319 research outputs found
Gold as hydrogen: Structural and electronic properties and chemical bonding in Si3Au3+/0/- and comparisons to Si3H3+/0/-
A single Au atom has been shown to behave like H in its bonding to Si in several mono- and disilicon gold clusters. In the current work, we investigate the AuâH analogy in trisilicon gold clusters, Si3Au+â0ââ3. Photoelectron spectroscopy and density functional calculations are combined to examine the geometric and electronic structure of Si3Auâ3. We find that there are three isomers competing for the ground state of Si3Auâ3 as is the case for Si3Hâ3. Extensive structural searches show that the potential energy surfaces of the trisilicon gold clusters (Si3Auâ3, Si3Au3, and Si3Au+3) are similar to those of the corresponding silicon hydrides. The lowest energy isomers for Si3Auâ3 and Si3Au3 are structurally similar to a Si3Au four-membered ring serving as a common structural motif. For Si3Au+3, the 2Ï aromatic cyclotrisilenylium auride ion, analogous to the aromatic cyclotrisilenylium ion (Si3H+3), is the most stable species. Comparison of the structures and chemical bonding between Si3Au+â0ââ3 and the corresponding silicon hydrides further extends the isolobal analogy between Au and H
Electronic structure of chromium oxides, CrOn- and CrOn (n=1-5) from photoelectron spectroscopy and density functional theory calculations
The electronic structure of CrOân and CrOn (n=1â5) was investigated using anion photoelectron spectroscopy and density functional theory. Photoelectron spectra of CrOân were obtained at several photon energies and yielded electron affinities, vibrational and electronic structure information about the neutral CrOn species. Density functional theory calculations were carried out for both the neutrals and anions and were used to interpret the experimental spectra. Several low-lying electronic states of CrO were observed and assigned from photodetachment of the CrOâ ground state(6â+) and an excited state (4â), which is only 0.1 eV higher. The main spectral features of CrOâ2 were interpreted based on a C2v CrOâ2 (4B1). A very weak Cr(O2)â isomer was also observed with lower electron binding energies. Relatively simple and vibrationally resolved spectra were observed for CrOâ3, which was determined to be D3h. The CrO3 neutral was calculated to be C3v with the Cr atom slightly out of the plane of the three O atoms. The spectrum of CrOâ4 revealed a very high electron binding energy. Several isomers of CrOâ4 were predicted and the ground state has a distorted tetrahedral structure (C2) without any OâO bonding. Only one stable structure was predicted forCrOâ5 with a superoxo O2 bonded to a C3v CrO3
Exact Variance Component Tests for Longitudinal Microbiome Studies
In metagenomic studies, testing the association of microbiome composition and clinical outcomes translates to testing the nullity of variance components. Motivated by a lung HIV (human immunodeficiency virus) microbiome project, we study longitudinal microbiome data by variance component models with more than two variance components. Current testing strategies only apply to the models with exactly two variance components and when sample sizes are large. Therefore, they are not applicable to longitudinal microbiome studies. In this paper, we propose exact tests (score test, likelihood ratio test, and restricted likelihood ratio test) to (1) test the association of the overall microbiome composition in a longitudinal design and (2) detect the association of one specific microbiome cluster while adjusting for the effects from related clusters. Our approach combines the exact tests for null hypothesis with a single variance component with a strategy of reducing multiple variance components to a single one. Simulation studies demonstrate that our method has correct type I error rate and superior power compared to existing methods at small sample sizes and weak signals. Finally, we apply our method to a longitudinal pulmonary microbiome study of human immunodeficiency virus (HIV) infected patients and reveal two interesting genera Prevotella and Veillonella associated with forced vital capacity. Our findings shed lights on the impact of lung microbiome to HIV complexities. The method is implemented in the open source, high-performance computing language Julia and is freely available at https://github.com/JingZhai63/VCmicrobiome
Competition between linear and cyclic structures in monochromium carbide clusters CrCn- and CrCn (n=2-8): A photoelectron spectroscopy and density functional study
Photoelectron spectroscopy (PES) is combined with density functional theory (DFT) to study the monochromium carbide clusters CrCân and CrCnâ(n=2â8). Well-resolved PES spectra were obtained, yielding structural, electronic, and vibrational information about both the anionic and neutral clusters. Experimental evidence was observed for the coexistence of two isomers for CrCâ2, CrCâ3, CrCâ4, and CrCâ6. Sharp and well-resolved PES spectra were observed for CrCânâ(n=4,6,8), whereas broad spectra were observed for CrCâ5 and CrCâ7. Extensive DFT calculations using the generalized gradient approximation were carried out for the ground and low-lying excited states of all the CrCân and CrCn species, as well as coupled-cluster calculations for CrCâ2 and CrC2. Theoretical electron affinities and vertical detachment energies were calculated and compared with the experimental data to help the assignment of the ground states and obtain structural information. We found that CrCâ2 and CrCâ3 each possess a close-lying cyclic and linear structure, which were both populated experimentally. For the larger CrCân clusters with n=4, 6, 8, linear structures are the overwhelming favorite, giving rise to the sharp PES spectral features. CrCâ7 was found to have a cyclic structure. The broad PES spectra of CrCâ5 suggested a cyclic structure, whereas the DFT results predicted a linear one
Implantation of neural stem cells embedded in hyaluronic acid and collagen composite conduit promotes regeneration in a rabbit facial nerve injury model
The implantation of neural stem cells (NSCs) in artificial scaffolds for peripheral nerve injuries draws much attention. NSCs were ex-vivo expanded in hyaluronic acid (HA)-collagen composite with neurotrophin-3, and BrdU-labeled NSCs conduit was implanted onto the ends of the transected facial nerve of rabbits. Electromyography demonstrated a progressive decrease of current threshold and increase of voltage amplitude in de-innervated rabbits after implantation for one, four, eight and 12 weeks compared to readouts derived from animals prior to nerve transection. The most remarkable improvement, observed using Electrophysiology, was of de-innervated rabbits implanted with NSCs conduit as opposed to de-innervated counterparts with and without the implantation of HA-collagen, NSCs and HA-collagen, and HA-collagen and neurotrophin-3. Histological examination displayed no nerve fiber in tissue sections of de-innervated rabbits. The arrangement and S-100 immunoreactivity of nerve fibers in the tissue sections of normal rabbits and injured rabbits after implantation of NSCs scaffold for 12 weeks were similar, whereas disorderly arranged minifascicles of various sizes were noted in the other three arms. BrdU+ cells were detected at 12 weeks post-implantation. Data suggested that NSCs embedded in HA-collagen biomaterial could facilitate re-innervations of damaged facial nerve and the artificial conduit of NSCs might offer a potential treatment modality to peripheral nerve injuries
Aromaticity and Antiaromaticity in Transition-Metal Systems
Aromaticity is an important concept in chemistry primarily for organic compounds, but it has been extended to compounds containing transition-metal atoms. Recent findings of aromaticity and antiaromaticity in all-metal clusters have stimulated further research in describing the chemical bonding, structures and stability in transition-metal clusters and compounds on the basis of aromaticity and antiaromaticity, which are reviewed here. The presence of d-orbitals endows much more diverse chemistry, structure and chemical bonding to transition-metal clusters and compounds. One interesting feature is the existence of a new type of aromaticity-d-aromaticity, in addition to s-and p-aromaticity which are the only possible types for main-group compounds. Another striking characteristic in the chemical bonding of transition-metal systems is the multifold nature of aromaticity, antiaromaticity or even conflicting aromaticity. Separate sets of counting rules have been proposed for cyclic transition-metal systems to account for the three types of s-, p-and d-aromaticity/antiaromaticity. The diverse transition-metal clusters and compounds reviewed here indicate that multiple aromaticity and antiaromaticity may be much more common in chemistry than one would anticipate. It is hoped that the current review will stimulate interest in further understanding the structure and bonding, on the basis of aromaticity and antiaromaticity, of other known or unknown transition-metal systems, such as the active sites of enzymes or other biomolecules which contain transition-metal atoms and clusters
Quantum theory of concerted electronic and nuclear fluxes associated with adiabatic intramolecular processes
An elementary molecular process can be characterized by the flow of particles
(i.e., electrons and nuclei) that compose the system. The flow, in turn, is
quantitatively described by the flux (i.e., the time-sequence of maps of the
rate of flow of particles though specified surfaces of observation) or, in
more detail, by the flux density. The quantum theory of concerted electronic
and nuclear fluxes (CENFs) associated with electronically adiabatic
intramolecular processes is presented. In particular, it is emphasized how the
electronic continuity equation can be employed to circumvent the failure of
the BornâOppenheimer approximation, which always predicts a vanishing
electronic flux density (EFD). It is also shown that all CENFs accompanying
coherent tunnelling between equivalent âreactantâ and âproductâ configurations
of isolated molecules are synchronous. The theory is applied to three systems
of increasing complexity. The first application is to vibrating, aligned
H2+(2ÎŁg+), or vibrating and dissociating H2+(2ÎŁg+, J = 0, M = 0). The EFD maps
manifest a rich and surprising structure in this simplest of systems; for
example, they show that the EFD is not necessarily synchronous with the
nuclear flux density and can alternate in direction several times over the
length of the molecule. The second application is to coherent tunnelling
isomerization in the model inorganic system B4, in which all CENFs are
synchronous. The contributions of core and valence electrons to the EFD are
separately computed and it is found that core electrons flow with the nuclei,
whereas the valence electrons flow obliquely to the core electrons in
distinctive patterns. The third application is to the Cope rearrangement of
semibullvalene, which also involves coherent tunnelling. An especially
interesting discovery is that the so-called âpericyclicâ electrons do not
behave in the manner typically portrayed by the traditional Lewis structures
with appended arrows. Indeed, it is found that only about 3 pericyclic
electrons flow, in contrast to the 6 predicted by the Lewis picture. It is
remarkable that the time scales of these three processes vary by 18 orders of
magnitude: femtoseconds (H2+(2ÎŁg+)); picoseconds (B4); kilosceconds
(semibullvalene). It is emphasized that results presented herein are appearing
in the literature for the first time
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