Eunja Kim

Computational study of hydrogen storage in organometallic compounds

The authors have performed a systematic computational study of the hydrogen storage capacity of model organometallic compounds consisting of Sc, Ti, and V transition metal atoms bound to CmHm rings (m=4-6). For all the complexes considered, the hydrogen storage capacity is limited by the 18-electron rule. The maximum retrievable H2 uptake predicted is 9.3 wt% using ScC4H4, slightly better than the 9.1 wt% hydrogen using TiC4H4, and much larger than the approximately 7 wt% hydrogen with VC4H4, where only four H2 molecules can be adsorbed. The kinetic stability of these hydrogen-covered organometallic complexes is reviewed in terms of the energy gap between the highest occupied and lowest unoccupied molecular orbitals and the strength and nature of successive H2 bindings.

Discovery of the recoverable high-pressure iron oxide Fe4O5

Phases of the iron–oxygen binary system are significant to most scientific disciplines, directly affecting planetary evolution, life, and technology. Iron oxides have unique electronic properties and strongly interact with the environment, particularly through redox reactions. The iron–oxygen phase diagram therefore has been among the most thoroughly investigated, yet it still holds striking findings. Here, we report the discovery of an iron oxide with formula Fe4O5, synthesized at high pressure and temperature. The previously undescribed phase, stable from 5 to at least 30 GPa, is recoverable to ambient conditions. First-principles calculations confirm that the iron oxide here described is energetically more stable than FeO + Fe3O4 at pressure greater than 10 GPa. The calculated lattice constants, equation of states, and atomic coordinates are in excellent agreement with experimental data, confirming the synthesis of Fe4O5. Given the conditions of stability and its composition, Fe4O5 is a plausible accessory mineral of the Earth’s upper mantle. The phase has strong ferrimagnetic character comparable to magnetite. The ability to synthesize the material at accessible conditions and recover it at ambient conditions, along with its physical properties, suggests a potential interest in Fe4O5 for technological applications.

Hydrogenation of Single-Wall Carbon Nanotubes Using Polyamine Reagents : Combined Experimental and Theoretical Study

We combine experimental observations with ab initio calculations to study the reversible hydrogenation of single-wall carbon nanotubes using high boiling polyamines as hydrogenation reagents. Our calculations characterize the nature of the adsorption bond and identify preferential adsorption geometries at different coverages. We find the barrier for sigmatropic rearrangement of chemisorbed hydrogen atoms to be approximately 1 eV, thus facilitating surface diffusion and formation of energetically favored, axially aligned adsorbate chains. Chemisorbed hydrogen modifies the structure and stability of nanotubes significantly and increases the inter-tube distance, thus explaining the improved dispersability in solvents like methanol, ethanol, chloroform, and benzene.

Nanoscale building blocks for the development of novel proton exchange membrane fuel cells.

We propose a new type of sulfonated aromatic polyarylenes as candidate building blocks for proton exchange membranes. Density functional theory calculations and ab initio molecular dynamics simulations suggest that desulfonation is limited at high temperatures, owing to the strong aryl-SO3H bond induced by the electron-deficient aromatic ring, and that the proposed polymers exhibit good thermomechanical stability due to the robust aromatic main-chain repeating unit. Simulations also emphasize the importance of the Grotthuss-type mechanism, with interconversion between Eigen (H9O4+) and Zundel cations (H5O2+) as limiting structures, for the hydrated proton transport in the vicinity of the sulfonic acid groups.

Technetium Dichloride: A New Binary Halide Containing Metal–Metal Multiple Bonds

Technetium dichloride has been discovered. It was synthesized from the elements and characterized by several physical techniques, including single crystal X-ray diffraction. In the solid state, technetium dichloride exhibits a new structure type consisting of infinite chains of face sharing [Tc(2)Cl(8)] rectangular prisms that are packed in a commensurate supercell. The metal-metal separation in the prisms is 2.127(2) Å, a distance consistent with the presence of a Tc≡Tc triple bond that is also supported by electronic structure calculations.

Bulk modulus of the C60 molecule via the tight binding method

Abstract The bulk modulus for the C 60 molecule is calculated using total energy minimization via the tight binding method. The calculated bulk modulus is 717 GPa, which is about 1.6 times larger than that of bulk diamond due to the high symmetry of the C 60 molecule; a possible explanation is given. This result is compared to a simple estimation from elasticity theory. Electronic structures and vibrational frequencies are also compared to theoretical and experimental results.

Synthesis and structure of technetium trichloride.

Technetium trichloride has been synthesized by reaction of Tc(2)(O(2)CCH(3))(4)Cl(2) with HCl(g) at 300 °C. The mechanism of formation mimics the one described earlier in the literature for rhenium. Tc(2)(O(2)CCH(3))(2)Cl(4) [P1̅; a = 6.0303(12) Å, b = 6.5098(13) Å, c = 8.3072(16) Å, α = 112.082(2)°, β = 96.667(3)°, γ = 108.792(3)°; Tc-Tc = 2.150(1) Å] is formed as an intermediate in the reaction at 100 °C. Technetium trichloride is formed above 250 °C and is isostructural with its rhenium homologue. The structure consists of Tc(3)Cl(9) clusters [R3̅m; a = b = 10.1035(19) Å, c = 20.120(8) Å], and the Tc-Tc separation is 2.444(1) Å. Calculations on TcX(3) (X = Cl, Br) have confirmed the stability of TcCl(3) and suggest the existence of a polymorph of TcBr(3) with the ReBr(3) structure.

Chemical bonding and aromaticity in trinuclear transition-metal halide clusters.

Trinuclear transition-metal complexes such as Re(3)X(9) (X = Cl, Br, I), with their uniquely featured structure among metal halides, have posed intriguing questions related to multicenter electron delocalization for several decades. Here we report a comprehensive study of the technetium halide clusters [Tc(3)(μ-X)(3)X(6)](0/1-/2-) (X = F, Cl, Br, I), isomorphous with their rhenium congeners, predicted from density functional theory calculations. The chemical bonding and aromaticity in these clusters are analyzed using the recently developed adaptive natural density partitioning method, which indicates that only [Tc(3)X(9)](2-) clusters exhibit aromatic character, stemming from a d-orbital-based π bond delocalized over the three metal centers. We also show that standard methods founded on the nucleus-independent chemical shift concept incorrectly predict the neutral Tc(3)X(9) clusters to be aromatic.

Technetium(IV) halides predicted from first-principles.

We report the crystal structures of the novel technetium tetrahalides TcX(4) [X = F, I], as predicted from first-principles calculations. Isomorphous with TcCl(4) and TcBr(4) crystals, TcF(4) is orthorhombic with the centro-symmetric space group Pbca, while TcI(4) crystallizes in the monoclinic space group P2(1)/c. The structures, [TcX(2)(mu-X)(4/2)](infinity), consist of distorted edge-sharing octahedral groups of composition TcX(6) linked into endless cis chains. A possible explanation for the differences between these structures is offered in terms of varying degrees of bonding within the chains.

β-Technetium Trichloride: Formation, Structure, and First-Principles Calculations

A second polymorph of technetium trichloride, β-TcCl(3), has been identified from the reaction between Tc metal and Cl(2) gas. The structure of β-TcCl(3) consists of infinite layers of edge-sharing octahedra, similar to its MoCl(3) and RuCl(3) analogues. The Tc-Tc distance [2.861(3) Å] between adjacent octahedra is indicative of metal-metal bonding. Earlier theoretical work predicted that β-TcCl(3) is less stable than α-TcCl(3). In agreement with the prediction, β-TcCl(3) slowly transforms into α-TcCl(3) (Tc(3)Cl(9)) over 16 days at 280 °C.