Complex nanoscale cage clusters built from uranyl polyhedra and phosphate tetrahedra

Five cage clusters that self-assemble in alkaline aqueous solution have been isolated and characterized. Each is built from uranyl hexagonal bipyramids with two or three equatorial edges occupied by peroxide, and three also contain phosphate tetrahedra. These clusters contain 30 uranyl polyhedra; 30 uranyl polyhedra and six pyrophosphate groups; 30 uranyl polyhedra, 12 pyrophosphate groups, and one phosphate tetrahedron; 42 uranyl polyhedra; and 40 uranyl polyhedra and three pyrophosphate groups. These clusters present complex topologies as well as a range of compositions, sizes, and charges. Two adopt fullerene topologies, and the others contain combinations of topological squares, pentagons, and hexagons. An analysis of possible topologies further indicates that higher-symmetry topologies are favored.     

Uranyl peroxide oxalate cage and core-shell clusters containing 50 and 120 uranyl ions

Cage clusters built from uranyl hexagonal bipyramids and oxalate ligands crystallize from slightly acidic aqueous solution under ambient conditions, facilitating structure analysis. Each cluster contains uranyl ions coordinated by peroxo ligands in a bidentate configuration. Uranyl ions are bridged by shared peroxo ligands, oxalate ligands, or through hydroxyl groups. U(50)Ox(20) contains 50 uranyl ions and 20 oxalate groups and is a topological derivative of the U(50) cage cluster that has a fullerene topology. U(120)Ox(90) contains 120 uranyl ions and 90 oxalate groups and is the largest and highest mass cluster containing uranyl ions that has been reported. It has a core-shell structure, in which the inner shell (core) consists of a cluster of 60 uranyl ions and 30 oxalate groups, identical to U(60)Ox(30), with a fullerene topology. The outer shell contains 12 identical units that each consist of five uranyl hexagonal bipyramids that are linked to form a ring (topological pentagon), with each uranyl ion also coordinated by a side-on nonbridging oxalate group. The five-membered rings of the inner and outer shells (the topological pentagons) are in correspondence and are linked through K cations. The inner shell topology has therefore templated the location of the outer shell rings, and the K counterions assume a structure-directing role. Small-angle X-ray scattering data demonstrated U(50)Ox(20) remains intact in aqueous solution upon dissolution. In the case of clusters of U(120)Ox(90), the scattering data for dissolved crystals indicates the U(60)Ox(30) core persists in solution, although the outer rings of uranyl bipyramids contained in the U(120)Ox(90) core-shell cluster appear to detach from the cluster when crystals are dissolved in water.     

Synthesis and structural studies of a bis(carbamoyl methyl) sulfoxide complex of uranyl nitrate

A new tri-functional ligand ⁱBu₂NCOCH₂SOCH₂CONⁱBu₂ was prepared and characterized. The coordination chemistry of this ligand with uranyl nitrate was studied with IR, ¹H NMR, electrospray mass–spectrometry, thermogravimetry, and elemental analysis. The structure of [UO₂(NO₃)₂(ⁱBu₂NCOCH₂SOCH₂CONⁱBu₂)] was determined by single-crystal X-ray diffraction. The uranium(VI) ion is surrounded by eight oxygens in a hexagonal bipyramidal geometry. Four oxygens from two nitrates and two oxygens from the ligand form a planar hexagon. The ligand is a bidentate chelate, bonding through sulfoxo and one of the carbamoyl groups to uranyl nitrate.     

Chelate effect and thermodynamics of metal complex formation in solution: a quantum chemical study

The accuracy of quantum chemical predictions of structures and thermodynamic data for metal complexes depends both on the quantum chemical methods and the chemical models used. A thermodynamic analogue of the Eigen-Wilkins mechanism for ligand substitution reactions (Model A) turns out to be sufficiently simple to catch the essential chemistry of complex formation reactions and allows quantum chemical calculations at the ab initio level of thermodynamic quantities both in gas phase and solution; the latter by using the conductor-like polarizable continuum (CPCM) model. Model A describes the complex formation as a two-step reaction: 1. [M(H2O)x](aq) + L(aq) <==>[M(H2O)x], L(aq); 2. [M(H2O)x], L(aq) <==>[M(H2O)(x-1)L],(H2O)(aq). The first step, the formation of an outer-sphere complex is described using the Fuoss equation and the second, the intramolecular exchange between an entering ligand from the second and water in the first coordination shell, using quantum chemical methods. The thermodynamic quantities for this model were compared to those for the reaction: [M(H2O)x](aq) + L(aq) <==>[M(H2O)(x-1)L](aq) + (H2O)(aq) (Model B), as calculated for each reactant and product separately. The models were tested using complex formation between Zn(2+) and ammonia, methylamine, and ethylenediamine, and complex formation and chelate ring closure reactions in binary and ternary UO(2)(2+)-oxalate systems. The results show that the Gibbs energy of reaction for Model A are not strongly dependent on the number of water ligands and the structure of the second coordination sphere; it provides a much more precise estimate of the thermodynamics of complex formation reactions in solution than that obtained from Model B. The agreement between the experimental and calculated data for the formation of Zn(NH(3))(2+)(aq) and Zn(NH(3))(2)(2+)(aq) is better than 8 kJ/mol for the former, as compared to 30 kJ/mol or larger, for the latter. The Gibbs energy of reaction obtained for the UO(2)(2+) oxalate systems using model B differs between 80 and 130 kJ/mol from the experimental results, whereas the agreement with Model A is better. The errors in the quantum chemical estimates of the entropy and enthalpy of reaction are somewhat larger than those for the Gibbs energy, but still in fair agreement with experiments; adding water molecules in the second coordination sphere improves the agreement significantly. Reasons for the different performance of the two models are discussed. The quantum chemical data were used to discuss the microscopic basis of experimental enthalpy and entropy data, to determine the enthalpy and entropy contributions in chelate ring closure reactions and to discuss the origin of the so-called "chelate effect". Contrary to many earlier suggestions, this is not even in the gas phase, a result of changes in translation entropy contributions. There is no simple explanation of the high stability of chelate complexes; it is a result of both enthalpy and entropy contributions that vary from one system to the other.     

Synthesis, characterization, and reactivity of a uranyl beta-diketiminate complex

Addition of 1 equiv of Li(Ar2nacnac) (Ar2nacnac = (2,6-(i)Pr2C6H3)NC(Me)CHC(Me)N(2,6-(i)Pr2C6H3)) to an Et2O suspension of UO2Cl2(THF)3 generates the uranyl dimer [UO2(Ar2nacnac)Cl]2 (1) in good yield. A second species can be isolated in low yield from the reaction mixtures of 1, namely [Li(OEt2)2][UO2(Ar2nacnac)Cl2] (2). The structures of both 1 and 2 have been confirmed by X-ray crystallography. Complex 1 reacts with Ph3PO to generate UO2(Ar2nacnac)Cl(Ph3PO) (3). In addition, 1 reacts with AgOTf and either 1 equiv of DPPMO2 or 2 equiv of Ph2MePO to provide [UO2(Ar2nacnac)(DPPMO2)][OTf] (4) and [UO2(Ar2nacnac)(Ph2MePO)2][OTf] (5), respectively. Both 4 and 5 have been fully characterized, including analysis by X-ray crystallography and cyclic voltammetry. Reduction of 4 with Cp2Co provides UO2(Ar2nacnac)(CH{Ph2PO}2) (6), a uranyl(VI) complex that is generated by the formal loss of H* from the DPPMO2 ligand. Labeling studies have been performed in an attempt to elucidate the mechanism of hydrogen loss. In contrast, reduction of 5 with Cp2Co provides UO2(Ar2nacnac)(Ph2MePO)2 (7), a rare example of a uranyl(V) complex. As expected, the solid-state molecular structure of 7 reveals slightly longer U-O(oxo) bond lengths relative to 5. Furthermore, complex 7 can be converted back into 5 by oxidation with AgOTf in toluene.     

Kinetic and quantum chemical studies of the mechanism of formation of 1,2-dialkyldiaziridines

The regularities of AlkNHBr consumption in the reaction of formation of 1,2-dialkyldiaziridines in aqueous media were studied for the first time by UV spectrometry. The rate constants of particular steps of the reaction were estimated starting from the possibility of formation of the precursor of 1,2-dialkyldiaziridine, N-halogenaminal, due to the amination of the intermediate iminium cation along with the parallel halogenation of the intermediate gem-diamine. The quantum chemical calculations (DFT, B3LYP, 6–31++G(d,p) and 3–21G basis sets) were performed for the spatial and electronic structures of the compounds and indices of the local reactivity and global electrophilicity of the key intermediates of the reactions. The results of the calculations allowed us to explain the retardation of the reaction when using EtNHBr instead of MeNHBr.     

A mechanism from quantum chemical studies for methane formation in methanogenesis

The mechanism for methane formation in methyl-coenzyme M reductase (MCR) has been investigated using the B3LYP hybrid density functional method and chemical models consisting of 107 atoms. The experimental X-ray crystal structure of the enzyme in the inactive MCR(ox1)(-)(silent) state was used to set up the initial model structure. The calculations suggest a mechanism not previously proposed, in which the most remarkable feature is the formation of an essentially free methyl radical at the transition state. The reaction cycle suggested starts from a Michaelis complex with CoB and methyl-CoM coenzymes bound and with a squareplanar coordination of the Ni(I) center in the tetrapyrrole F(430) prosthetic group. In the rate-limiting step the methyl radical is released from methyl-CoM, induced by the attack of Ni(I) on the methyl-CoM thioether sulfur. In this step, the metal center is oxidized from Ni(I) to Ni(II). The resulting methyl radical is rapidly quenched by hydrogen-atom transfer from the CoB thiol group, yielding the methane molecule and the CoB radical. The estimated activation energy is around 20 kcal/mol, which includes a significant contribution from entropy due to the formation of the free methyl. The mechanism implies an inversion of configuration at the reactive carbon. The size of the inversion barrier is used to explain the fact that CF(3)-S-CoM is an inactive substrate. Heterodisulfide CoB-S-S-CoM formation is proposed in the final step in which nickel is reduced back to Ni(I). The suggested mechanism agrees well with experimental observations.     

The coordination of uranyl in water: a combined quantum chemical and molecular simulation study

The coordination environment of uranyl in water has been studied using a combined quantum mechanical and molecular dynamics approach. Multiconfigurational wave function calculations have been performed to generate pair potentials between uranyl and water. The quantum chemically determined energies have been used to fit parameters in a polarizable force field with an added charge transfer term. Molecular dynamics simulations have been performed for the uranyl ion and up to 400 water molecules. The results show a uranyl ion with five water molecules coordinated in the equatorial plane. The U-O(H(2)O) distance is 2.40 A, which is close to the experimental estimates. A second coordination shell starts at about 4.7 A from the uranium atom. No hydrogen bonding is found between the uranyl oxygens and water. Exchange of waters between the first and second solvation shell is found to occur through a path intermediate between association and interchange. This is the first fully ab initio determination of the solvation of the uranyl ion in water.     

Spectroscopic and quantum chemical studies of isocytosine

The methods of electronic and vibrational (IR) spectroscopy were used to study the spectral properties of isocytosine in H₂O, D₂O, chloroform, and hexane in a wide concentration interval. Quantum chemical calculations of tautomeric forms and dimers of isocytosine were carried out. The bands of the calculated and experimental spectra were assigned. The results of the quantum calculations were compared with the experimental data. The spectral bands were classified according to the type of tautomer or dimer to which they belong.Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 24, No. 1, pp. 29–36, January–February, 1988.The authors are grateful to I. M. Ginzberg and L. F. Strelkova for their participation in the discussion of the infrared spectroscopy results.     

CO adsorption on pure and binary-alloy gold clusters: a quantum chemical study

We performed density-functional theory analysis of nondissociative CO adsorption on 22 binary Au-alloy (Au(n)M(m)) clusters: n=0-3, m=0-3, and m+n=2 (dimers) or 3 (trimers), M=Cu/Ag/Pd/Pt. We report basis-set superposition error corrections to adsorption energies and include both internal energy of adsorption (DeltaU(ads)) and Gibbs free energy of adsorption (DeltaG(ads)) at standard conditions (298.15 K and 1 atm). We found onefold (atop) CO binding on all the clusters except Pd2 (twofold/bridged), Pt2 (twofold/bridged), and Pd3 (threefold). In agreement with the experimental results, we found that CO adsorption is thermodynamically favorable on pure Au/Cu clusters but not on pure Ag clusters and also observed the following adsorption affinity trend: Pd>Pt>Au>Cu>Ag. For alloy dimers we found the following patterns: Au2>M Au>M2 (M=Ag/Cu) and M2>M Au>Au2 (M=Pd/Pt). Alloying Ag/Cu dimers with (more reactive) Au enhanced adsorption and the opposite effect was observed for PdPt dimers. The Ag-Au, Cu-Au, and Pd-Au trimers followed the trends observed on dimers: Au3>M Au2>M2Au>M3 (M=Ag/Cu) and Pd3>Pd2Au>PdAu2>Au3. Interestingly, Pt-Au trimers reacted differently and alloying with Au systematically increased the adsorption affinity: PtAu2>Pt2Au>Pt3>Au3. A strikingly different behavior of Pt is also manifested by the triplet spin state and onefold (atop) binding in Pt3-CO which is in contradiction with the singlet spin state and threefold binding in Pd3-CO. We found a linear correlation between CO binding energy (BE) and elongation of the CO bond. For Ag-Au and Cu-Au clusters, the increase in CO BE (and elongation of the C-O bond which is probably due to the back donation) is accompanied by the decrease in the cluster-CO distance suggesting that the donation (from 5sigma highest occupied molecular orbital in CO to cluster lowest unoccupied molecular orbital) mechanism also contributes to the BE. For Pd-Au clusters, the cluster-CO distance (and CO bond length) increases with increase in the BE, suggesting that the donation mechanism may not be important for those clusters. No clear trend was observed for Pt-Au clusters.     

Raman spectral studies of uranyl sulphate and its urea complex structural isomers

Using Raman spectroscopy, the authors have studied two forms of the dioxouranium(VI) sulphate complexes of the urea molecule, the bis(urea)dioxouranium(VI) sulphates α- and β-UO₂SO₄·2CO(NH₂)₂. Raman spectra were recorded using λ = 488 and 647.1 nm laser excitation lines. For λ = 448 nm, the crystals exhibit strong, structured luminescence, with peaks separated by 857 cm⁻¹ (α-form) and 853 cm⁻¹ (β-form). For λ = 647.1 nm, very intense Raman spectra are obtained which are similar but slightly different and can be used to distinguish each form. A comparison with the urea and uranyl sulphate hydrate (USH) spectra has clarified the Raman band assignment of the two parent α- and β forms. Contributions of the urea donor ligand and the sulphate have been identified. Charge transfer to uranium and the uranium—oxygen force constant in the uranyl ion have been evaluated. The results agree with previous X-ray structural investigations.     

Hydrocarbon adsorption on gold clusters: Experiment and quantum chemical modeling

Heats of adsorption Q of n-alkanes C₆–C₉ on ZrO₂ modified with gold and nickel nanoparticles were determined experimentally. The Q values were found to be higher on average by 7 kJ/mol on the modified samples as compared to the pure support. Density functional theory with the PBE functional and the pseudopotential for gold effectively allowing for relativistic corrections was used to model the adsorption of saturated hydrocarbons on Au and Au + Ni, as exemplified by the interaction of alkanes C₁–C₃ with Au _m , Au _{m − 1}Ni (m = 3, 4, 5) clusters as well as the interaction of C₁–C₈ with Au₂₀. Based on the calculation results, the probable coordination centers of alkanes on nanoparticle surfaces were found to be vertices and edges, whereas face localization was less probable.     

Redox behavior of cyclo[6]pyrrole in the formation of a uranyl complex

A uranyl complex, the first metal complex to be formed from the cyclo[n]pyrrole series of expanded porphyrins, is formed when cyclo[6]pyrrole is treated with the uranyl cation under aerobic conditions. Spectroscopic, spectroelectrochemical, and electron spin resonance data of this species are consistent with the ligand in the complex being oxidized to an antiaromatic form.     

Synthesis and studies of a tetradecanuclear manganese(II)/(III) cage

A novel Mn14 cluster is reported; this is a new nuclearity for manganese cages and highly unusual in that the ligands are not exclusively oxygen donors.     

On calculating a polymer's enthalpy of formation with quantum chemical methods

Calculating the enthalpy of formation of a polymer with ab initio methods requires two choices. The first decision is whether to use oligomeric extrapolation or periodic boundary conditions to model the extended system, and the second choice is between formation reactions to be modeled, for example, formation from atoms, formation from standard states, or formation from some set of molecular systems. Utilizing trans-polyacetylene and polyethylene as examples, the oligomeric and periodic techniques are contrasted, leading to a discussion of the larger than minimal unit cell required when frequency calculations only include in-phase vibrations, that is, only the k = 0 frequencies, in an enthalpy of formation calculation. The accuracy of calculating the enthalpy of formation, in light of density functional theory's increased error with larger systems and with respect to various reference states, is also discussed. The calculation of the enthalpy of formation for a polymer is most accurate when the reference states are chosen carefully and most efficient when using periodic boundary conditions.     

Molecular dynamics and quantum chemical studies on nitroglycerine

A constant temperature molecular dynamics study has been performed on PM3 (RHF) geometry optimized nitroglycerine molecule. The dynamics was carried out by using MM+ method at 550 K which is above the explosion point of nitroglycerine. Some molecular orbital characteristics of nitroglycerine at elevated temperatures were computed.     

A chemical bond theory of quantum size effects of semiconductor clusters

Size dependence effects in semiconductor clusters have been a subject of extensive studies for the last two decades. However, it is still difficult to employ the existing theoretical models to give reliable results of energies for clusters in the whole nanometer region. Here we offer a new theoretical method for the quantum size effects based on the idea that the energy gap shift of the cluster arises from the sum of the surface effect shift and quantum effect shift parts. We express the effects through algebraic relations rather than through variational solutions of the wave equation, without the use of any special adjustable parameter. Results reveal for the first time that the shape of the energy gap shift curve is dominated by the surface energy shift. Our method can also predict quantitatively the size dependence of dielectric constant. The new theoretical findings in the ultrasmall (<1 nm) anatase TiO(2) and the silicon clusters cannot be explained using previous theories.     

Optical and chemical studies of II–VI compound clusters

Recent experimental results on the structural, chemical, and optical properties of II–VI compound clusters containing between 2 and 24 molecules are presented. Stability patterns in the mass spectra of MgO clusters correlate with stable structures predicted for ionic clusters. The time dependence of H₂O adsorption onto MgO clusters is measured and found to vary strongly as a function of the number of adsorbed molecules. A sharp peak in the photodissociation probability is observed for metal-excess SrO and CaO cluster ions near 2.0 eV.     

Fluorescein dye derivative: Synthesis,characterization, quantum chemical and promising antimicrobial activity studies

The dimethyl sulfite as an alkylating as well as alkoxylating reagent is well known in synthetic organic as well as organometallic chemistry for a long time. Herein, we have utilized dimethyl sulfite as an alkylating reagent and reacted with fluorescein derivative, N-fluorescein-lactam-hydrazine ( B ). This reaction leads to the generation of tetramethylated fluorescein lactam hydrazine ( A ). The newly designed and synthesized molecule ( A ) has been characterized by ¹H, ¹³C NMR and HRMS data. The antimicrobial activity of the synthesized alkylated fluorescein derivative has been tested against E. coli, P. aeruginosa, Salmonella typhimurium and S. aureus microbial agents. Time-kill assay results also confirmed that fluorescein derivative is a potent antimicrobial agent and revealed that the time-kill assay of fluorescein derivative is value-added than the well-known antibiotic. In addition, quantum chemical study was used to analyze its activity trend and correlated with the experimental data. The computed results of DFT revealed that the lipophilicity as well as the LUMO-HOMO band gap (ΔE_LUMO-HOMO = 4.75 eV) of compound A make it more suitable as an antimicrobial agent to match with in vitro antimicrobial experimental results. The MIC and MBC values of compound A were observed to be lower in contrast to fluorescein and ampicillin for all the tested bacterial strains, which were approximately 3- to 4-fold lower than MIC and MBC values of the later. Results obtained from the study clearly indicate that compound A has better antimicrobial and bactericidal activity in comparison to ampicillin and fluorescein. Synthesized compound can be a better substitute of traditionally used antibiotics, Ampicillin.