Computational infrared and Raman spectra by hybrid QM/MM techniques: a study on molecular and catalytic material systems.

Vibrational spectroscopy is one of the most well-established and important techniques for characterizing chemical systems. To aid the interpretation of experimental infrared and Raman spectra, we report on recent theoretical developments in the ChemShell computational chemistry environment for modelling vibrational signatures. The hybrid quantum mechanical and molecular mechanical approach is employed, using density functional theory for the electronic structure calculations and classical forcefields for the environment. Computational vibrational intensities at chemical active sites are reported using electrostatic and fully polarizable embedding environments to achieve more realistic vibrational signatures for materials and molecular systems, including solvated molecules, proteins, zeolites and metal oxide surfaces, providing useful insight into the effect of the chemical environment on the signatures obtained from experiment. This work has been enabled by the efficient task-farming parallelism implemented in ChemShell for high-performance computing platforms.  This article is part of a discussion meeting issue 'Supercomputing simulations of advanced materials'.

Encapsulation of Silver Nanoparticles in an Amine-Functionalized Porphyrin Metal-Organic Framework and Its Use as a Heterogeneous Catalyst for CO2 Fixation under Atmospheric Pressure.

A new porphyrin-based compound, [Zn3 (C40 H24 N8 )(C20 H8 N2 O4 )2 (DEF)2 ](DEF)3 (1; DEF=N,N-diethylformamide), has been synthesized by employing 5,10,15,20-tetrakis(4-pyridyl)porphyrin, 1,2-diamino-3,6-bis(4-carboxyphenyl)benzene, and Zn2+ salt at 100 °C under solvothermal conditions. The structure, as determined by single-crystal XRD studies, is three-dimensional with threefold interpenetration. The usefulness of free -NH2 groups in the ligand was exploited for anchoring silver nanoparticles through a simple solution-based route. The silver-loaded sample, Ag@1, was characterized by powder XRD, energy-dispersive X-ray spectroscopy, high-resolution TEM, SEM, X-ray photoelectron spectroscopy, and inductively coupled plasma MS analysis, which clearly indicated that silver nanoparticles with a size of 3.83 nm were uniformly distributed within the metal-organic framework (MOF). The Ag@1 sample was evaluated for possible catalytic activity for the carboxylation of a terminal alkyne by employing CO2 under atmospheric pressure; this gave excellent results. The Ag@1 catalyst was found to be robust, active, and recyclable. The present studies suggest that porphyrin MOFs not only exhibit interesting structures, but also show good heterogeneous catalytic activity towards the fixation of CO2 .

Chemical Fixation of CO2 and Other Heterogeneous Catalytic Studies by Employing a Layered Cu-Porphyrin Prepared Through Single-Crystal to Single-Crystal Exchange of a Zn Analogue.

A solvothermal reaction of Zn(NO3 )2 ⋅6 H2 O, tetra-(4-pyridyl)porphyrin (H2 TPyP), and 4,4'-oxybis(benzoic acid) (H2 OBA) resulted in a new two-dimensional Zn- porphyrin metal-organic framework compound, [Zn2 (C40 H24 N8 )0.5 (C14 H8 O5 )(DMA)](DMA)(H2 O)6 (1; DMA=N,N-dimethylacetamide). The ZnII ions present in 1 could be exchanged by using a solution of Cu(NO3 )2 ⋅3 H2 O in DMA at room temperature to give [Cu2 (C40 H24 N8 )0.5 (C14 H8 O5 )(DMA)](DMA)(H2 O)3 (Cu∈1). The extra-framework solvent molecules have been shown to be reversibly removed or exchanged without collapse of the framework. Solvent-free Cu∈1 was explored as an active heterogeneous catalyst towards three different organic reactions: 1) the chemical fixation of CO2 into cyclic carbonate at room temperature and 1 atm; 2) the nitroaldol reaction under solvent-free conditions, and 3) the three-component coupling of aminopyridine, benzaldehyde, and aryl alkynes followed by 5-exo-dig cyclization to produce the important pharmacophore imidazopyridine.

Impact of Coulomb Correlations on Magnetic Anisotropy in Mn3Ga Ferrimagnet.

Traditional density functional theory (DFT) miserably fails to reproduce the experimental volume and magnetic anisotropy of D022 Mn3Ga, which has recently become one of the most sought-after materials in order to achieve a stable spin switching at low current density. Despite great progress over the last 10 years, this issue has hitherto remained unsolved. Here, taking into account the effects of strong electronic correlations beyond what is included in standard DFT, we show by comparison with the experiment that the DFT+U method is capable of quantitatively describing the volume and the magnetic anisotropy energy (MAE) in this alloy with physically meaningful choice of onsite Coulomb-U parameter. For the first time using a plane-wave code, we decompose MAE into spin channel-resolved components in order to determine spin-flip and spin-conserving contributions. The Mn atom at the tetrahedral site is identified as the primary source of the high perpendicular MAE with the most dominant spin-orbit coupling (SOC) occurring between its two orbital pairs: ↑↑ coupling and ↓↓ coupling between [Formula: see text] and d xy , and ↑↓ coupling between d yz and [Formula: see text]. Using the SOC-perturbation theory model, we provide interpretation of our numerical results. These results are important for quantitative microscopic understanding of the large perpendicular MAE observed in this material, and should assist in harnessing its potential for applications in futuristic spintronic devices.

Assembling Porphyrins into Extended Network Structures by Employing Aromatic Dicarboxylates: Synthesis, Metal Exchange, and Heterogeneous Catalytic Studies.

Three new metal-organic porphyrinic framework compounds of zinc and 5,10,15,20-tetrakis(4-pyridyl)porphyrin (TPyP) have been synthesized under solvothermal conditions. The compounds [Zn5 (C40 H24 N8 )(C8 H4 O4 )2 (NO3 )6 (DMA)2 ] (DMA)3 (H2 O)8 (1; DMA=dimethylacetamide), [Zn3 (C40 H24 N8 )(C8 H4 O4 )2 (DMF)](DMF)5 (H2 O)12 (2), and [Zn3 (C40 H24 N8 )(C12 H6 O4 )2 (DMA)2 ](H2 O)7 (3) have two (1) and three dimensionally (2 and 3) extended structures. All the three structures contain porphyrinic units connected through the carboxylate linkers. The nitrates bind the metal centers and are not hydrogen bonded. The different binding modes of nitrate in the structure of 1 are observed for the first time in a porphyrin-based MOF. The openness of the structure allowed us to explore metal exchange through a room-temperature metathetic route. Compound 2 undergoes 100 % exchange with copper, whereas compound 3 exchanges 70 % with copper. The copper-exchanged compounds Cu∈2 and Cu∈3 were observed to be good heterogeneous catalysts for many important organic reactions. The chemo and regioselective enamination of ß-ketoesters, preparation of propargylamine derivatives as well as regioselective cycloadditions of alkyne and azide (click reactions) have been carried out with good yields and selectivity. All the compounds have been characterized by PXRD, IR, UV/Vis, atomic absorption spectroscopy (AAS), and energy-dispersive X-ray spectroscopy (for Cu exchange).

Expected and unexpected transformations of manganese(III) tris(4-nitrophenyl)corrole.

The synthesis and characterization of manganese complexes of 5,10,15-tris(4-nitrophenyl) corrole in different oxidation states are described. Adoption of established protocols for the synthesis of (nitrido)manganese(V) complexes led to the expected corrole derivative as a minor product, accompanied by nitrogen atom insertion into the macrocycle. The major product was fully characterized via the combination of spectroscopy and X-ray crystallography as (nitrido)manganese(V) 6-azahemiporphycene.

Activation of carbon dioxide over zinc oxide by localised electrons.

The unique mechanism of activation of carbon dioxide over zinc oxide is unravelled using advanced quantum mechanical methods. The key process is the CO(2) chemisorption catalysed by a highly localized electron carrier trapped at a vacant oxygen interstitial surface site. At the top of the reaction barrier CO(2) pulls the electron from the vacancy and thus becomes active.

Noble metal ionic catalysts: correlation of increase in CO oxidation activity with increasing effective charge on Pd ion in Pd ion substituted Ce1-xMxO2-delta (M = Ti, Zr and Hf).

Pd ion substituted Ce1-xMxO2-delta (M = Ti, Zr, Hf) have been prepared by a single step solution combustion method. Two atom% Pd ion substitution in the title compounds is confirmed by X-ray diffraction (XRD) and Pd ion charge state and redox properties have been determined by X-ray photoelectron spectroscopy (XPS) and H2/TPR studies. While Pd ion in CeO2 (Ce0.98Pd0.02O2-delta) showed higher catalytic activity for CO oxidation than Pd metal impregnated over CeO2, further increase in the catalytic activity is observed with Pd ion in Ce1-xTi(Hf)xO2 and a decrease in the activity with Pd ion in Ce1-xZrxO2. Effective charge on Pd ion could be varied by its substitution in these solid solutions Ce1-xMxO2 (M = Ti, Zr & Hf) and also in TiO2 compared to Pd ion in PdO. Effective positive charge on Pd ion is determined from the core level binding energy shift of Pd(3d5/2) peak with respect to Pd metal. Rate of CO oxidation increased and activation energy decreased with increase in effective charge on the Pd ion in the Pd ion substituted fluorites.