865 research outputs found
Spectroscopic Fingerprints of sp1 Hybridized C in Surface-Grown Molecular Assemblies
No full textThe advent of on-surface chemistry and molecular self-assembly opened the way to the realization of organic materials whose lack of stability in vacuo would otherwise forbid their synthesis. Recently the on-surface coupling of the precursors has also been used to synthesize adsorbed organic systems with chemically vulnerable linear carbon chains with sp 1 hybridization [1]. A notable advantage of on-surface synthesis is that the full realm of the experimental and theoretical surface science toolbox is available for the investigation. While the method of choice for surface-supported architectures is the scanning tunneling microscope and ultimate resolution is achieved by low-T non-contact atomic force microscopy, electron core-level spectroscopy can provide an even more local probe of the electronic properties of the material. In particular, polarized near-edge X-ray absorption fine structure (NEXAFS) spectroscopy is here suggested to discriminate sp 1 /sp 2 character in the structures. We present an ab initio study of the polarized NEXAFS spectrum of model and real sp 1 /sp 2 materials. Calculations are performed within density functional theory with plane waves and pseudopotentials, and spectra are computed by core-excited C potentials as validated by previous studies [2,3]. We evaluate the dichroism in the spectrum for ideal carbynes and highlight the main differences relative to typical sp 2 systems. We then consider a mixed polymer alternating sp 1 C 4 units with sp 2 biphenyl groups, recently synthesized on Au(111) [4], as well as other linear structures and two-dimensional networks, pointing out a spectral line shape specifically due to the the presence of linear C chains [5]. Our study suggests that the measurements of polarized NEXAFS spectra could be used to distinctly fingerprint the presence of sp 1 hybridization in surface-grown C structures. [1] Q. Sun, R. Zhang, J. Qiu, R. Liu, and W. Xu, Adv. Mater. 2018, 30, 1705630 [2] G. Fratesi, V. Lanzilotto, L. Floreano, and G. P. Brivio, J. Phys. Chem. C 2013, 117, 6632 [3] G. Fratesi, V. Lanzilotto, S. Stranges, M. Alagia, G. P. Brivio, and L. Floreano, Phys. Chem. Chem. Phys. 2014, 16, 14834 [4] Q. Sun, L. Cai, H. Ma, C. Yuan, and W. Xu, ACS Nano 2016, 10, 7023 [5] G. Fratesi, S. Achilli, N. Manini, G. Onida, A. Baby, A. Ravikumar, A. Ugolotti, G.P. Brivio, A. Milani, and C.S. Casari, Materials 2018, 11, 255
In situ Ru K-edge EXAFS of CO adsorption on a Ru modified Pt/C fuel cell catalyst
No full textThe Ru–CO bond of CO adsorbed on a Ru modified Pt/C fuel cell catalyst has been directly probed by in situ EXAFS at the Ru K-edge, providing evidence of a CO:metal surface atom ratio greater than 1:1 and that CO is adsorbed at bridging sites associated with Ru atoms at the surface of the catalyst nanoparticles. This result illustrates the limitations of single crystal models as representations of the bonding of adsorbed species at nanoparticle surfaces.<br/
Ambiguity and communication
No full textThe ambiguity of a nondeterministic finite automaton (NFA) N for input size n is the maximal number of accepting computations of N for an input of size n. For all k, r 2 N we construct languages Lr,k which can be recognized by NFA's with size k poly(r) and ambiguity O(nk), but Lr,k has only NFA's with exponential size, if ambiguity o(nk) is required. In particular, a hierarchy for polynomial ambiguity is obtained, solving a long standing open problem (Ravikumar and Ibarra, 1989, Leung, 1998)
Role of c-Cbl in Invasion of Mammalian Cells by Rickettsia Conorii
No full text[SPECIFIC AIMS] Rickettsia conorii is an intracellular bacterium that causes Mediterranean spotted fever. R. conorii is transmitted from ticks to humans and invades vascular endothelial cells. A recent publication (Martinez et al., 2005) has identified Ku70 as a host cell receptor that binds to rOmpB, an R. conorii surface ligand. The engagement of Ku70 by rOmpB leads to a rapid ubiquitination of Ku70 by c-Cbl, followed by the entry of R. conorii into the host cell. However, the role of c-Cbl in ubiquitinating Ku70 and communicating with the invading bacterium remain to be clarified. Based on the report of Martinez et al and other groups on the roles played by R. conorii surface ligands rOmpA and rOmpB, we propose the following hypothesis: [HYPOTHESIS] c-Cbl ubiquitinates Ku70 in response to signals from a Rickettsia conorii cell surface protein called rOmpA. This ubiquitination event enables Ku70 and the attached bacteria to be endocytosed into the cell. [SPECIFIC AIMS] 1. To analyse the effect of c-Cbl - mediated ubiquitination on Ku70 localisation. We will investigate if ubiquitination of Ku70 by c-Cbl is necessary for Ku70 to localize to the bacterial entry foci. We will culture c-Cbl deficient cells and observe the effects of R. conorii infection on them. 2. To map the putative signal(s) involved in the communication between R. conorii rOmpA outside the cell and c-Cbl inside the cell. We will identify the putative rOmpA receptor on host cells and protein(s) that may act as bridges of communication between rOmpA and c-Cbl. 3. To investigate the mechanism of how ubiquitination of Ku70 enables R. conorii to enter a host cell. We will determine if any c-Cbl interacting proteins and/or endocytic proteins are recruited to the cell surface by ubiquitinated Ku70 to form a phagosome. If endocytic proteins are recruited, we will proceed
On some variations of two-way probabilistic finite automata models
No full textAbstractRabin [M. Rabin, Probabilistic finite automata, Information and Control (1963) 230–245] initiated the study of probabilistic finite automata (pfa). Rabin’s work showed a crucial role of the gap in the error bound (for accepting and non-accepting computations) in the power of the model. Further work resulted in the identification of qualitatively different error models (one-sided error, bounded and unbounded errors, no error etc.) Karpinski and Verbeek [M. Karpinski, R. Verbeek, There is no polynomial deterministic simulation of probabilistic space with two-way random-tape generator, Information and Control 67 (1985) 158–162] and Nisan [N. Nisan, On read-once vs. multiple access to randomness in logspace, in: Proc. of Fifth IEEE Structure in Complexity Theory, Barcelona, Spain, 1990, pp. 179–184] studied a model of probabilistic automaton in which the tape containing random bits can be read by a two-way head. They presented results comparing models with one-way vs. two-way access to randomness. Dwork and Stockmeyer [C. Dwork, L. Stockmeyer, Interactive proof systems with finite state verifiers, IBM Report RJ 6262, 1988] and Condon et al. [A. Condon, et al., On the power of finite automata with both nondeterministic and probabilistic states, SIAM Journal on Computing (1998) 739–762] studied a model of 2-pfa with nondeterministic states (2-npfa). In this paper, we present some results about the above mentioned variations of probabilistic finite automata, as well as a model of 2-pfa augmented with a pebble studied in [B. Ravikumar, Some observations on two-way probabilistic finite automata, in: Proc. of the Foundations of Software Technology and Theoretical Computer Science, 1992, pp. 392–403]. Our observations indicate that these models exhibit subtle variations in their computational power. We also mention many open problems about these models. Complete characterizations of their power will likely provide deeper insights about the role of randomness in space-bounded computations
Chain-End Controlled Depolymerization Selectivity in α,α-Disubstituted Propionate PHAs with Dual Closed-Loop Recycling and Record-High Melting Temperature
No full textWithin the large poly(3-hydroxyalkanoate) (PHA) family, C3 propionates are much less studied than C4 butyrates, with the exception of α,α-disubstituted propionate PHAs, particularly poly(3-hydroxy-2,2-dimethylpropionate), P3H(Me)2P, due to its high melting temperature (Tm ∼ 230 °C) and crystallinity (∼76%). However, inefficient synthetic routes to its monomer 2,2-dimethylpropiolactone [(Me)2PL] and extreme brittleness of P3H(Me)2P largely hinder its broad applications. Here, we introduce simple, efficient step-growth polycondensation (SGP) of a hydroxyacid or methyl ester to afford P3H(Me)2P with low to medium molar mass, which is then utilized to produce lactones through base-catalyzed depolymerization. The ring-opening polymerization (ROP) of the 4-membered lactone leads to high-molar-mass P3H(Me)2P, which can be depolymerized by hydrolysis to the hydroxyacid in 99% yield or methanolysis to the hydroxyester in 91% yield, achieving closed-loop recycling via both SGP and ROP routes. Intriguingly, the chain end of the SGP-P3H(Me)2P determines the depolymerization selectivity toward 4- or 12-membered lactone formation, while both can be repolymerized back to P3H(Me)2P. Through the formation of copolymers P3H(Me/R)2P (R = Et, nPr), PHAs with high tensile strength and ductility, coupled with high barriers to water vapor and oxygen, have been created. Notably, the PHA structure-property study led to P3H(nPr)2P with a record-high Tm of 266 °C within the PHA family
Unusual C–C Bond Cleavage in the Formation of Amine-Bis(phenoxy) Group 4 Benzyl Complexes: Mechanism of Formation and Application to Stereospecific Polymerization
No full textGroup 4 tetrabenzyl compounds MBn4 (M = Zr, Ti), upon protonolysis with an equimolar amount of the tetradentate amine-tris(phenol) ligand N[(2,4-tBu2C6H2(CH2)OH]3 in toluene from −30 to 25 °C, unexpectedly lead to amine-bis(phenoxy) dibenzyl complexes, BnCH2N[(2,4-tBu2C6H2(CH2)O]2MBn2 (M = Zr (1), Ti (2)) in 80% (1) and 75% (2) yields. This reaction involves an apparent cleavage of the >NCH2–ArOH bond (loss of the phenol in the ligand) and formation of the >NCH2–CH2Bn bond (gain of the benzyl group in the ligand). Structural characterization of 1 by X-ray diffraction analysis confirms that the complex formed is a bis(benzyl) complex of Zr coordinated by a newly derived tridentate amine-bis(phenoxy) ligand arranged in a mer configuration in the solid state. The abstractive activation of 1 and 2 with B(C6F5)3·THF in CD2Cl2 at room temperature generates the corresponding benzyl cations {BnCH2N[(2,4-tBu2C6H2(CH2)O]2MBn(THF)}+[BnB(C6F5)3]− (M = Zr (3), Ti, (4)). These cationic complexes, along with their analogues derived from (imino)phenoxy tri- and dibenzyl complexes, [(2,6-iPr2C6H3)N═C(3,5-tBu2C6H2)O]ZrBn3 (5) and [2,4-Br2C6H2(O)(6-CH2(NC5H9))CH2N═CH(2-adamantyl-4-MeC6H2O)]ZrBn2 (6), have been found to effectively polymerize the biomass-derived renewable β-methyl-α-methylene-γ-butyrolactone (βMMBL) at room temperature into the highly stereoregular polymer PβMMBL with an isotacticity up to 99% mm. A combined experimental and DFT study has yielded a mechanistic pathway for the observed unusual C–C bond cleavage in the present protonolysis reaction between ZrBn4 and N[(2,4-tBu2C6H2(CH2)OH]3 for the formation of complex 1, which involves the benzyl radical and the Zr(III) species, resulting from thermal and photochemical decomposition of ZrBn4, followed by a series of reaction sequences consisting of protonolysis, tautomerization, H-transfer, oxidation, elimination, and radical coupling
Public Infrastructure Spillovers and Growth: Theory and Time Series Evidence for Australia.
No full textIn this paper, growth of per capita income can be exogenous and/or endogenous due to aggregate public infrastructure spillover. The deterministic Glomm and Ravikumar (1994) model is augmented in this paper to produce a stochastic growth counterpart which has useful time series implications.INCOME ; TIME SERIES ; CAPITAL
Fingerprints of sp1-carbon hybridization in the core-level spectra of surface-grown materials
No full textThe advent of on-surface chemistry and molecular self-assembly opened the way to the realization of organic materials whose lack of stability in vacuo would otherwise forbid their synthesis. A famous example is the formation of graphene nanoribbons with tunable width by polimerization of properly designed molecular precursors. Recently the on-surface coupling of the precursors have also been used to synthesize adsorbed organic systems with chemically vulnerable linear carbon chains with sp1 hybridization. [1] Another advantage of on-surface synthesis is that the full realm of the experimental and theoretical surface science toolbox is available for the investigation.
While the method of choice for surface-supported architectures is the scanning tunneling microscope and ultimate resolution is achieved by low-T non-contact atomic force microscopy, electron core-level spectroscopy can provide an even more local and sometimes insightful probe of the electronic properties of the material, as we show here.
We present ab initio investigations of the core-level spectral properties of a paradigmatic mixed sp1/sp2 C polymer, recently synthesized on Au(111) in linear/2D forms, and constituted by alternated biphenyl and linear C4 units, to discuss the appearance of the sp1 hybridization typical of linear C chains in the spectra. X-ray photoemission (XPS) and polarized near-edge X-ray absorption fine structure (NEXAFS) spectra are evaluated for the polymer in vacuo and on the metal surface, in the framework of density functional theory simulations with core-excited C potentials that have successfully reproduced spectral dichroisms [2]. We find that, at variance with the XPS spectra that do not easily show the signature of sp1 hybridization, NEXAFS spectra can distinctly fingerprint the sp1 part of the polymer through polarized spectra. Such capability is further facilitated by the different degree of hybridization of the molecular orbitals at the interface for the sp1 and sp2 states.
[1] Q. Sun, R. Zhang, J. Qiu, R. Liu, and W. Xu, Adv. Mater. 2018, 30, 1705630
[2] G. Fratesi, V. Lanzilotto, L. Floreano, and G. P. Brivio, J. Phys. Chem. C 2013, 117, 663
On-Chip Memory Architecture Exploration Of Embedded System On Chip
No full textToday’s feature-rich multimedia products require embedded system solution with complex System-on-Chip (SoC) to meet market expectations of high performance at low cost and lower energy consumption. SoCs are complex designs with multiple embedded processors, memory subsystems, and application specific peripherals. The memory architecture of embedded SoCs strongly influences the area, power and performance of the entire system. Further, the memory subsystem constitutes a major part (typically up to 70%) of the silicon area for the current day SoC.
The on-chip memory organization of embedded processors varies widely from one SoC to another, depending on the application and market segment for which the SoC is deployed. There is a wide variety of choices available for the embedded designers, starting from simple on-chip SPRAM based architecture to more complex cache-SPRAM based hybrid architecture. The performance of a memory architecture also depends on how the data variables of the application are placed in the memory. There are multiple data layouts for each memory architecture that are efficient from a power and performance viewpoint. Further, the designer would be interested in multiple optimal design points to address various market segments. Hence a memory architecture exploration for an embedded system involves evaluating a large design space in the order of 100,000 of design points and each design points having several tens of thousands of data layouts. Due to its large impact on system performance parameters, the memory architecture is often hand-crafted by experienced designers exploring a very small subset of this design space. The vast memory design space prohibits any possibility for a manual analysis.
In this work, we propose an automated framework for on-chip memory architecture exploration. Our proposed framework integrates memory architecture exploration and data layout to search the design space efficiently. While the memory exploration selects specific memory architectures, the data layout efficiently maps the given application on to the memory architecture under consideration and thus helps in evaluating the memory architecture. The proposed memory exploration framework works at both logical and physical memory architecture level. Our work addresses on-chip memory architecture for DSP processors that is organized as multiple memory banks, with each back can be a single/dual port banks and with non-uniform bank sizes. Further, our work also address memory architecture exploration for on-chip memory architectures that is SPRAM and cache based. Our proposed method is based on multi-objective Genetic Algorithm based and outputs several hundred Pareto-optimal design solutions that are interesting from a area, power and performance viewpoints within a few hours of running on a standard desktop configuration
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