Row Transition Metal Atoms Research Articles

Partial substitution of the Mn atoms in CaMnO3by first row transition metal atoms: effect on oxygen vacancy formation

AbstractSustainable hydrogen production is one of the most challenging topics in modern energy economics. Electrochemical and thermal splitting of water are promising techniques, but are highly energy demanding. Efficient hydrogen evolution reaction catalysts can play a key role to lower the electrolysis potential and to make water splitting more feasible. Among other perovskites, CaMnO3has been identified as efficient electrode material due to its relatively high oxygen vacancy concentrations at elevated temperatures. But this compound needs to be further improved for technical use. In this study, the effect of Mn substitution in CaMnO3by 3d metals M = Sc, Ti, V, Cr, Fe, Co, Ni, Cu and Zn on the oxygen vacancy formation energy is investigated theoretically at DFT level. Vacancy formation energies, enthalpies and free enthalpies are calculated with a combination of hybrid and GGA density functionals. Configuration entropy is taken into account by calculating all possible configurations of M and oxygen vacancy site in supercell models. The calculated oxygen vacancy formation energies are strongly affected by Mn/M substitution, the most promising heteroelement being Cu. CaMn0.875Cu0.125O2.875and CaMn0.875Cu0.125O2.75are in equilibrium at 537 K, compared to 1231 K for CaMnO2.875/CaMnO2.75and 1350 K for CaMnO3/CaMnO2.875.

Estimations of Fe0/-1-N2 interaction energies of iron(0)-dicarbene and its reduced analogue by EDA-NOCV analyses: crucial steps in dinitrogen activation under mild conditions.

Metal complexes containing low valence iron atoms are often experimentally observed to bind with the dinitrogen (N2) molecule. This phenomenon has attracted the attention of industrialists, chemists and bio-chemists since these N2-bonded iron complexes can produce ammonia under suitable chemical or electrochemical conditions. The higher binding affinity of the Fe-atom towards N2 is a bit ‘mysterious’ compared to that of the other first row transition metal atoms. Fine powders of α-Fe0 are even part of industrial ammonia production (Haber–Bosch process) which operates at high temperature and high pressure. Herein, we report the EDA-NOCV analyses of the previously reported dinitrogen-bonded neutral molecular complex (cAACR)2Fe0–N2 (1) and mono-anionic complex (cAACR)2Fe−1–N2 (2) to give deeper insight of the Fe–N2 interacting orbitals and corresponding pairwise intrinsic interaction energies (cAACR = cyclic alkyl(amino) carbene; R = Dipp or Me). The Fe0 atom of 1 prefers to accept electron densities from N2via σ-donation while the comparatively electron rich Fe−1 centre of 2 donates electron densities to N2via π-backdonation. However, major stability due to the formation of an Fe–N2 bond arises due to Fe → N2 π-backdonation in both 1 and 2. The cAACR ligands act as a charge reservoir around the Fe centre. The electron densities drift away from cAAC ligands during the binding of N2 molecules mostly via π-backdonation. EDA-NOCV analysis suggests that N2 is a stronger π-acceptor rather than a σ-donor.

First-principles study of adsorption of 3d and 4d transition metal atoms on aluminene

Using the first principles calculations based on density functional theory, we study the structural, electronic, and magnetic properties of composite systems of first and second row transition metal (TM) atoms adsorbed aluminene. This study aims to probe the stability of the system as a result of adsorption of TM atoms on the two-dimensional (2D) surface of planar aluminene. We also study the same for two other well-known 2D single-atom-monolayer (SAML) structures, namely, silicene and arsenene, and we compare the results with those of aluminene. The TM atoms are adsorbed on these 2DSAML structures in three different crystallographically inequivalent sites, namely, the hollow (H), the top (T) and the bridge (Br) sites. The stability of the composite systems, which are a combination of a 2D monolayer structure and a TM atom each, is systematically analyzed in terms of binding energy per atom, for eighteen different TM atoms, with 3 d or 4 d electrons in their valence shell. We compare the results of optimized geometry and stability of the above-mentioned 2D structures when the TM atoms are adsorbed at the H, T and Br sites. We further analyze the interaction energy to discuss about and compare the ease of adsorption of various TM atoms on the three different 2DSAML structures. Interestingly, in most of the cases, adsorption of TM atoms typically yields higher interaction energy in case of aluminene when compared with silicene and arsenene systems. Among all the TM adatoms, few atoms have been identified to have higher interaction energy compared to the others. From our calculations of the density of states and band structure of the composite systems, almost all the composite systems have been found to be metallic, except the composite system of Sc atom and silicene, which exhibits a half-metallic state. Further, though all the three 2DSAML structures are non-magnetic in pristine form, a few composite systems are found to be magnetic in nature.

Carbon-Hydrogen Activation in Zerovalent Bis(1,5-cyclooctadiene) Complexes of the First Row Transition Metals: A Theoretical Study.

Stepwise interaction of first row transition metal atoms with 1,5-cyclooctadiene to give (C8H12)2M complexes is studied using the M06-L/DZP density functional method. The experimentally known (C8H12)2Ni is the thermodynamically most favorable complex, with a predicted geometry consistent with its experimental structure as determined by X-ray crystallography. The other transition metal atoms from scandium to zinc also interact exothermically with 1,5-cyclooctadiene to give (C8H12)2M derivatives, but these exhibit lower symmetry than the S4 symmetry exhibited by (C8H12)2Ni. Carbon-hydrogen activation of CH2 groups in a C8H12 ligand is predicted for most systems. Thus, conversion of (η2,2-C8H12)2M to (η3,2-C8H11)(η2,1-C8H13)M, through a hydride intermediate (η3,2-C8H11)(η2,2-C8H12)MH, is predicted for scandium, vanadium, chromium, manganese, and cobalt. For titanium with a low-lying empty orbital, further C-H activation through a hydride intermediate (η6-C8H10)(η2,1-C8H13)TiH is predicted, leading ultimately to (η6-C8H10)(η1,1-C8H14)Ti, in which the hexahapto η6-C8H10 ligand is shown by NICS to be aromatic. These two C-H activation processes on a titanium center represent the dehydrogenation of 1,5-cyclooctadiene to 1,3,5-cyclooctatriene with the second 1,5-cyclooctadiene ligand as the hydrogen acceptor. For zinc C-H activation terminates at (η1-C8H11)(C8H12)ZnH, which has a C-Zn-H three-center bond. No energetically favorable C-H activation processes are predicted for the iron, nickel, and copper (η2,2-C8H12)2M derivatives.

First row transition metal atoms embedded in multivacancies in a rippled graphene system

Ab-initio calculations based on density functional theory (DFT) have been performed to study systems where a first row transition metal atom is embedded in a rippled graphene due to the existence of an 8-order multivacancy. In addition to these cases, also the inclusion of a zinc atom, with a 3d10 electron configuration, was also studied. Structural distortions and magnetic response for each system were studied. A correlation was found for the magnitude of the rippling and the distortion in the vacancy. Variation in the trends was found for Cu and Zn cases, which were explained on the basis of the filling of the 3dx2-y2 orbital. All the systems exhibit lower magnetic moment in comparison to the metal-less system. The quenching of the magnetic moment due to the carbon atoms in the vacancy is observed for Sc and Cu.

Auxiliary-field quantum Monte Carlo calculations of the molybdenum dimer.

Chemical accuracy is difficult to achieve for systems with transition metal atoms. Third row transition metal atoms are particularly challenging due to strong electron-electron correlation in localized d-orbitals. The Cr2 molecule is an outstanding example, which we previously treated with highly accurate auxiliary-field quantum Monte Carlo (AFQMC) calculations [W. Purwanto et al., J. Chem. Phys. 142, 064302 (2015)]. Somewhat surprisingly, computational description of the isoelectronic Mo2 dimer has also, to date, been scattered and less than satisfactory. We present high-level theoretical benchmarks of the Mo2 singlet ground state (X(1)Σg (+)) and first triplet excited state (a(3)Σu (+)), using the phaseless AFQMC calculations. Extrapolation to the complete basis set limit is performed. Excellent agreement with experimental spectroscopic constants is obtained. We also present a comparison of the correlation effects in Cr2 and Mo2.

Communication: Multireference equation of motion coupled cluster: A transform and diagonalize approach to electronic structure

The novel multireference equation-of-motion coupled-cluster (MREOM-CC) approaches provide versatile and accurate access to a large number of electronic states. The methods proceed by a sequence of many-body similarity transformations and a subsequent diagonalization of the transformed Hamiltonian over a compact subspace. The transformed Hamiltonian is a connected entity and preserves spin- and spatial symmetry properties of the original Hamiltonian, but is no longer Hermitean. The final diagonalization spaces are defined in terms of a complete active space (CAS) and limited excitations (1h, 1p, 2h, …) out of the CAS. The methods are invariant to rotations of orbitals within their respective subspaces (inactive, active, external). Applications to first row transition metal atoms (Cr, Mn, and Fe) are presented yielding results for up to 524 electronic states (for Cr) with an rms error compared to experiment of about 0.05 eV. The accuracy of the MREOM family of methods is closely related to its favorable extensivity properties as illustrated by calculations on the O2-O2 dimer. The computational costs of the transformation steps in MREOM are comparable to those of closed-shell Coupled Cluster Singles and Doubles (CCSD) approach.

Metallacarboranes: Toward Promising Hydrogen Storage Metal Organic Frameworks

Using first principles calculations, we show the high hydrogen storage capacity of metallacarboranes, where the transition metal (TM) atoms can bind up to 5 H(2)-molecules. The average binding energy of ∼0.3 eV/H favorably lies within the reversible adsorption range. Among the first row TM atoms, Sc and Ti are found to be the optimum in maximizing the H(2) storage (∼8 wt %) on the metallacarborane cluster. Being an integral part of the cage, TMs do not suffer from the aggregation problem, which has been the biggest hurdle for the success of TM-decorated graphitic materials for hydrogen storage. Furthermore, the presence of carbon atom in the cages permits linking the metallacarboranes to form metal organic frameworks, which are thus able to adsorb hydrogen via Kubas interaction, in addition to van der Waals physisorption.

The Shell Structure of Atoms

The total electron density distribution of an isolated atom or an atom in a molecule does not reveal an atomic shell structure. Many localization functions, such as the radial averaged electron density, the Laplacian of the electron density, or the electron localization function have been proposed to visualize and analyze the shell structure of atoms. It was found that for light main group elements the correct number of shells is revealed by such functions. Later it was recognized that for heavy main group elements and for transition metals many of these diagnostic tools fail to reveal the full set of electronic shells as expected from the periodic table. In this work we focus on the radial structure of isolated atoms as revealed by the Laplacian of the electron density. We will demonstrate that it is the nodal structure of the orbitals of the inner shells which is responsible for the diminishing of at least one valence shell of third row transition metal atoms. Particular attention is paid to the effect of different electronic configurations on the shell structure of atoms and the question if the changes observed in the Laplacian of the radial density are sufficiently large for experimental studies on the topology of the electron density. Our presentation is as general as possible and, hence, employs a fully relativistic, i.e., four-component picture and a multiconfigurational ansatz for the wave function, which is thus valid for the whole periodic table of elements.

Interconfigurational energies and ionization potentials: Test of a correlation energy functional

A recently proposed correlation energy functional [S. Ragot, P. Cortona, J. Chem. Phys. 121 (2004) 7671] has been tested by calculating the interconfigurational energies and the ionization potentials of the third row transition metal atoms and the ionization potentials of a selected set of molecules. A great and almost systematic improvement, with respect to the results of standard local-density calculations, has been found for the ionization potentials: the accuracy of the new approximation is similar to that of the B3LYP hybrid functional. The interconfigurational energies are also improved, but the improvement is smaller than in the case of the ionization potentials. This is due to the fact that the local-density results largely benefit from the compensation of the errors. Calculations performed correcting the exchange-correlation functional for the self-interaction effects (by the D-SIC method) have also been done for the transition metal atoms: a further general improvement of the results is found.

New Relativistic ANO Basis Sets for Transition Metal Atoms

New basis sets of the atomic natural orbital (ANO) type have been developed for the first, second, and third row transition metal atoms. The ANOs have been obtained from the average density matrix of the ground and lowest excited states of the atom, the positive and negative ions, and the atom in an electric field. Scalar relativistic effects are included through the use of a Douglas-Kroll-Hess Hamiltonian. Multiconfigurational wave functions have been used with dynamic correlation included using second order perturbation theory (CASSCF/CASPT2). The basis sets are applied in calculations of ionization energies, electron affinities, and excitation energies for all atoms and polarizabilities for spherically symmetric atoms. These calculations include spin-orbit coupling using a variation-perturbation approach. Computed ionization energies have an accuracy better than 0.2 eV in most cases. The accuracy of computed electron affinities is the same except in cases where the experimental values are smaller than 0.5 eV. Accurate results are obtained for the polarizabilities of atoms with spherical symmetry. Multiplet levels are presented for some of the third row transition metals.

Structure of Nb/Ta layered ternary tellurides containing first row transition metal atoms

Structure of Nb/Ta layered ternary tellurides containing first row transition metal atoms

Structure of Nb/Ta layered ternary tellurides containing first row transition metal atoms

A summary of research on the structure of Nb/Ta layered tellurides in the past period is reported. 14 compounds, which have been structurally characterized by X-ray diffraction work, are presented according to their structural features. The first two compounds, Nb2CrTe4 and Nb2Cu1.48Te4, are characterized in that the ternary atoms are inserted in the different layers from the Nb atoms. While in the other compounds, both kinds of metal atoms are inserted in the same layer. The six compounds with formula M2M¢2Te4 (M = Nb/Ta; M¢ = Ni, Co, Fe) are characterized in that their structure can be described as construction by using cluster units “M2M¢2Te10” as building blocks. In the two metal-rich compounds, TaCo2Te2 and TaNi2Te2, Ta atom has a distorted mono-capped pentagonal prism configuration. The structure of TaFeTe3, TaNi2Te3 and NbNi2.34Te3 can be described as building by the arrangement of double octahedral chains (DOC). In this connection, a selenide Ta2Ni2Se5 is also included by using the second type of DOC arrangement as the basis to build the structure.

Matrix isolation and density functional studies of novel transition metal complexes. NO + Fe, Co, Ni, Cu, and Zn in argon matrices

We have undertaken a study of the interaction and reaction of nitric oxide with the first row transition metal atoms. Such studies are applicable to catalytic processes (i.e. NO x reduction) as well as direct corrosion of metals. NO and transition metal vapors were trapped in cryogenic argon matrices and their interactions were probed using FTIR spectroscopy. A number of complexes of varying stoichiometry were detected; however, there was no detectable interaction between NO and Zn. Density functional studies using several functional combinations were performed to determine structures, vibrational frequencies, and binding energies of the MNO complexes. There is no obvious discernible trend in the binding properties across the row. DFT calculations do support the Zn NO results by predicting a negative binding energy for the ZnNO complex. A comparison with MCO complexes is offered.

Why do the second row transition metal atoms prefer 5s14dm+1 to 5s24dm?

Numerical Hartree-Fock (NHF) calculations have been performed for 332 ground and low-lying excited states of the fifth period atoms Rb through Xe, with our special interest in the states arising from the 5s 24d m , 5s 14d m +1, and 5s 0 4d m +2 configurations of the second row transition metal atoms. Among various properties, orbital energies and mean values ofr of the outermost orbitals of each symmetry are presented as well as total energies. It is discussed in some detail why the second row transition metal atoms have a tendency to prefers 1 d m +1 as the ground configuration in contrast to the preferreds 2 d m configuration in the first row transition metal atoms. Our systematic NHF computations reported in this and the previous papers conclude that the Hartree-Fock method correctly predicts the experimental ground state of the atoms He through Xe with the sole exception for Zr.

Second row transition metal mixed hydride-halide triatomic molecules

The ground state structures and bond energies have been obtained for the triatomic MHX systems where M is the entire sequence of second row transition metal atoms and X is a halide. The most interesting results of this study appear when these systems are compared to the triatomic MH2 and MX2 systems. It turns out that the structure of the MHX systems are quite similar to the corresponding MH2 systems in general. Most of the MHX systems to the right thus have bent low-spin ground states, indicating large covalent contributions to the bonding. RuHX is a special case and has a high-spin linear ground state. For the systems to the left ionicity dominates the bonding. An important result, with implications for halide ligand effects on carbonyl and olefin insertion into M-H and M-R bonds, is that the M-H bonds for the systems to the right have a different character and are significantly weaker for the MHX than for the MH2 systems. A similar effect is noted when the M-Cl bond strengths of MCl2 are compared to the ones in MHCl. Both these effects can be explained by a more cationic metal with mores 0-state character when halide ligands are present.

The bonding in second row transition metal dihydrides, difluorides and dichlorides

The dihydrides, the difluorides and the dichlorides of the second row transition metal atoms from yttrium to palladium have been studied with methods including electron correlation of all valence electrons. Comparisons are made to the previously studied corresponding diatomic systems. It is found that the general trends of the binding energies of the second hydride and halide remain the same as in the diatomic hydrides and halides. The second ligand binding energies for the dihalides thus vary much more than for the dihydrides. This is due to important attractive effects between the halide lone-pairs and empty 4d-orbitals to the left and strong repulsions towards occupied 4d-orbitals to the right. For some systems the second ligand binds much more than the first ligand, as for RuF2 where the difference is 34.3 kcal/mol, whereas for other systems the reverse is true, as for PdCl2 where the first ligand binds more than the second with 20.4 kcal/mol. The results can be explained by strong ligand field effects and differences in the atomic spectra.

The activation of the C-H bond in acetylene by second row transition metal atoms

The C-H activation reaction of acetylene by second row transition metal atoms has been studied including electron correlation of all valence electrons. Binding energies have been computed for both π-coordinated complexes and C-H insertion products. It is found that for most atoms the π-coordinated complexes are thermodynamically favoured, just as in the case for the corresponding ethylene reaction. The barrier height for the C-H insertion increases from acetylene to ethylene and to methane. This is in line with the experimental finding that there should be an inverse relation between C-H bond strengths and the difficulty to activate these bonds. To explain the detailed differences between the C-H activation of acetylene and ethylene, the interaction with two, rather than one, π- and π*-orbitals for acetylene is of key importance. The barrier height for the acetylene reaction increases significantly between niobium and molybdenum going to the right in the periodic table, just as for all oxidative addition reactions previously studied. The origin of this increase is that noibium has one empty 4d-orbital but for molybdenum all 4d-orbitals are occupied. Rhodium has the lowest barrier for C-H activation for all systems studied.

Theoretical study of the activation of the nitrogen-hydrogen bond in ammonia by second row transition metal atoms

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTTheoretical study of the activation of the nitrogen-hydrogen bond in ammonia by second row transition metal atomsMargareta R. A. Blomberg, Per E. M. Siegbahn, and Mats SvenssonCite this: Inorg. Chem. 1993, 32, 20, 4218–4225Publication Date (Print):September 1, 1993Publication History Published online1 May 2002Published inissue 1 September 1993https://doi.org/10.1021/ic00072a012RIGHTS & PERMISSIONSArticle Views271Altmetric-Citations52LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) Get e-Alerts Get e-Alerts

The olefin insertion reaction into a metal-hydrogen bond for second-row transition-metal atoms, including the effects of covalent ligands

The olefin insertion into a transition metal-hydrogen bond has been studied for the entire sequence of second row transition metal atoms. Energies including correlation of all valence electrons have been obtained for geometry optimized structures of the reactants, of the π-bonded olefin complexes, of the transition states, and finally of the product metal-alkyl systems. To study the effects of covalent ligands the calculations were repeated with one and two additional hydride ligands