pinecone/core-2020-05-10-deduplication huggingface dataset:
Quenching Echo Modulations in NMR Spectroscopy

In NMR spectroscopy, homonuclear scalar couplings normally lead to modulations of spin echoes that tend to interfere with the accurate determination of transverse relaxation rates by Carr–Purcell–Meiboom–Gill (CPMG) multiple refocusing experiments. Surprisingly, the echo modulations are largely cancelled when the refocusing pulses applied to the coupling partner deviate slightly from ideal π rotations due to tilted effective radio-frequency (RF) fields, even at offsets that are much smaller than the radio-frequency amplitude. Experiments and simulations illustrate these effects for two-spin IS systems containing donor and acceptor 15N nuclei I=N D and S=NA in RNA Watson–Crick base pairs with homonuclear scalar couplings JIS=2hJ(ND, NA) across the hydrogen bonds
Effect of Branching on Two-Photon Absorption in Triphenylbenzene Derivatives

Claudine Katan ‘s present address : CNRS UMR6082 FOTON, INSA de Rennes, 20 avenue des Buttes de Coësmes, CS 70839, 35708 RENNES cedex 7, FranceInternational audienceThe photophysical and linear and nonlinear spectral properties of octupolar compounds with a triphenylbenzene core are investigated and compared with properties of corresponding dipolar branches. A correlation is found between the solvatochromic behavior and the two-photon absorption cross section. Moreover, the nature of the core is found to be responsible for the nature of the coupling between branches; in the studied case only (weak) electrostatic interactions are effective, while other cores, like the triphenylamine moiety, are able to promote coherent coupling between the branches, leading to strongly nonadditive properties
Indirect Detection of Nitrogen-14 in Solid-State NMR Spectroscopy

Other ; This thesis describes new methods for the detection of 14N nuclei by solid-state nuclear magnetic resonance. So far, the low natural abundance (0.4 %) 15N isotope has been widely used to study nitrogen-containing samples because of its spin-1/2 nature. The limited use of the spin-1 isotope 14N (natural abundance 99.6 %) in NMR is due to its strong quadrupolar coupling constant, which leads to very broad spectra that are difficult to excite uniformly and equally difficult to detect, requiring broad receiver bandwidths. The new methods presented in this work result from two main projects covering the indirect detection of 14N via carbon and via protons. The indirect detection of 14N can be achieved by heteronuclear multiple- or single-quantum correlation (HMQC or HSQC) experiments, which rely on the transfer of coherence from a neighboring spin S = 1/2 (13C or 1H) to single- (SQ) or double-quantum (DQ) transitions of 14N nuclei. These methods allow one to observe 14N powder patterns with enhanced sensitivity. These spectra depend on the quadrupolar coupling constant CQ (typically a few MHz), the asymmetry parameter ηQ, and on the orientation of the internuclear vector rIS between the I (14N) and S (13C or 1H) nuclei with respect to the quadrupolar tensor. These parameters reveal valuable information about the structure and dynamics of nitrogen-containing solids. The indirect detection methods are discussed in detail, covering the optimization of experimental parameters; subsequently, applications to the study of amino acids at different magnetic fields are demonstrated
Quenching and Recoupling of Echo Modulations in NMR Spectroscopy

International audience ; Trains of spin echoes are normally modulated by homonuclear scalar couplings. It has long been known that echo modulations are quenched when the pulse-repetition rates are much larger than the offsets of the coupling partners, because the spin systems behave as if they consisted of magnetically equivalent spins when the offsets are suppressed. This type of quenching of the echo modulations can occur when the radio-frequency (RF) pulses are ideal, that is, when they are perfectly homogeneous, properly calibrated to induce rotations through an angle, pi, and have an RF amplitude, omega(1)=-gammaB(1), that is strong compared to the largest offset, Omega(S)=omega(0S)-omega(RF), with respect to the carrier frequency. Recently, it was discovered that echo modulations can also be quenched when the RF pulses are nonideal, that is, when they are too weak to bring about an ideal rotation of the magnetization of the coupling partners, so that the effective fields associated with the RF pulses are tilted in the rotating frame. This phenomenon typically occurs when the pulse-repetition rates are much slower than the offset of the coupling partner. Under such conditions, it turns out, however, that for certain offsets, when the phase, Phi(S) (which arises from a free precession of the magnetization of the coupling partner, S, in the pulse interval, 2tau, and the pulse length, tau(pi)), approaches a multiple of 2pi, the echo modulations are restored. However, the frequencies of these echo modulations are not simply determined by the homonuclear scalar coupling, J(IS). The Fourier transforms of the echo trains (the so-called "J spectra") reveal surprising multiplet patterns, and the amplitudes of the echo modulations depend on the offsets of the coupling partners. Herein, we present a unified theory, based on an average-Hamiltonian approach, to describe these effects for two-spin systems. Experimental evidence of echo modulations in a system of two spins is presented. Experiments with three and more spins, backed up by extensive numerical simulations, will be presented elsewhere
The gas-phase reaction between O3 and HO radical: a theoretical study

6 pages, 3 figures.-- PMID: 17566136 [PubMed].-- Supplementary information (10 pages, figures S1-S2, tables S1-S5) available at the publisher's site:http://www.wiley-vch.de/contents/jc_2267/2007/f700115_s.pdfWe report a theoretical study on the reaction of ozone with hydroxyl radical, which is important in the chemistry of the atmosphere and in particular participates in stratospheric ozone destruction. The reaction is a complex process that involves, in the first stage, a pre-reactive hydrogen-bonded complex (C1), which\ud
is formed previous to two transition states (TS1 and TS2) involving the addition of the hydroxyl radical to ozone, and leads to the formation of HO4 polyoxide radical before the release of the\ud
products HO2 and O2. The reaction is computed to be exothermic by 42.72 kcal mol-1, which compares quite well with the experimental\ud
estimate, and the energy barriers of TS1 and TS2 with respect to C1 are computed to be 1.80 and 2.26 kcal mol 1 at 0 K. A kinetic study based on the variational transition state theory (VTST) predicts a rate constant, at 298 K, of 7.37B\ud
10 14 cm3 molecule-1 s-1, compared to the experimentally recommended value of 7.25x10^(-14) cm3 molecule-1 s-1.This research has been supported by the Spanish Dirección General de Investigación Científica y Técnica (DGYCIT, grant CTQ2005-07790) and by the Generalitat de Catalunya (Grant 2005SGR00111). The calculations described in this work were carried out at the Centre de Supercomputació de Catalunya (CESCA) and at an AMD Opteron cluster of our group. A.M. thanks the Spanish Ministerio de Educación y Ciencia for a fellowship (BES-\ud
2003-1352).Peer reviewe
Structure and dynamics of a discotic liquid-crystalline charge-transfer complex

The structure of the charge-transfer complex hexakis(n-hexyloxy)triphenylene-2,4,7-trinitro-9-fluorenone (HAT6-TNF) has been characterized by neutron scattering, X-ray diffraction (XRD), optical microscopy, and dielectric relaxation spectroscopy (DRS). On the basis of these data and the 1:1 stoichiometry, a consistent structure for the complex is proposed. This structure differs markedly from structures previously proposed for similar materials, because the TNF molecules are found to be situated between the discotic columns rather than sandwiched between the discotic molecules of a given column. The addition of TNF to HAT6 is found to stiffen the structure, and quasi-elastic neutron scattering shows that the local dynamics of the discotic molecules in HAT6-TNF is slowed by the presence of the TNF molecules. This scenario is consistent with the observation of two VFT-type (VFT=Vogel-Fulcher-Tamman) dielectric relaxation processes that relate to the columnar glass transition and a polyethylene-like hindered glass transition originating from the nano-phase-separated fraction of the aliphatic tails.status: publishe
X-ray spectroscopic and diffraction study of the structure of the active species in the Ni-II-catalyzed polymerization of isocyanides

Contains fulltext :
                  36705.pdf (postprint version ) (Open Access)The structure of the active complex in the Ni-cotolyzed polymerization of isocyonides to give polyisocyanides is investigated. It is shown by X-ray absorption spectroscopy (XAS), including EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure), and single-crystal X-ray diffraction, to contain a carbene-like ligand. This is the first structural characterization of a crucial intermediate in the so-coiled merry-go-round mechanism for Ni-catolyzed isocyanide polymerization
Extending the Scope of Singlet-State Spectroscopy

International audience ; Different decoupling sequences are tested-using various shaped radio-frequency (RF) pulses-to achieve the longest possible lifetimes of singlet-state populations over the widest possible bandwidths, that is, ranges of offsets and relative chemical shifts of the nuclei involved in the singlet states. The use of sinc or refocusing broadband universal rotation pulses (RE-BURP) for decoupling during the intervals where singlet-state populations are preserved allows one to extend the useful bandwidth with respect to prior state-of-the-art methods based on composite-pulse WALTZ decoupling. The improved sinc decoupling sequences afford a more reliable and sensitive measure of the lifetimes of singlet states in pairs of spins that have widely different chemical shifts, such as the two aromatic protons H(5) and H(6) in uracil. Similar advantages are expected for nucleotides in RNA and DNA. Alternative approaches, in particular frequency-modulated decoupling sequences, also appear to be effective in preserving singlet-state populations, even though the profiles of the apparent relaxation rate constants as a function of the offset are somewhat perturbed. The best decoupling sequences prove their utility in sustaining longer lifetimes of singlet states than previously achieved for the side-chain tyrosine protons in bovine pancreatic trypsin inhibitor (BPTI) at 600 MHz (14.1 T), where the differences of chemical shifts between coupled protons are a challenge
Listening to Lanthanide Complexes: Determination of the Intrinsic Luminescence Quantum Yield by Nonradiative Relaxation

The photophysical properties of lanthanide complexes have been studied extensively; however, fundamental parameters such as the intrinsic quantum yield as well as radiative and nonradiative decay rates are difficult or even impossible to measure experimentally. Herein, a photoacoustic (PA) method is proposed to determine the intrinsic quantum yield of lanthanide complexes with lifetimes in the order of milliseconds. This method is used to determine the intrinsic quantum yields for europium(III)-containing metallomesogens as well as terbium(III) complexes. The results show that the PA signal is sensitive to both the lifetime and the ratio of the fast-to-slow heat component of the samples. It is found that there is an efficient ligand sensitization and a moderate intrinsic quantum yield for the complexes. The intrinsic quantum yield of Eu(3+) in the metallomesogens exhibits an obvious increase upon the isotropic liquid to smectic A transition. The proposed PA method is quite simple, and can contribute to a clearer understanding of the photophysical processes in luminescent lanthanide complexes.status: publishe
Room temperature electronic template effect of pre-structured SmSi(111)-8x2 interface yielding self-aligned organic molecules

International audienceThis work describes an innovative concept for the development of organized molecular systems thanks to the template effect of the pre-structured semi-conductive SmSi(111) interface. This substrate was selected because Sm deposition in the submonolayer range leads to a 8x2-reconstruction, which is a well-defined one-dimensional semi-metallic structure. Adsorption of aromatic molecules (1,4-di-(9-ethynyltriptycenyl)-benzene) on SmSi(111)-8x2 and Si(111)-7x7 interfaces has been investigated by scanning tunneling microscopy (STM) at room temperature. Density functional theory (DFT) and semi-empirical (ASED+) calculations have been performed to define the nature of the molecular adsorption sites of the target molecule on SmSi as well as their self-alignment on this interface. Experimental data and theoretical results are in good agreement
Probing Dynein and Kinesin Stepping with Mechanical Manipulation in a Living Cell

Molecular motors: By combining optical tweezers and high-speed particle tracking, individual steps of microtubule motor proteins transporting organelles can be detected under known force loads in living mammalian cells (see figure).

We report a label-free assay for simultaneous optical manipulation and tracking of endogenous lipid droplets as actively transported cargoes in a living mammalian cell with sub-millisecond time resolution. Using an EM-CCD camera as a highly sensitive quadrant detector, we can detect steps of dynein- and kinesin-driven cargoes under known force loads. We can distinguish single and multiple motor-driven cargoes and show that the stall forces for inward and outward transported cargoes are similar. By combining the stall force observable with the ability to detect individual steps, we can characterize kinesin- and dynein-driven active transport in different force regimes.Chemistry and Chemical Biolog
The Role of Oxygen at the Interface between Titanium and Carbon Nanotubes

International audienceWe study the interface between carbon nanotubes (CNTs) and surface-deposited titanium using electron microscopy and photoemission spectroscopy, supported by density functional calculations. Charge transfer from the Ti atoms to the nanotube and carbide formation is observed at the interface which indicates strong interaction. Nevertheless, the presence of oxygen between the Ti and the CNTs significantly weakens the Ti–CNT interaction. Ti atoms at the surface will preferentially bond to oxygenated sites. Potential sources of oxygen impurities are examined, namely oxygen from any residual atmosphere and pre-existing oxygen impurities on the nanotube surface, which we enhance through oxygen plasma surface pretreatment. Variation in literature data concerning Ohmic contacts between Ti and carbon nanotubes is explained via sample pre-treatment and differing vacuum levels, and we suggest improved treatment routes for reliable Schottky barrier-free Ti–nanotube contact formation
Coupled Modulated Bilayers

We propose a model addressing the coupling mechanism between two spatially
modulated monolayers. We obtain the mean-field phase diagrams of coupled
bilayers when the two monolayers have the same preferred modulation wavelength.
Various combinations of the monolayer modulated phases are obtained and their
relative stability is calculated. Due to the coupling, a spatial modulation in
one of the monolayers induces a similar periodic structure in the second one.
We have also performed numerical simulations for the case when the two
monolayers have different modulation wavelengths. Complex patterns may arise
from the frustration between the two incommensurate but annealed structures.Comment: 20 pages, 6 figure
Boron Nitride Nanomesh: A template for Nano-ice

Using variable temperature scanning tunneling microscopy and dI/dz barrier
height spectroscopy, the structure of water on h-BN/Rh(111) nanomesh has been
investigated. Below its desorption temperature, two distinct phases of water
self-assemble within the 3.2 nm unit cell of the nanomesh. In the 2 nm holes,
an ordered phase of nano-ice crystals with about 40 molecules is found. The ice
crystals arrange in a bilayer honeycomb lattice, where the hydrogen atoms of
the lower layer point to the substrate. The phase on the 1 nm wires, is a low
density gas phase, which is characterized by contrast modulations and streaky
noise in the STM images. Tunneling barrier measurements infer the proton
positions in the nano-ice clusters.Comment: 4 figure
Reversible carrier-type transition in gas-sensing oxides and
  nanostructures

Despite many important applications of a-Fe2O3 and Fe doped SnO2 in
semiconductors, catalysis, sensors, clinical diagnosis and treatments, one
fundamental issue that is crucial to these applications remains theoretically
equivocal- the reversible carrier-type transition between n- and p-type
conductivities during gas-sensing operations. Here, we give unambiguous and
rigorous theoretical analysis in order to explain why and how the oxygen
vacancies affect the n-type semiconductors, a-Fe2O3 and Fe doped SnO2 in which
they are both electronically and chemically transformed into a p-type
semiconductor. Furthermore, this reversible transition also occurs on the oxide
surfaces during gas-sensing operation due to physisorbed gas molecules (without
any chemical reaction). We make use of the ionization energy theory and its
renormalized ionic displacement polarizability functional to reclassify,
generalize and to explain the concept of carrier-type transition in solids, and
during gas-sensing operation. The origin of such a transition is associated to
the change in ionic polarizability and the valence states of cations in the
presence of (a) oxygen vacancies and (b) physisorped gas molecules.Comment: To be published in ChemPhysChe
Triplication of the photocurrent in dye solar cells by increasing the elongation of the π-conjugation in Zn-porphyrin sensitizers

Porphyrins are promising sensitizers for dye solar cells (DSCs) but narrow absorption bands at 400-450 and 500-650 nm limit their light-harvesting properties. Increasing elongation of the π-conjugation and loss of symmetry causes broadening and a red-shift of the absorption bands, which considerably improves the performance of the DSC. Herein we use an oligothienylenevinylene to bridge a Zn-porphyrin system and the anchoring group of the sensitizer. We separately study the performance of the two basic units: oligothienylenevinylene and Zn-porphyrin. The combined system provides a three-fold enhancement of the photocurrent with respect to parent dyes. This is caused by an additional strong absorption in the region 400-650 nm that leads to flat IPCE of 60%. Theoretical calculations support that the addition of the oligothienylenevinylene unit as a linking bridge creates a charge transfer band that transforms a Zn-porphyrin dye into a push-pull type system with highly efficient charge injection propertie
Inelastic Neutron Scattering Study of PtII Complexes Displaying Anticancer Properties

The well-known platinum(II) chemotherapeutic drugs cisplatin [cis-(NH(3))(2)PtCl(2)] and carboplatin [Pt(NH(3))(2)C(6)O(4)H(6)], as well as the analogous transplatin [trans-(NH(3))(2)PtCl(2)], were studied by inelastic neutron scattering (INS) spectroscopy, coupled to quantum mechanical methods, and some ancillary work with X-ray diffraction on powders. An assignment of the experimental spectra was carried out based on the calculated INS transition frequencies and intensities (at the DFT level), thereby achieving a good correspondence between the calculated and observed data. Unusually good-quality INS spectra were obtained from about 250 mg, which is the smallest sample of a hydrogenous compound for which a successful INS interpretation has been reported. The knowledge of the local configuration of this kind of complexes is essential for an accurate understanding of their activity, which will pave the way for the rational design of novel third-generation drugs comprising cisplatin- and carboplatin-like moieties
A confidence level algorithm for the determination of absolute configuration using vibrational circular dichroism or raman optical activity

Spectral comparison is an important part of the assignment of the absolute configuration (AC) by vibrational circular dichroism (VCD), or equally by Raman optical activity (ROA). In order to avoid bias caused by personal interpretation, numerical methods have been developed to compare measured and calculated spectra. Using a neighbourhood similarity measure, the agreement between a computed and measured VCD or ROA spectrum is expressed numerically to introduce a novel confidence level measure. This allows users of vibrational optical activity (VOA) techniques (VCD and ROA) to assess the reliability of their assignment of the AC of a compound. To that end, a database of successful AC determinations is compiled along with neighbourhood similarity values between the experimental spectrum and computed spectra for both enantiomers. For any new AC determination, the neighbourhood similarities between the experimental spectrum and the computed spectra for both enantiomers are projected on the database allowing an interpretation of the reliability of their assignment
Nitrogen-doped carbon nanotubes: growth, mechanism and structure

Nitrogen‐doped bamboo‐structured carbon nanotubes have been successfully grown using a series of cobalt/molybdenum catalysts. The morphology and structure of the nanotubes were analysed by transmission electron microscopy and Raman spectroscopy. The level of nitrogen doping, as determined by X‐ray photoelectron spectroscopy, was found to range between 0.5 to 2.5 at. %. The growth of bamboo‐structured nanotubes in the presence of nitrogen, in preference to single‐walled and multi‐walled nanotubes, was due to the greater binding energy of nitrogen for cobalt in the catalyst compared to the binding strength of carbon to cobalt, as determined by density functional theory
Three‐Dimensional Super‐Resolution Imaging of the Midplane Protein FtsZ in Live  Caulobacter crescentus  Cells Using Astigmatism

Single‐molecule super‐resolution imaging provides a non‐invasive method for nanometer‐scale imaging and is ideally suited to investigations of quasi‐static structures within live cells. Here, we extend the ability to image subcellular features within bacteria cells to three dimensions based on the introduction of a cylindrical lens in the imaging pathway. We investigate the midplane protein FtsZ in  Caulobacter crescentus  with super‐resolution imaging based on fluorescent‐protein photoswitching and the natural polymerization/depolymerization dynamics of FtsZ associated with the Z‐ring. We quantify these dynamics and determine the FtsZ depolymerization time to be <100 ms. We image the Z‐ring in live and fixed  C. crescentus  cells at different stages of the cell cycle and find that the FtsZ superstructure is dynamic with the cell cycle, forming an open shape during the stalked stage and a dense focus during the pre‐divisional stage.     3D is here to stay:  Three‐dimensional super‐resolution astigmatic optical imaging provides images of the FtsZ Z‐ring in bacterial cells in various stages of the cell cycle. The picture shows, left to right: live stalked cell, live pre‐divisional cell, fixed stalked cell, fixed pre‐divisional cell. Scale bar: 200 nm
Chemical Etching of Zinc Oxide for Thin-Film Silicon Solar Cells

Chemical etching is widely applied to texture the surface of sputter-deposited zinc oxide for light scattering in thin-film silicon solar cells. Based on experimental findings from the literature and our own results we propose a model that explains the etching behavior of ZnO depending on the structural material properties and etching agent. All grain boundaries are prone to be etched to a certain threshold, that is defined by the deposition conditions and etching solution. Additionally, several approaches to modify the etching behavior through special preparation and etching steps are provided