggest that the possibility of integrating this new oxidation mechanism into alkane oxidation catalytic cycles. We also give guidelines to choose the systems in which this oxidation mechanism may play an important role.</p
Neural Pattern Similarity and Visual Perception
Get PDFThis thesis addresses the question of whether people actually see the same visual stimuli somehow differently, and under what conditions, if so. It is an experimental contribution to the basic understanding of visual and especially face perception, and its neural correlates, with an emphasis on comparing patterns of neural activity driven by visual stimuli across trials and across individuals. We make extensive use of functional magnetic resonance imaging (fMRI); all inferences about neural activity are made via this intermediary. The thesis is organized into two parts:
In Part I, we investigate the nature of face familiarity and distinctiveness at perceptual and neural levels. We first address the question of how the faces of those people personally familiar to a viewer appear different than they would to an unfamiliar viewer. The main result is that they appear more distinctive, i.e., dissimilar to and distinguishable from other faces, and more so the higher the level of familiarity. Having established this connection between face familiarity and distinctiveness, we ask next what is different about the perception of such faces, as compared with indistinct and unfamiliar faces, at the level of brain activation. We find that familiar and distinctive faces are represented more consistently: compared with indistinct faces, which evoke slightly different patterns of activity with each new presentation, these faces evoke slightly similar patterns. Combined with the observation that consistency can enhance memory encoding (a result reported by Xue et al. [102]), this suggests a cyclic process for the learning of unfamiliar faces in which consistent representation and the presence of newly formed memories mutually feedback on each other.
Whereas in Part I we focus on individual differences in neural activity, principally by experimentally manipulating stimulus familiarity, in Part II, we shift our focus to similarities across individuals and extend our investigation beyond faces to the perception of visual objects in general and moving images. We begin with an experiment involving the perception of static images selected from 44 object categories, where we find that the distances between these categories, induced from activity in cortical visual object areas, correlate highly between subjects, and also to distances inferred from a behavioral clustering task, and that this correlation remains significant even among subsets of closely related categories. We also show that one subject's brain activity can be accurately modeled using another's, and that this allows us to predict which image a subject is viewing based on his/her brain activity. Then, in a different experiment investigating the perception of dynamic/video stimuli, we find evidence that when watching videos with sound, visual attention is likely blurred at times and transferred to audition; subjects relatively temporally decorrelate in visual areas compared to the muted case, in which the patterns of neural activity correlate across subjects at an average of 78% the level found with oneself later in time.
The findings reported in this thesis thus offer quantitative lower bounds on how similarly different individuals neurally experience visual stimuli, and an explanation for how they perceptually and neurally diverge when familiarity with a (face) stimulus varies, suggesting a possible mechanism for the encoding of new visual objects into memory. We conclude with a discussion of some of the questions raised by this work and directions for future research.</p
Neutron Stars and NuSTAR: A Systematic Survey of Neutron Star Masses in High Mass X-ray Binaries & Characterization of CdZnTe Detectors for NuSTAR
Get PDFMy thesis centers around the study of neutron stars, especially those in massive binary systems. To this end, it has two distinct components: the observational study of neutron stars in massive binaries with a goal of measuring neutron star masses and participation in NuSTAR, the first imaging hard X-ray mission, one that is extremely well suited to the study of massive binaries and compact objects in our Galaxy.
The Nuclear Spectroscopic Telescope Array (NuSTAR) is a NASA Small Explorer mission that will carry the first focusing high energy X-ray telescope to orbit. NuSTAR has an order-of-magnitude better angular resolution and has two orders of magnitude higher sensitivity than any currently orbiting hard X-ray telescope. I worked to develop, calibrate, and test CdZnTe detectors for NuSTAR. I describe the CdZnTe detectors in comprehensive detail here — from readout procedures to data analysis. Detailed calibration of detectors is necessary for analyzing astrophysical source data obtained by the NuSTAR. I discuss the design and implementation of an automated setup for calibrating flight detectors, followed by calibration procedures and results.
Neutron stars are an excellent probe of fundamental physics. The maximum mass of a neutron star can put stringent constraints on the equation of state of matter at extreme pressures and densities. From an astrophysical perspective, there are several open questions in our understanding of neutron stars. What are the birth masses of neutron stars? How do they change in binary evolution? Are there multiple mechanisms for the formation of neutron stars? Measuring masses of neutron stars helps answer these questions. Neutron stars in high-mass X-ray binaries have masses close to their birth mass, providing an opportunity to disentangle the role of "nature" and "nurture" in the observed mass distributions. In 2006, masses had been measured for only six such objects, but this small sample showed the greatest diversity in masses among all classes of neutron star binaries. Intrigued by this diversity — which points to diverse birth masses — we undertook a systematic survey to measure the masses of neutron stars in nine high-mass X-ray binaries. In this thesis, I present results from this ongoing project.
While neutron stars formed the primary focus of my work, I also explored other topics in compact objects. Appendix A describes the discovery and complete characterization of a 1RXS J173006.4+033813, a polar cataclysmic variable. Appendix B describes the discovery of a diamond planet orbiting a millisecond pulsar, and our search for its optical counterpart.</p
Network Coding and Distributed Compression over Large Networks: Some Basic Principles
Get PDFThe fields of Network Coding and Distributed Compression have focused primarily on finding the capacity for families of problems defined by either a broad class of networks topologies (e.g., directed, acyclic networks) under a narrow class of demands (e.g., multicast), or a specific network topology (e.g. three-node networks) under different types of demands (e.g. Slepian-Wolf, Ahlswede-Körner). Given the difficulty of the general problem, it is not surprising that the collection of networks that have been fully solved to date is still very small. This work investigates several new approaches to bounding the achievable rate region for general network source coding problems - reducing a network to an equivalent network or collection of networks, investigating the effect of feedback on achievable rates, and characterizing the role of side information.
We describe two approaches aimed at simplifying the capacity calculations in a large network. First, we prove the optimality of separation between network coding and channel coding for networks of point-to-point channels with a Byzantine adversary. Next, we give a strategy for calculating the capacity of an error-free network by decomposing that network into smaller networks. We show that this strategy is optimal for a large class of networks and give a bound for other cases.
To date, the role of feedback in network source coding has received very little attention. We present several examples of networks that demonstrate that feedback can increases the set of achievable rates in both lossy and lossless network source coding settings. We derive general upper and lower bounds on the rate regions for networks with limited feedback that demonstrate a fundamental tradeoff between the forward rate and the feedback rate. For zero error source coding with limited feedback and decoder side information, we derive the exact tradeoff between the forward rate and the feedback rate for several classes of sources. A surprising result is that even zero rate feedback can reduce the optimal forward rate by an arbitrary factor.
Side information can be used to reduce the rates required for reliable information. We precisely characterize the exact achievable region for multicast networks with side information at the sinks and find upper and lower bounds on the achievable rate region for other demand types.</p
Structure and Reactivity of Copper Dipyridyl Carbinol- and Rhenium Diphosphine Complexes
Get PDFTwo series of terphenyl-based ligands were designed and synthesized, and the reactivity of their transition metal complexes was examined in the context of small molecule activation.
Ligands bearing pyridyl donors and pendant hydroxyl groups were synthesized in order to accommodate multiple copper centers with varying steric environments. Copper(I) complexes of these ligands were prepared and characterized, in which copper is coordinated only via the pyridyl nitrogen donors. Treatment of the Cu(I) species with O2 or deprotonation of the ligand followed by metallation with Cu(II) generates multinuclear copper cores supported by bridging alkoxides. Mono-, di-, tetra-, and hexanuclear copper complexes have been synthesized by changing the protonation state and backbone geometry of the ligand. The resulting copper complexes have been characterized by a variety of techniques including single-crystal X-ray diffraction and 1H NMR spectroscopy.
Rhenium(I) carbonyl coordination chemistry was explored using a series of terphenyl diphosphine ligands with varying steric properties and substituents. Structural differences between complexes of the meta and para ligand variants have been characterized by single crystal X-ray diffraction and 31P-NMR spectroscopy. Cyclic voltammetry studies reveal differences in redox properties, though neither complex appears to exhibit catalytic activity under CO2. Subsequently, we investigate the chemistry of rhenium complexes of 2-methoxy-5-tert-butyl- or 2-methoxy-5- dimethylamino modified ligands.</p
The Implementation of Optofluidic Microscopy on a Chip Scale and Its Potential Applications in Biology
Get PDFThis thesis presents an effort to miniaturize conventional optical microscopy to a chip level using microfluidic technology. Modern compound microscopes use a set of bulk glass lenses to form magnified images from biological objects. This limits the possibility of shrinking the size of a microscope system. The invention of micro/nanofabrication technology gives hope to engineers who want to rethink the way we build optical microscopes. This advancement can fundamentally reform the way clinicians and biologists conduct microscopy. Optofluidic microscopy (OFM) is a miniaturized optical imaging method which utilizes a microfluidic flow to deliver biological samples across a 1-D or 2-D array of sampling points defined in a microfluidic channel for optical scanning. The optical information of these sampling points is collected by a CMOS imaging sensor on the bottom of the microfluidic channel. Although the size of the OFM device is as small as a US dime, it can render high resolution images of less than 1 μm with quality comparable to that of a bulky, standard optical microscope. OFM has a good potential in various biological applications. For example, the integration of an OFM system with high-speed hydrodynamic focusing technology will allow very large scale imaging-based analysis of cells or microorganisms; the compactness and low cost nature of OFM systems can enable portable or even disposable biomedical diagnostic tools for future telemedicine and personalized health care
High-Performance Silicon Nanowire Electronics
Get PDFThis thesis explores 10-nm wide Si nanowire (SiNW) field-effect transistors (FETs) for logic applications via the fabrication and testing of SiNW-based ring oscillators. Both SiNW surface treatments and dielectric annealing are reported for producing SiNW FETs that exhibit high performance in terms of large on/off-state current ratio (~108), low drain-induced barrier lowering (~30 mV), high carrier mobilities (~269 cm2/V•s), and low subthreshold swing (~80 mV/dec). The performance of inverter and ring-oscillator circuits fabricated from these nanowire FETs is explored as well. The inverter demonstrates the highest voltage gain (~148) reported for a SiNW-based NOT gate, and the ring oscillator exhibits near rail-to-rail oscillation centered at 13.4 MHz. The static and dynamic characteristics of these NW devices indicate that these SiNW-based FET circuits are excellent candidates for various high-performance nanoelectronic applications.
A set of novel charge-trap non-volatile memory devices based on high-performance SiNW FETs are well investigated. These memory devices integrate Fe2O3 quantum dots (FeO QDs) as charge storage elements. A template-assisted assembly technique is used to align FeO QDs into a close-packed, ordered matrix within the trenches that separate highly aligned SiNWs, and thus store injected charges. A Fowler-Nordheim tunneling mechanism describes both the program and erase operations. The memory prototype demonstrates promising characteristics in terms of large threshold voltage shift (~1.3 V) and long data retention time (~3 × 106 s), and also allows for key components to be systematically varied. For example, varying the size of the QDs indicates that larger diameter QDs exhibit a larger memory window, suggesting the QD charging energy plays an important role in the carrier transport. The device temperature characteristics reveal an optimal window for device performance between 275K and 350K.
The flexibility of integrating the charge-trap memory devices with the SiNW logic devices offers a low-cost embedded non-volatile memory solution. A building block for a SiNW-based field-programmable gate array (FPGA) is proposed in the future work.</p
Simulations of Compressible, Diffusive, Reactive Flows with Detailed Chemistry Using a High-Order Hybrid WENO-CD Scheme
Get PDFA hybrid weighted essentially non-oscillatory (WENO)/centered-difference (CD) numerical method, with low numerical dissipation, high-order shock-capturing, and structured adaptive mesh refinement (SAMR), has been developed for the direct numerical simulation (DNS) of the multicomponent, compressive, reactive Navier-Stokes equations. The method enables accurate resolution of diffusive processes within reaction zones. This numerical method is verified with a series of one- and two-dimensional test problems, including a convergence test of a two-dimensional unsteady reactive double Mach reflection problem. Validation of the method is conducted with experimental comparisons of three applications all of which model multi-dimensional, unsteady reactive flow: an irregular propane detonation, shock and detonation bifurcations, and spark ignition deflagrations.
The numerical approach combines time-split reactive source terms with a high-order, shock-capturing scheme specifically designed for diffusive flows. A description of the order-optimized, symmetric, finite difference, flux-based, hybrid WENO / centered-difference scheme is given, along with its implementation in a high-order SAMR framework. The implementation of new techniques for discontinuity flagging, scheme-switching, and high-order prolongation and restriction is described. In particular, the refined methodology does not require upwinded WENO at grid refinement interfaces for stability, allowing high-order prolongation and thereby eliminating a significant source of numerical diffusion within the overall code performance.
A minimally reduced irregular detonation mixture mechanism (22 species and 53 reversible reactions) is developed and combined with the WENO-CD numerical method to accurately model two-dimensional hydrocarbon (propane) detonations with detailed chemistry and transport. First of its kind, resolved double Mach reflection (DMR) detonation simulations with a large hyrdocarbon mixture are presented. Detailed discussions and comparisons of the influence of grid resolution, lower-order numerical methods, and inviscid approximations are made in addition to the detailed presentation of fluid dynamics found in an unsteady, highly unstable, reactive DMR simulation. Also conducted are direct experimental comparisons to soot foils and schlieren images with an unresolved large-scale propane detonation channel simulation.
The numerical method is also applied to the DNS of two other problems, detonation/shock bifurcations and spark ignited deflagrations. Through the resolution of viscous/diffusive scales, new insights into how a bifurcated foot develops after a detonation end wall reflection, and how geometry can influence the development of a flame kernel after spark ignition are found.</p