Monica Kosa

Negative Linear Compressibility of a Metal–Organic Framework

A 3D hybrid zinc formate framework, [NH(4)][Zn(HCOO)(3)], possessing an acs topology, shows a high degree of mechanical anisotropy and negative linear compressibility (NLC) along its c axis. High-pressure single-crystal X-ray diffraction studies and density functional theory calculations indicate that contraction of the Zn-O bonds and tilting of the formate ligands with increasing pressure induce changes in structure that result in shrinkage of the a and b axes and the NLC effect along c.

Phase selection and energetics in chiral alkaline Earth tartrates and their racemic and meso analogues: synthetic, structural, computational, and calorimetric studies.

The hydrothermal reactions of calcium, strontium, and barium with l-, meso-, and d,l-tartaric acid were examined from room temperature to 220 degrees C. We report the synthesis of 13 new phases and crystal structures of 11 alkaline earth tartrates, including an unusual I(3)O(0) framework, [Ba(d,l-Tar)] (Tar = C(4)H(4)O(6)(2-)), with 3-D inorganic connectivity. Each alkaline earth exhibits different phase behavior in the reactions with the three forms of tartaric acid. Calcium forms unique l-, meso-, and d,l-tartrate phases which persist to 220 degrees C. Strontium forms three unique phases at lower temperatures, but above 180 degrees C reactions with l- and d,l-tartaric acid yield the meso phase. Likewise, Ba forms three unique low-temperature phases, but above 200 degrees C reactions with l- and meso-tartaric acid yield the d,l phase. Computational and calorimetric studies of the anhydrous calcium phases, [Ca(l-Tar)] and [Ca(meso-Tar)], strontium phases, [Sr(l-Tar)] and [Sr(meso-Tar)], and barium phases, [Ba(l-Tar)] and [Ba(d,l-Tar)], were performed to determine relative phase stabilities and elucidate the role of thermodynamic and kinetic factors in controlling phase behavior. The computational and calorimetric results were in excellent agreement. The [Ca(meso-Tar)] phase was found to be 9.1 kJ/mol more stable than the [Ca(l-Tar)] phase by computation (total electronic energies) and 2.9 +/- 1.6 kJ/mol more stable by calorimetry (enthalpies of solution). The [Sr(meso-Tar)] phase was found to be 13.4 and 8.1 +/- 1.4 kJ/mol more stable than [Sr(l-Tar)] by computation and calorimetry, respectively. Finally, the [Ba(l-Tar)] phase was found to be 6.4 and 7.0 +/- 1.0 kJ/mol more stable than the [Ba(d,l-Tar)] phase. Our results suggest that the calcium and strontium meso phases are the most thermodynamically stable phases in their systems over the temperature range studied. The phase transitions are controlled by relative thermodynamic stabilities but also by a kinetic factor, likely the barrier to isomerization/racemization of the tartaric acid, which is hypothesized to preclude phase transformations at lower temperatures. In the barium system we find the [Ba(l-Tar)] phase to be the most thermodynamically stable phase at low temperatures, while the [Ba(d,l-Tar)] phase becomes the thermodynamic product at high temperatures, due to a larger entropic contribution.

Trisilaallene and the Relative Stability of Si3H4 Isomers

A theoretical quantum-mechanical study of trisilaallene, H2Si [Formula: see text] Si [Formula: see text] SiH2, and of 15 other Si3H4 isomers was carried out using ab initio and DFT methods with a variety of basis sets. Values given below are at B3LYP/6-31G(d,p). Unlike H2C [Formula: see text] C [Formula: see text] CH2 which is linear, H2Si [Formula: see text] Si [Formula: see text] SiH2 is highly bent at the central silicon atom, with a SiSiSi bending angle of 69.4°. The Si [Formula: see text] Si bond length is 2.269 Å, longer than a regular Si [Formula: see text] Si double bond (2.179 Å) but shorter than a Si-Si single bond (2.351 Å). The distance between the terminal silicon atoms is 2.583 Å, significantly longer than a Si-Si single bond. The geometry and electronic properties of H2Si [Formula: see text] Si [Formula: see text] SiH2 are similar to those of the corresponding trisilacyclopropylidene, which is only 2.7 kcal/mol higher in energy. A barrier of only 0.1 kcal/mol separates trisilacyclopropylidene and trisilaallene which can be described as bond-stretch isomers. Sixteen minima were located on the Si3H4 PES, most of them within a narrow energy range of ca. 10 kcal/mol. Six of the Si3H4 isomers are analogous to the classic C3H4 minima structures; however, the other Si3H4 isomers do not have carbon analogues, and they are characterized by hydrogen-bridged structures.

Bifunctional Catalysis by Natural Cinchona Alkaloids: A Mechanism Explained

The use of bifunctional chiral catalysts, which are able to simultaneously bind and activate two reacting partners, currently represents an efficient and reliable strategy for the stereoselective preparation of valuable chiral compounds. Cinchona alkaloids such as quinine and quinidine, simple organic molecules generously provided by Nature, were the first compounds to be proposed to operate through a cooperative catalysis. To date, a full mechanistic characterization of the dual catalysis mode of cinchona alkaloids has proven a challenging objective, due to the transient, non-covalent nature of the involved catalyst-substrate interactions. Here, we propose a mechanistic rationale on how natural cinchona alkaloids act as efficient bifunctional catalysts by using a broad range of computational methods, including classical molecular dynamics, a mixed quantum mechanical/molecular mechanics (QM/MM) approach, and correlated ab-initio calculations. We also unravel the origin of enantio- and diastereoselectivity, which is due to a specific network of hydrogen bonds that stabilize the transition state of the rate-determining step. The results are validated by experimental evidence.

Metallocorroles as Nonprecious‐Metal Catalysts for Oxygen Reduction

The future of affordable fuel cells strongly relies on the design of earth-abundant (non-platinum) catalysts for the electrochemical oxygen reduction reaction (ORR). However, the bottleneck in the overall process occurs therein. We have examined herein trivalent Mn, Fe, Co, Ni, and Cu complexes of β-pyrrole-brominated corrole as ORR catalysts. The adsorption of these complexes on a high-surface-area carbon powder (BP2000) created a unique composite material, used for electrochemical measurements in acidic aqueous solutions. These experiments disclosed a clear dependence of the catalytic activity on the metal center of the complexes, in the order of Co>Fe>Ni>Mn>Cu. The best catalytic performance was obtained for the Co(III) corrole, whose onset potential was as positive as 0.81 V versus the reversible hydrogen electrode (RHE). Insight into the properties of these systems was gained by spectroscopic and computational characterization of the reduced and oxidized forms of the metallocorroles.

Probing the Mechanical Properties of Hybrid Inorganic–Organic Frameworks: A Computational and Experimental Study

Hybrid framework materials are modular compounds consisting of metal ions and organic linkers, variation of which has given rise to a myriad of structures with technologically relevant properties. To date, extensive experimental and theoretical studies have been carried out to understand key factors that affect processes such as gas adsorption, gas separation and heterogeneous catalysis in nanoporous metal-organic frameworks (MOFs). [1] Dense hybrid frameworks are also of growing interest on account of their unique physical properties, such as multiferroics, electronic conductivity and photoluminescence, among others. [2] For all viable applications, the robustness of the materials and, in particular, a detailed understanding of their mechanical properties, are necessary for successful utilization. This field, however, remains largely unexplored. Recent experimental studies [3] have demonstrated that the elastic properties, especially the bulk modulus (B) [4] and the Young’s modulus (E) [5] of nanoporous and dense hybrid frameworks, can be correlated to their density, dimensionality and their underlying chemical structures. In addition, recent computational studies have reported that the bulk moduli of a family of isoreticular metal-organic frameworks (IRMOFs) depend on the size of the aromatic organic linkers (which determines the density). [6] Herein, we employed a combination of computational and experimental approaches to probe the elastic properties of a dense and anisotropic hybrid framework material: zinc phosphate phosphonoacetate hydrate, Zn3(PO4)(O2CCH2PO3)(H2O), 1. [7] We propose an efficient computational scheme for the approximate analysis of the Young’s modulus and the Poisson’s ratio (n) along the principal directions of an anisotropic crystal. Notably, this approach circumvents the intricacies involved in computing the full elastic stiffness tensor. [8] The validity of our theoretical calculations was addition, theoretical studies were performed by subjecting the anisotropic framework to hydrostatic compression to reveal the role of the basic building blocks. Studies to date indicate that both the local density approximation (LDA) and the general gradient approximation (GGA) levels of density functional theory (DFT) have over-predicted the bulk modulus (B) of MOF materials. By way of an example, the B value of the lightweight MOF-5 (density of 0.59 gcm ! 3 ) was calculated to be in the range of 16‐20 GPa, [6] which is notably higher than measurements obtained from other related MOF-type structures of considerably higher densities. Specifically, high pressure experiments have determined B values at only 6.5 GPa (0.93 gcm ! 3 ) and 14 GPa (1.54 gcm ! 3 )f or MOF materials with zeolitic imidazolate framework (ZIF) structures. [3c,9] Likewise, a large discrepancy exists in terms of the Young’s modulus (E). Theoretical studies on cubic crystals of MOF-5 have reported E values in the range of 14.8‐

Putting DFT to the test: a first-principles study of electronic, magnetic, and optical properties of Co3O4.

First-principles density functional theory (DFT) and a many-body Greens function method have been employed to elucidate the electronic, magnetic, and photonic properties of a spinel compound, Co3O4. Co3O4 is an antiferromagnetic semiconductor composed of cobalt ions in the Co(2+) and Co(3+) oxidation states. Co3O4 is believed to be a strongly correlated material, where the on-site Coulomb interaction (U) on Co d orbitals is presumably important, although this view has recently been contested. The suggested optical band gap for this material ranges from 0.8 to 2.0 eV, depending on the type of experiments and theoretical treatment. Thus, the correlated nature of the Co d orbitals in Co3O4 and the extent of the band gap are still under debate, raising questions regarding the ability of DFT to correctly treat the electronic structure in this material. To resolve the above controversies, we have employed a range of theoretical methods, including pure DFT, DFT+U, and a range-separated exchange-correlation functional (HSE06) as well as many-body Greens function theory (i.e., the GW method). We compare the electronic structure and band gap of Co3O4 with available photoemission spectroscopy and optical band gap data and confirm a direct band gap of ca. 0.8 eV. Furthermore, we have also studied the optical properties of Co3O4 by calculating the imaginary part of the dielectric function (Im(ε)), facilitating direct comparison with the measured optical absorption spectra. Finally, we have calculated the nearest-neighbor interaction (J1) between Co(2+) ions to understand the complex magnetic structure of Co3O4.

Structural trends in hybrid perovskites [Me2NH2]M[HCOO]3 (M = Mn, Fe, Co, Ni, Zn): computational assessment based on Bader charge analysis

Topological analysis of the electron density of hybrid perovskites with different transition metal atoms indicates that the variation in the cell size depends on the extent of charge transfer from metal to oxygen rather than on the identity of the transition metal atom alone. The metal–oxygen interaction is less polarized and thus a greater covalent vs. ionic contribution is expected along the first row transition metal series.

Were reactions of triplet silylenes observed

The observation that (iPr3Si)(tBu3Si)Si: (1) yields an intramolecular C-H bond insertion product at room temperature, together with earlier computational predictions that triplet 1 is slightly more stable (or isoenergetic) than singlet 1 and additional considerations, led previous investigators to conclude that 1 is the first silylene to exhibit triplet electronic state reactivity. In this paper we test, using DFT and ab initio methods, whether the occurrence of intramolecular C-H bond insertion indeed indicates the presence of a triplet-state silylene. DFT calculations at the B3LYP/6-31+G(d,p)//B3LYP/6-31+G(d,p) level show that singlet (iPr3Si)(tBuMe2Si)Si: (9), a close model of singlet 1, inserts intramolecularly into a C-H bond of the tBu group with a barrier of 22.7 kcal/mol (22.2 kcal/mol at SCS-MP2/cc-PVTZ). However, for triplet9 the barrier of this insertion reaction is significantly higher, 34.6 kcal/mol (41.9 kcal/mol at SCS-MP2/cc-PVTZ). This implies that at room temperature the intramolecular insertion reaction of the singlet is 10(9)-10(12) faster than that of the triplet. We conclude, in contrast to previous conclusions, that the observed intramolecular C-H bond insertion reactions of 1 as well as of (tBu3Si)2Si: (2) occur from the singlet state. Furthermore, the occurrence of an intramolecular C-H bond insertion cannot serve as evidence for the presence of a triplet silylene, either in cases where the singlet and triplet states are nearly isoenergetic (e.g., 1 and 9) or even for silylenes where the triplet state is the ground state (e.g., 2), because the corresponding singlet silylenes insert intramolecularly much faster. The search for a genuine reaction of a triplet silylene has to continue.

Controlling dye aggregation, injection energetics and catalytic recombination in organic sensitizer based dye cells using a single electrolyte additive

Organic dyes have been used extensively in recent years as sensitizers for Dye Sensitized Solar Cells (DSSCs) due to their high molar extinction coefficients, straightforward synthetic routes and readily available synthetic precursors. Though widely used, these dyes have some drawbacks, such as a tendency to aggregate and to catalyze electron recombination, thereby compromising both photovoltage and photocurrent. To circumvent the above-mentioned shortcomings of organic dyes, we adopt a novel strategy based on the addition of substituted benzene co-solvents to the electrolyte. This approach has several advantageous features which enhance cell performance: first, the substituted benzene molecules penetrate the dye layer to form stable complexes, thereby screening the excited state quenching and increasing the charge separation efficiency in the cell. Second, the benzene additive inhibits the catalytic recombination processes between electrons in TiO2 and the oxidized electrolyte, which increases the device Voc. Finally, despite not being adsorbed to the surface, the benzene derivatives shift the TiO2 conduction band positively, which improves the Jsc. SQ-1 sensitized DSSCs obtained using this strategy show a Jsc of 10.7 mA cm−2, a Voc of 657 mV and a total efficiency of 4.7% which is the best efficiency reported so far for such dyes in DSSCs.