Página 12 dos resultados de 2015 itens digitais encontrados em 0.009 segundos

Highly Enantioselective Cascade Transformations by Merging Heterogeneous Transition Metal Catalysis with Asymmetric Aminocatalysis

Deiana, Luca; Afewerki, Samson; Palo-Nieto, Carlos; Verho, Oscar; Johnston, Eric V.; Córdova, Armando
Fonte: Nature Publishing Group Publicador: Nature Publishing Group
Tipo: Artigo de Revista Científica
Publicado em 14/11/2012 EN
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The concept of combining heterogeneous transition metal and amine catalysis for enantioselective cascade reactions has not yet been realized. This is of great advantage since it would allow for the recycling of expensive and non-environmentally friendly transition metals. We disclose that the use of a heterogeneous Pd-catalyst in combination with a simple chiral amine co-catalyst allows for highly enantioselective cascade transformations. The preparative power of this process has been demonstrated in the context of asymmetric cascade Michael/carbocyclization transformations that delivers cyclopentenes bearing an all carbon quaternary stereocenters in high yields with up to 30:1 dr and 99% ee. Moreover, a variety of highly enantioselective cascade hetero-Michael/carbocyclizations were developed for the one-pot synthesis of valuable dihydrofurans and pyrrolidines (up to 98% ee) by using bench-stable heterogeneous Pd and chiral amines as co-catalysts.

Recent advances in transition-metal-catalyzed intermolecular carbomagnesiation and carbozincation

Murakami, Kei; Yorimitsu, Hideki
Fonte: Beilstein-Institut Publicador: Beilstein-Institut
Tipo: Artigo de Revista Científica
Publicado em 11/02/2013 EN
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Carbomagnesiation and carbozincation reactions are efficient and direct routes to prepare complex and stereodefined organomagnesium and organozinc reagents. However, carbon–carbon unsaturated bonds are generally unreactive toward organomagnesium and organozinc reagents. Thus, transition metals were employed to accomplish the carbometalation involving wide varieties of substrates and reagents. Recent advances of transition-metal-catalyzed carbomagnesiation and carbozincation reactions are reviewed in this article. The contents are separated into five sections: carbomagnesiation and carbozincation of (1) alkynes bearing an electron-withdrawing group; (2) alkynes bearing a directing group; (3) strained cyclopropenes; (4) unactivated alkynes or alkenes; and (5) substrates that have two carbon–carbon unsaturated bonds (allenes, dienes, enynes, or diynes).

Lifetime of combustion-generated environmentally persistent free radicals on Zn(II)O and other transition metal oxides

Vejerano, Eric; Lomnicki, Slawo; Dellinger, Barry
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
EN
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Previous studies indicated that Environmentally Persistent Free Radicals (EPFRs) are formed in the post-flame, cool zone of combustion. They result from the chemisorption of gas-phase products of incomplete combustion (particularly hydroxyl- and chlorine-substituted aromatics) on Cu(II)O, Fe(III)2O3, and Ni(II)O domains of particulate matter (fly ash or soot particles). This study reports our detailed laboratory investigation on the lifetime of EPFRs on Zn(II)O/silica surface. Similarly, as in the case of other transition metals, chemisorption of the adsorbate on the Zn(II)O surface and subsequent transfer of electron from the adsorbate to the metal forms a surface-bound EPFR and a reduced metal ion center. The EPFRs are stabilized by their interaction with the metal oxide domain surface. The half-lives of EPFRs formed on Zn(II)O domains were the longest observed among the transition metal oxides studied and ranged from 3 to 73 days. These half-lives were an order of magnitude longer than those formed on nickel and iron oxides, and were 2 orders of magnitude longer compared to the EPFRs on copper oxide which have half-lives only on the order of hours. The longest-lived radicals on Zn(II)O correspond to the persistency in ambient air particles of almost a year. The half-life of EPFRs was found to correlate with the standard reduction potential of the associated metal.

India - Orissa in Transition : Challenges for 2006-2010

World Bank
Fonte: Washington, DC Publicador: Washington, DC
Tipo: Economic & Sector Work :: Other Public Sector Study; Economic & Sector Work
ENGLISH; EN_US
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This report assesses the ongoing transition in Orissa. It examines how and why the successes were achieved. It attempts to outline the dimensions of the challenge ahead, as Orissa marches forward into the second phase of policy and institutional reforms, building on its improved fiscal position to deliver rapid and inclusive growth. It highlights key issues and binding or soon-to-be binding constraints. The concluding section identifies priorities for public expenditure and public policy outcomes in the immediate, medium-term, and long-term future. The report is intended as a contribution to the public debate and consultation initiated by Government of Orissa on the state's long-term vision and development strategy until 2020. The first two chapters focus respectively on economic growth performance and fiscal performance during the past 5-6 years. The subsequent three chapters focus respectively on key aspects of the outstanding challenge facing the state, namely: infrastructure, human development, and public accountability for service delivery. The final chapter summarizes the main findings and recommendations and poses considerations about priorities and sequencing.

LUMINESCENT TRANSITION METAL COMPLEXES BASED ON N-HETEROCYCLIC AND N^C-CHELATE 4-COORDINATE ORGANOBORYL LIGANDS

Wang, NAN
Fonte: Quens University Publicador: Quens University
Tipo: Tese de Doutorado
EN; EN
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The objective of this thesis is to examine the photophysical and structural properties of Ru(II)/Re(I) based bimetallic complexes based on p-[N-2-(2’-pyridyl)benzimidazolyl]-[N-2-(2’-pyridyl)indolyl]-benzene (L1) ligand, as well as the photophysical and photochemical properties of N^C-chelate 4-coordinate organoboron compounds that contain a metal acetylide group. Ligand L1 was synthesized and fully characterized. Due to the incorporation of two distinct chelating sites, an N^N-chelate site and an N^C-chelate site, L1 has been found to be very effective in selective binding to two different metal ions. Two new heterobimetallic complexes Ru(II)-Pt(II) and Ru(II)-Pd(II) using L1 as the bridging ligand were prepared and fully characterized. All Ru(II)-containing complexes have been found to be luminescent. The Pt(II) unit appears to enhance phosphorescent efficiency of the Ru(II) unit while the Pd(II) unit has little influence. Using L1 as the bridging unit, two new Re(I) based heterobimetallic complexes Re(I)-Pt(II) and Re(I)-Pd(II) were also successfully synthesized. Results indicate that there is communication between the two different metal centers. The preliminary results indicated that the mononuclear Re(I) complex based on L1 is a promising candidate for the electrocatalytic CO2 reduction. Pd(II) and Pt(II) complexes were synthesized with an atropisomeric bis-pyridyl chelate ligand bis{3...

Discovering the electronic circuit diagram of life: structural relationships among transition metal binding sites in oxidoreductases

Kim, J. Dongun; Senn, Stefan; Harel, Arye; Jelen, Benjamin I.; Falkowski, Paul G.
Fonte: The Royal Society Publicador: The Royal Society
Tipo: Artigo de Revista Científica
Publicado em 19/07/2013 EN
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Oxidoreductases play a central role in catalysing enzymatic electron-transfer reactions across the tree of life. To first order, the equilibrium thermodynamic properties of these proteins are governed by protein folds associated with specific transition metals and ligands at the active site. A global analysis of holoenzyme structures and functions suggests that there are fewer than approximately 500 fundamental oxidoreductases, which can be further clustered into 35 unique groups. These catalysts evolved in prokaryotes early in the Earth's history and are largely responsible for the emergence of non-equilibrium biogeochemical cycles on the planet's surface. Although the evolutionary history of the amino acid sequences in the oxidoreductases is very difficult to reconstruct due to gene duplication and horizontal gene transfer, the evolution of the folds in the catalytic sites can potentially be used to infer the history of these enzymes. Using a novel, yet simple analysis of the secondary structures associated with the ligands in oxidoreductases, we developed a structural phylogeny of these enzymes. The results of this ‘composome’ analysis suggest an early split from a basal set of a small group of proteins dominated by loop structures into two families of oxidoreductases...

Topological Crystalline Insulators in Transition Metal Oxides

Kargarian, Mehdi; Fiete, Gregory A.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
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Topological crystalline insulators (TCI) possess electronic states protected by crystal symmetries, rather than time-reversal symmetry. We show that the transition metal oxides with heavy transition metals are able to support nontrivial band topology resulting from mirror symmetry of the lattice. As an example, we consider pyrochlore oxides of the form A$_2$M$_2$O$_7$. As a function of spin-orbit coupling strength, we find two $Z_2$ topological insulator phases can be distinguished from each other by their mirror Chern numbers, each of which indicates a different TCI. We also derive an effective $\bf{k\cdot p}$ Hamiltonian, similar to the model introduced for $\mathrm{Pb_{1-x}Sn_{x}Te}$, and discuss the effect of an on-site Hubbard interaction on the topological crystalline insulator phase using slave-rotor mean-field theory, which predicts new classes of topological quantum spin liquids.; Comment: 5 pages, 3 figures

Site preference of transition-metal elements in L10-TiAl: A first-principles study

Jiang, Chao
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 16/04/2008
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The site preference of 3d (Ti-Cu), 4d (Zr-Ag) and 5d (Hf-Au) transition-metal elements in L10 TiAl is studied using a combination of first-principles supercell calculations and the statistical mechanical Wagner-Schottky model. Our key finding is that both temperature and alloy stoichiometry can strongly affect the site occupancy behavior of ternary alloying elements in L10 TiAl. We further predict that the tendency of transition metals to occupy the Al sites in TiAl increases with increasing d-electron number along a series.

Role of local geometry in spin and orbital structure of transition metal compounds

Khomskii, D. I.; Kugel, K. I.; Sboychakov, A. O.; Streltsov, S. V.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 19/10/2015
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We analyze the role of local geometry in the spin and orbital interaction in transition metal compounds with orbital degeneracy. We stress that the tendency observed for the most studied case (transition metals in O$_6$ octahedra with one common oxygen -- common corner of neighboring octahedra and with $\sim 180^{\circ}$ metal--oxygen--metal bonds), that ferro-orbital ordering renders antiferro-spin coupling, and, {\it vice versa}, antiferro-orbitals give ferro-spin ordering, is not valid in general case, in particular for octahedra with common edge and with $\sim 90^{\circ}$ M--O--M bonds. Special attention is paid to the ``third case'', neighboring octahedra with common face (three common oxygens) -- the case practically not considered until now, although there are many real systems with this geometry. Interestingly enough, the spin--orbital exchange in this case turns out to be to be simpler and more symmetric than in the first two cases. We also consider, which form the effective exchange takes for different geometries in case of strong spin--orbit coupling.; Comment: 31 pages, 9 figures, submitted to JETP

First examples of stable transition metal complexes of an all-metal antiaromatic molecule (Al$_4$Li$_4$)

Datta, Ayan; Pati, Swapan K
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 20/09/2004
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We propose new methodologies for stabilizing all-metal antiaromatic clusters like: Al$_{4}$Li$_{4}$. We demonstrate that these all-metal species can be stabilized by complexation with 3d-transition metals very similar to its organic counterpart, C$_4$H$_4$. Complexation to transition metal ions reduce the frontier orbital energies and introduces aromaticity. We consider a series of such complexes [$\eta$$^4$(Al$_4$Li$_4$)-Fe(CO)$_3$, $\eta$$^2$$\sigma$$^2$(Al$_4$Li$_4$)-Ni and (Al$_{4}$Li$_{4}$)$_{2}$Ni] and make a comparison between the all-metal species and the organometallic compounds to prove conclusively our theory. Fragmentation energy analysis as well as NICS support similar mechanism of complexation induced stability in these all-metal molecules.

Electronic structure and magnetism of transition metal doped Zn12O12 clusters: Role of defects

Ganguli, Nirmal; Dasgupta, Indra; Sanyal, Biplab
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 02/12/2010
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We present a comprehensive study of the energetics and magnetic properties of ZnO clusters doped with 3d transition metals (TM) using ab initio density functional calculations in the framework of generalized gradient approximation + Hubbard U (GGA+U) method. Our results within GGA+U for all 3d dopants except Ti indicate that antiferromagnetic interaction dominates in a neutral, defect-free cluster. Formation energies are calculated to identify the stable defects in the ZnO cluster. We have analyzed in details the role of these defects to stabilize ferromagnetism when the cluster is doped with Mn, Fe, and Co. Our calculations reveal that in the presence of charged defects the transition metal atoms residing at the surface of the cluster may have an unusual oxidation state, that plays an important role to render the cluster ferromagnetic. Defect induced magnetism in ZnO clusters without any TM dopants is also analyzed. These results on ZnO clusters may have significant contributions in the nanoengineering of defects to achieve desired ferromagnetic properties for spintronic applications.; Comment: Accepted for publication in "Journal of Applied Physics", 9 pages, 9 figures

Types of single particle symmetry breaking in transition metal oxides due to electron correlation

Wagner, Lucas K.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
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Very accurate wave functions are calculated for small transition metal oxide molecules. These wave functions are decomposed using reduced density matrices to study the underlying correlation of electrons. The correlation is primarily of left-right type between the transition metals and the oxygen atoms, which is mediated by excitations from the nominal single Slater ground state into antibonding and d-type orbitals. In a localized representation, this correlation manifests itself in a 2-electron hopping term that is off-diagonal. This term is of similar magnitude to the commonly considered Hubbard-type on-site interaction.

Spin-correlations and magnetic structure in an Fe monolayer on 5d transition metal surfaces

Simon, E.; Palotás, K.; Ujfalussy, B.; Deák, A.; Stocks, G. M.; Szunyogh, L.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
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We present a detailed first principles study on the magnetic structure of an Fe monolayer on different surfaces of 5d transition metals. We use the spin-cluster expansion technique to obtain parameters of a spin model, and predict the possible magnetic ground state of the studied systems by employing the mean field approach and in certain cases by spin dynamics calculations. We point out that the number of shells considered for the isotropic exchange interactions plays a crucial role in the determination of the magnetic ground state. In the case of Ta substrate we demonstrate that the out-of-plane relaxation of the Fe monolayer causes a transition from ferromagnetic to antiferromagnetic ground state. We examine the relative magnitude of nearest neighbour Dzyaloshinskii-Moriya (D) and isotropic (J) exchange interactions in order to get insight into the nature of magnetic pattern formations. For the Fe/Os(0001) system we calculate a very large D/J ratio, correspondingly, a spin spiral ground state. We find that, mainly through the leading isotropic exchange and Dzyaloshinskii-Moriya interactions, the inward layer relaxation substantially influences the magnetic ordering of the Fe monolayer. For the Fe/Re(0001) system characterized by large antiferromagnetic interactions we also determine the chirality of the $120^{\circ}$ N\'eel-type ground state.; Comment: 15 pages...

Calculation of thermal expansion coefficient of Fe/sub 3/Al with the addition of transition metal elements

Seletskaia, Tatiana; Muratov, Leonid; Cooper, Bernard
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 25/11/2002
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The addition of transition metal elements can significantly modify physical properties of intermetalic compounds. We studied the influence of Molybdenum and Vanadium additives on thermal expansion coefficient (CTE) of Fe/sub 3/Al and FeAl over the wide range of temperatures. The site preference of both transition metals was determined by full-potential LMTO method within the grandcanonical formalism. At low temperatures CTEs were found directly from the FP-LMTO calculations by incorporating them into the Debye model of a solid. The obtained thermal expansion for pure Fe/sub 3/Al and FeAl is within 10% of its experimentally measured values. At high temperatures we performed molecular dynamics simulations based on our many-body atomistic potentials. The parameters were fitted to reproduce the total energy of a crystal under various types of deformations obtained by FP-LMTO method and were tested with respect to different structures and vacancy formation energies. Our calculations show that addition of V decreases the CTEs of both iron-aluminides, while the addition of Mo makes Fe/sub 3/Al DO3 structure unstable.; Comment: 8 pages, 6 figures, to be presented at MRS Fall'2002

Tunable magnetization damping in transition metal ternary alloys

Ingvarsson, S.; Xiao, Gang; Parkin, S. S. P.; Koch, R. H.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 27/08/2004
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We show that magnetization damping in Permalloy, Ni80Fe20 (``Py''), can be enhanced sufficiently to reduce post-switching magnetization precession to an acceptable level by alloying with the transition metal osmium (Os). The damping increases monotonically upon raising the Os-concentration in Py, at least up to 9% of Os. Other effects of alloying with Os are suppression of magnetization and enhancement of in-plane anisotropy. Magnetization damping also increases significantly upon alloying with the five other transition metals included in this study (4d-elements: Nb, Ru, Rh; 5d-elements: Ta, Pt) but never as strongly as with Os.; Comment: 4 pages, submitted to Appl. Phys. Lett

Systematic {\it ab initio} study of the magnetic and electronic properties of all 3d transition metal linear and zigzag nanowires

Tung, J. C.; Guo, G. Y.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 07/08/2007
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It is found that all the zigzag chains except the nonmagnetic (NM) Ni and antiferromagnetic (AF) Fe chains which form a twisted two-legger ladder, look like a corner-sharing triangle ribbon, and have a lower total energy than the corresponding linear chains. All the 3d transition metals in both linear and zigzag structures have a stable or metastable ferromagnetic (FM) state. The electronic spin-polarization at the Fermi level in the FM Sc, V, Mn, Fe, Co and Ni linear chains is close to 90% or above. In the zigzag structure, the AF state is more stable than the FM state only in the Cr chain. It is found that the shape anisotropy energy may be comparable to the electronic one and always prefers the axial magnetization in both the linear and zigzag structures. In the zigzag chains, there is also a pronounced shape anisotropy in the plane perpendicular to the chain axis. Remarkably, the axial magnetic anisotropy in the FM Ni linear chain is gigantic, being ~12 meV/atom. Interestingly, there is a spin-reorientation transition in the FM Fe and Co linear chains when the chains are compressed or elongated. Large orbital magnetic moment is found in the FM Fe, Co and Ni linear chains.

Magnetic moment and magnetic anisotropy of linear and zigzag 4{\it d} and 5{\it d} transition metal nanowires: First-principles calculations

Tung, J. C.; Guo, G. Y.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 11/03/2010
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An extensive {\it ab initio} study of the physical properties of both linear and zigzag atomic chains of all 4$d$ and 5$d$ transition metals (TM) within the GGA by using the accurate PAW method, has been carried out. All the TM linear chains are found to be unstable against the corresponding zigzag structures. All the TM chains, except Nb, Ag and La, have a stable (or metastable) magnetic state in either the linear or zigzag or both structures. Magnetic states appear also in the sufficiently stretched Nb and La linear chains and in the largely compressed Y and La chains. The spin magnetic moments in the Mo, Tc, Ru, Rh, W, Re chains could be large ($\geq$1.0 $\mu_B$/atom). Structural transformation from the linear to zigzag chains could suppress the magnetism already in the linear chain, induce the magnetism in the zigzag structure, and also cause a change of the magnetic state (ferromagnetic to antiferroamgetic or vice verse). The calculations including the spin-orbit coupling reveal that the orbital moments in the Zr, Tc, Ru, Rh, Pd, Hf, Ta, W, Re, Os, Ir and Pt chains could be rather large ($\geq$0.1 $\mu_B$/atom). Importantly, large magnetic anisotropy energy ($\geq$1.0 meV/atom) is found in most of the magnetic TM chains, suggesting that these nanowires could have fascinating applications in ultrahigh density magnetic memories and hard disks. In particular...

Magnetically hindered chain formation in transition-metal break junctions

Thiess, Alexander; Mokrousov, Yuriy; Heinze, Stefan; Blügel, Stefan
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 19/11/2009
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Based on first-principles calculations, we demonstrate that magnetism impedes the formation of long chains in break junctions. We find a distinct softening of the binding energy of atomic chains due to the creation of magnetic moments that crucially reduces the probability of successful chain formation. Thereby, we are able to explain the long standing puzzle why most of the transition-metals do not assemble as long chains in break junctions and provide thus an indirect evidence that in general suspended atomic chains in transition-metal break junctions are magnetic.; Comment: 5 pages, 3 figures

Density Functional Calculations On First-Row Transition Metals

Russo, Thomas V.; Martin, Richard L.; Hay, P. Jeffrey
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 10/03/1994
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The excitation energies and ionization potentials of the atoms in the first transition series are notoriously difficult to compute accurately. Errors in calculated excitation energies can range from 1--4 eV at the Hartree-Fock level, and errors as high as 1.5eV are encountered for ionization energies. In the current work we present and discuss the results of a systematic study of the first transition series using a spin-restricted Kohn-Sham density-functional method with the gradient-corrected functionals of Becke and Lee, Yang and Parr. Ionization energies are observed to be in good agreement with experiment, with a mean absolute error of approximately 0.15eV; these results are comparable to the most accurate calculations to date, the Quadratic Configuration Interaction (QCISD(T)) calculations of Raghavachari and Trucks. Excitation energies are calculated with a mean error of approximately 0.5eV, compared with $\sim 1\mbox{eV}$ for the local density approximation and 0.1eV for QCISD(T). These gradient-corrected functionals appear to offer an attractive compromise between accuracy and computational effort.; Comment: Journal of Chemical Physics, 29, LA-UR-93-4258

Theoretical investigation on the transition metal borides with Ta3B4-type structure: a class of hard and refractory materials

Miao, Naihua; Sa, Baisheng; Zhou, Jian; Sun, Zhimei
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 06/03/2015
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Based on density functional theory, we have systematically studied the structural stability, mechanical properties and chemical bonding of the transition metal borides M3B4 (M=Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W) for the first time. All the present studied M3B4 have been demonstrated to be thermodynamically and mechanically stable. The bulk modulus, shear modulus, Young's modulus, Poisson's ratio, microhardness, Debye temperature and anisotropy have been derived for ideal polycrystalline M3B4 aggregates. In addition, the relationship between Debye temperature and microhardness has been discussed for these isostructral M3B4. Furthermore, the results of the Cauchy pressure, the ratio of bulk modulus to shear modulus, and Poisson's ratio suggest that the valence electrons of transition metals play an important role in the ductility of M3B4. The calculated total density of states for M3B4 indicates that all these borides display a metallic conductivity. By analyzing the electron localization function, we show that the improvement of the ductility in these M3B4 might attribute to the decrease of their angular bonding character.