Electronic structure and bonding of cobalt monoxide, CoO, and its ions CoO+ and CoO-: an ab initio study

We present a systematic and high-level ab initio study of CoO and its ions, CoO(+) and CoO(-). Employing variational multireference (MRCI) and single-reference coupled-cluster methods combined with basis sets of quintuple quality, we have calculated 50, 31, and 7 bound states for CoO, CoO(+), and CoO(-), respectively. For all these states, complete potential energy curves have been constructed at the MRCI level of theory, whereas for a few low-lying states core subvalence and scalar relativistic effects have been taken into account. We report energetics, spectroscopic parameters, dipole moments, and spin-orbit coupling constants. The ground states of CoO, CoO(+), and CoO(-) are X(4)Δ, X(5)Δ, and X(5)Δ, respectively, the latter established for the first time. The CoO is quite ionic with a Co to O Mulliken charge transfer of ~0.6 electrons and a dipole moment μ(X(4)Δ) = 4.5 ± 0.1 D. The overall agreement between theory and experiment is good, but there are also important deviations. Despite the seeming simplicity of these diatomic species, reliable results can only be obtained at a high level of theory.     

Tracer-diffusion of Zn2+ ions in cobalt sulphate

Tracer-diffusion of Zn²⁺ ions in the presence of CoSO₄ is studied at 25°C in 1% agar gel over a concentration range of 10^–5 to 0.25M using a zone-diffusion technique. The deviations observed between experimental and theoretical values of diffusion coefficients are explained by considering different types of interactions occurring in the ion-gel-water system. Further, study of the obstruction, effect in the diffusion of Zn²⁺ ions at different concentrations of CoSO₄ reveals that the -value decreases with increasing concentration of the electrolyte. This observation is accounted on the basis of competitive hydration between ions and agar molecules.     

First principles study of magnetism in nanographenes

Magnetism in nanographenes [also known as polycyclic aromatic hydrocarbons (PAHs)] is studied with first principles density functional calculations. We find that an antiferromagnetic (AFM) phase appears as the PAH reaches a certain size. This AFM phase in PAHs has the same origin as the one in infinitely long zigzag-edged graphene nanoribbons, namely, from the localized electronic state at the zigzag edge. The smallest PAH still having an AFM ground state is identified. With increased length of the zigzag edge, PAHs approach an infinitely long ribbon in terms of (1) the energetic ordering and difference among the AFM, ferromagnetic, and nonmagnetic phases and (2) the average local magnetic moment at the zigzag edges. These PAHs serve as ideal targets for chemical synthesis of nanographenes that possess magnetic properties. Moreover, our calculations support the interpretation that experimentally observed magnetism in activated carbon fibers originates from the zigzag edges of the nanographenes.     

First principles study on the structural,electronic, and optical properties of Sc-doped AlN

Based on the density functional pseudo-potential method, the structural properties, the band structure, the density of states and the optical properties of the pure and Sc-doped AlN are calculated. The calculation results indicate that the defect of Sc(Al) exists steadily with a certain solubility in the doped system. Sc substitution of the Al site induces effective reduction of the band gap of AlN and the band gap being continuously reduced when increasing Sc concentrations. The existence of the strong hybridization between Sc 3d and N 2p indicates the transport of electrons from Sc atoms to N atoms. Besides, it is shown that the insertion of Sc atom leads to redshift of the optical absorption edge. The intensity of both the imaginary part of the dielectric function and the optical absorption of Al_{1 ? x} Sc _x N are found to decrease with increasing Sc concentrations in the low energy range.     

First principles study of small palladium cluster growth and isomerization

Structures and physical properties of small palladium clusters Pd_n up to n = 15 and several selected larger clusters were studied using density functional theory under the generalized gradient approximation. It was found that small Pd_n clusters begin to grow 3‐dimensionally at n = 4 and evolve into symmetric geometric configurations, such as icosahedral and fcc‐like, near n = 15. Several isomers with nearly degenerate average binding energies were found to coexist and the physical properties of these clusters were calculated. For several selected isomers, relatively moderate energy barriers for structural interchange for a given cluster size were found, implying that isomerization could readily occur under ambient conditions. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Synthesis and hydride transfer reactions of cobalt and nickel hydride complexes to BX3 compounds

Hydrides of numerous transition metal complexes can be generated by the heterolytic cleavage of H(2) gas such that they offer alternatives to using main group hydrides in the regeneration of ammonia borane, a compound that has been intensely studied for hydrogen storage applications. Previously, we reported that HRh(dmpe)(2) (dmpe = 1,2-bis(dimethylphosphinoethane)) was capable of reducing a variety of BX(3) compounds having a hydride affinity (HA) greater than or equal to the HA of BEt(3). This study examines the reactivity of less expensive cobalt and nickel hydride complexes, HCo(dmpe)(2) and [HNi(dmpe)(2)](+), to form B-H bonds. The hydride donor abilities (ΔG(H(-))°) of HCo(dmpe)(2) and [HNi(dmpe)(2)](+) were positioned on a previously established scale in acetonitrile that is cross-referenced with calculated HAs of BX(3) compounds. The collective data guided our selection of BX(3) compounds to investigate and aided our analysis of factors that determine favorability of hydride transfer. HCo(dmpe)(2) was observed to transfer H(-) to BX(3) compounds with X = H, OC(6)F(5), and SPh. The reaction with B(SPh)(3) is accompanied by the formation of dmpe-(BH(3))(2) and dmpe-(BH(2)(SPh))(2) products that follow from a reduction of multiple B-SPh bonds and a loss of dmpe ligands from cobalt. Reactions between HCo(dmpe)(2) and B(SPh)(3) in the presence of triethylamine result in the formation of Et(3)N-BH(2)SPh and Et(3)N-BH(3) with no loss of a dmpe ligand. Reactions of the cationic complex [HNi(dmpe)(2)](+) with B(SPh)(3) under analogous conditions give Et(3)N-BH(2)SPh as the final product along with the nickel-thiolate complex [Ni(dmpe)(2)(SPh)](+). The synthesis and characterization of HCo(dedpe)(2) (dedpe = Et(2)PCH(2)CH(2)PPh(2)) from H(2) and a base is also discussed, including the formation of an uncommon trans dihydride species, trans-[(H)(2)Co(dedpe)(2)][BF(4)].     

First principles investigation of the electronic structure of the iron carbide cation, FeC+

We have studied 40 states of the diatomic iron carbide cation FeC(+) by multireference methods coupled with relatively large basis sets. For most of the states, we have constructed complete potential energy curves, reporting dissociation energies, usual spectroscopic parameters, and bonding mechanisms for the lowest of the studied states. The ground state is of (2)Delta symmetry, with the first excited state (a(4)Sigma(-)) lying 18 kcal/mol higher. The X(2)Delta state displays a triple-bond character, with an estimated D(0) value of 104 kcal/mol with respect to the adiabatic products or 87 kcal/mol with respect to the ground-state fragments.     

First principles study of photostability within hydrogen-bonded amino acids

The photochemistry and photophysics of a two-glycine minimal model is studied at the CASPT2//CASSCF level of theory. Different photoinduced processes are discussed, on the basis of the calculated minimum energy paths and the characterization of the electronic state crossings. Two main processes could provide UV-photostability to the hydrogen-bonded peptide system: (i) forward-backward photoinduced electron/proton transfer involving the H in the hydrogen bond, (ii) singlet-singlet energy transfer between two amino acids, providing ultrafast population of the low-energy n,π* state.     

First principles study of the structure,electronic state,and stability of CmN2 clusters

Geometric and electronic properties of C_mN₂ (m = 1–14) clusters have been investigated by density functional theory using the hybrid B3LYP functional and the 6‐311G(d) basis set. Harmonic frequencies for these clusters are given to aid in the characterization of the ground states. These results show that C_mN₂ (m = 1–14) clusters form linear structures with D_∞h symmetry. Two N atoms favor to bond at ends in linear isomers. The chains with odd m have triplet ground states whereas the ones with even m have singlet ground states. The calculated HOMO–LUMO gaps and ionization potentials all show that the C_mN₂ (m = 1–14) clusters with even m are more stable than those with odd m, which is consistent with the observed even–odd alternation of the time‐of‐flight signal intensities. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Ab initio investigations of the electronic and magnetic structures of CoH and CoH2

First principles investigation of the structural, electronic and magnetic properties study of cobalt and the hydrides CoH_x (x = 1, 2) show significant volume expansion effect versus Co–H bonding. As hydrogen is incorporated in the cobalt lattice, the density of states undergoes gradual modifications within the valence band and particularly near the Fermi level. A resulting strong reduction of magnetization characterizes the dihydride whereas the monohydride is revealed as a strong ferromagnet, like Co.     

First principles study of dielectric and vibrational properties of pyrochlore hafnates

We present the first principles study of dielectric, and vibrational properties of pyrochlore hafnates, RE₂Hf₂O₇ (RE = La, Nd, Sm, Eu, Gd, Tb) in their cubic structure using density functional theory and density functional perturbation theory, incorporating ultrasoft pseudopotentials and plane waves. The full set of zone centre Raman and infra red frequencies, along with the contribution of all infra red active modes towards dielectric constant and mode effective charges is calculated and discussed. In all the studied hafnates dominant contribution to static dielectric constant is found to come from three infrared active phonons around 125 cm⁻¹, 170 cm⁻¹ and 305 cm⁻¹. In addition, calculated structural properties are in very good agreement with experimental results. Mixed ionic-covalent bonding is found from the analysis of dynamical charges, static charges and density of states.     

Sorption of nickel and cobalt ions onto cobalt and nickel ferrites

The corrosion of the metal parts in the primary circuit of pressurized water reactors leads to the release of colloidal particles (NiFe(2)O(4), CoFe(2)O(4), NiO, Ni...) and ionic species (Co, Ni, Cr...). Particles can interact with ionic species in the primary medium, contributing to their transport and to their deposition onto surfaces outside the neutron flux generating radioactive contamination. Sorption and zetametry experiments at 25 °C were performed on the Ni(2+)/CoFe(2)O(4) and Co(2+)/NiFe(2)O(4) systems in order to determine the behaviour of corrosion products in the fluid of the primary circuit. Sorption appears as surface complexation starting from pH 6 and is followed by precipitation of hydroxide above pH 7.5. Complexation and solubility constants were obtained from the modelling of sorption curves. The two oxide systems present a very similar sorption behaviour, but some differences, due to their different isoelectric points, could be observed on zetametric measurements.     

First principles investigation of chromium carbide, CrC

We have investigated the electronic structure of 14 states of the experimentally unknown diatomic molecule chromium carbide, CrC, using standard multireference configuration interaction methods and high quality basis sets. We report potential curves, binding energies, and a number of spectroscopic parameters. The ground state of CrC, X 3Sigma-, displays triple-bond character with a binding energy of D(e)=89 kcal/mol and an internuclear separation of r(e)=1.63 A. The first excited state (1 5Sigma-) lies 9.2 kcal/mol higher. All the states studied are fairly ionic, featuring an electron transfer of 0.3-0.5e- from the metal atom to the carbon atom.     

First principles molecular dynamics study of filled ice hydrogen hydrate

We investigated structural changes, phase diagram, and vibrational properties of hydrogen hydrate in filled-ice phase C(2) by using first principles molecular dynamics simulation. It was found that the experimentally reported "cubic" structure is unstable at low temperature and∕or high pressure: The "cubic" structure reflects the symmetry at high (room) temperature where the hydrogen bond network is disordered and the hydrogen molecules are orientationally disordered due to thermal rotation. In this sense, the "cubic" symmetry would definitely be lowered at low temperature where the hydrogen bond network and the hydrogen molecules are expected to be ordered. At room temperature and below 30 GPa, it is the thermal effects that play an essential role in stabilizing the structure in "cubic" symmetry. Above 60 GPa, the hydrogen bonds in the framework would be symmetrized and the hydrogen bond order-disorder transition would disappear. These results also suggest the phase behavior of other filled-ice hydrates. In the case of rare gas hydrate, there would be no guest molecules' rotation-nonrotation transition since the guest molecules keep their spherical symmetry at any temperature. On the contrary methane hydrate MH-III would show complex transitions due to the lower symmetry of the guest molecule. These results would encourage further experimental studies, especially nuclear magnetic resonance spectroscopy and neutron scattering, on the phases of filled-ice hydrates at high pressures and∕or low temperatures.     

First principles study of structure and properties of La- and Mn-modified BiFeO3

First principles calculations have been performed to study the effects of the La³⁺ and Mn³⁺ substitutions in the multiferroic BiFeO₃. The real compositions Bi_1−xLa_xFeO₃ and BiFe_1−xMn_xO₃ with x = 0.0, 0.1, 0.2, 0.3 were modeled by substitution of one, two and three Bi³⁺ or Fe³⁺ by La³⁺ or Mn³⁺ in the orthorhombic BiFeO₃ structure, respectively. Density functional theory within the generalized gradient approximation with Hubbard correction of Dudarev (GGA + U) and plane wave pseudo-potential approach has been used to track the changes that occur in the structural parameters, electronic structure, magnetic, optical and polarization properties of the modified BiFeO₃. The substitution of one Bi³⁺ with La³⁺ increases the band gap energy whereas the augmentation of La³⁺ substitutes decreases it. The substitutions of Fe³⁺ with Mn³⁺ do not change the band gap energy. The calculations predicted larger polarization of the modified BiFeO₃, antiferromagnetism for Bi_1−xLa_xFeO₃ and small ferrimagnetism for BiFe_1−xMn_xO₃. Better multiferroic properties are expected for BiFe_1−xMn_xO₃ materials (x = 0.1, 0.2) due to the increasing polarization and ferrimagnetic behavior. The optical properties were estimated by the calculated imaginary and real parts of the dielectric function. The increase of La³⁺ and Mn³⁺ substitutes lead to lower absorption intensity at energy range 2–7 eV.     

Theoretical study of the structurally nonrigid ions LiP 2 + , HP 2 + , and H3P 2 +

New Chemical Problems Institute, USSR Academy of Sciences. Translated from Zhurnal Strukturnoi Khimii, Vol. 30, No. 4, pp. 164–167, July–August, 1989.     

Theoretical study of hydrated Be2+ ions

Hydrated Be²⁺ ions [Be(H₂O)_n]²⁺, n = 1−4 and 6, were examined theoretically. The structure of the hydrated ions was determined and the hydration energy estimated with and without electron correlation. The bond between the Be²⁺ ion and the oxygen of water is very strong and has the nature of a dative bond. The non-additivity of the binding energy is so profound that without taking it into account the structure and dynamics of Be²⁺ ions cannot be explained. The hydration number in water is found to be 4. The fifth and sixth water molecules prefer forming the second coordination shell to the Be²⁺ ion. The result is in agreement with X-ray analysis of the aqueous solution, but not with a recent molecular dynamics simulation. In addition, the harmonic vibrational frequencies for the complexes are evaluated and compared with some experiments.     

Efficient electrocatalytic hydrogen production from H+ ions using specially designed boron-capped cobalt clathrochelates

Specially designed hexachlorine-containing cobalt(II) tris-dioximate clathrochelates were found to efficiently electrocatalyze the production of molecular hydrogen from H(+) ions without the overpotential of this process.     

First principles and experimental 1H NMR signatures of solvated ions: The case of HCl(aq).

A combined experimental and ab initio study is presented of the 1H NMR chemical shift distribution of aqueous hydrogen chloride solution as a function of acid concentration, based on Car-Parrinello molecular dynamics simulations and fully periodic NMR chemical-shift calculations. The agreement of computed and experimental spectra is very good. From first-principles calculations, we can show that the individual contributions of Eigen and Zundel ions, regular water molecules, and the chlorine solvation shell to the NMR line are very distinct and almost independent of the acid concentration. From the computed instantaneous NMR distributions, it is further possible to characterize the average variation in hydrogen-bond strength of the different complexes.     

First principles study of the reaction of formic and acetic acids with hydroxyl radicals

The oxidation of formic and acetic acids with hydroxyl radicals was studied as a model for the oxidation of larger carboxylic acids using first principles calculations. For formic acid, the CBS-QB3 activation barriers of 14.1 and 12.4 kJ/mol for the acid and for the formyl channel, respectively, are within 3 kJ/mol of benchmark W1U values. Tunneling significantly enhances the rate coefficient for the acid channel and is responsible for the dominance of the acid channel at 298 K. At 298 K, tunneling correction factors of 339 and 2.0 were calculated for the acid and the formyl channel using the small-curvature tunneling method and the CBS-QB3 potential energy surface. The Wigner, Eckart, and zero-curvature tunneling methods severely underestimate the importance of tunneling for the acid channel. The resulting reaction rate coefficient of 0.98 x 10(5) m(3)/(mol x s) at 298 K is within a factor 2-3 of experimental values. For acetic acid, an activation barrier of 11.0 kJ/mol and a tunneling correction factor of 199 were calculated for the acid channel. Two mechanisms compete for hydrogen abstraction at the methyl group, with activation barriers of 11.9 and 12.5 kJ/mol and tunneling correction factors of 9.1 and 4.1 at 298 K. The resulting rate coefficient of 1.2 x 10(5) m(3)/(mol x s) at 298 K and branching ratio of 94% compare well with experimental data.