Structural increments for 11-vertex nido-phospha- and aza(carba)boranes and -borates; dependence of energy penalties on the extent of Electron Localization

Relevant structural features and corresponding energy penalties were determined that allow to easily estimate the relative stabilities of 11-vertex nido-phospha- and aza-substituted boranes, borates, carbaboranes, and carbaborates. For this purpose, density functional theory computations at the B3LYP/6-311+G(d,p)//B3LYP/6-31G(d)+ZPE level were carried out to determine the relative energies of 95 phospha- and 46 aza(carba)boranes and -borates. Energy penalties assigned to disfavoring structural features show additive behavior and excellent precision with respect to the computed results, as in the case of 6- and 11-vertex nido-carboranes and -borates. An unsubstituted phosphorus atom was found to possess energy penalties quite similar to those of the three-electron-donating H-C group. A bare nitrogen atom has energy penalties much larger than those of a bare phosphorus atom. Four-electron-donating RP and RN moieties, however, have even more adverse energy penalties. The disfavoring effects of heteroatoms in a borane cluster are determined by the amount of electron localization, that is, primarily by the number of skeletal electrons that formally originate from the heterogroup and secondarily by the electronegativity. Heteroatom energy penalties are independent of the type of the other heteroatoms present in the same cluster. Some novel phospha(carba)borane geometries with bare and exo-substituted phosphorus atoms in the same cluster have favorable thermodynamic stabilities competitive with those of known isomers.     

Stabilizing heteroatom-centered 16-vertex group 11 tetrahedral architectures: Bonding and structural considerations toward versatile endohedral species

Density functional theory (DFT) calculations were carried out on a series of clusters made of a centered tetrahedral 16-atom superatomic cage having 20 or 18 jellium electrons (je) and structurally related to [Au₂₀], namely [X@M₁₆] (M = group 11; X = group 2, 4, 12, 14 element). Such species provide further information of how two different electron counts offer a more preferred endohedral situation for specific group elements. Calculations show that the encapsulated atom provides supplementary orbitals to stabilize the bonding M₁₆ MO's. Different favored electron counts are found depending on the nature of the encapsulated atom, as observed by the formation of 20-je species when encapsulating a group 14 element and 18-je species when encapsulating a group 2 element. In addition, the capabilities to enable reactive sites along the cage structure are found via the formation of σ holes at the coinage-metal edges, as shown by their electrostatic potential surface. Such naked species, which constitute an interesting addition to libraries of examples as small models for doped M(111) surfaces of fcc metals, reveal that different superatomic electronic configurations can favor the encapsulation of certain group elements. These results can guide further design of endohedral species.     

The relative stabilities of tetrafluoroethylene and Hexafluoro-2-butyne adducts of fluoro- or chlorobis- triphenylphosphine)carbonyliridium

Tetracyanoethylene, tetrafluoroethylene and hexafluoro-2-butyne form 1/1 adducts with trans-IrF(CO)(PPh₃)₂. Enthalpies of dissociation for the reactions:

(X = F or Cl; L = C₂F₄ or C₄F₆) have been measured. More stable adducts are formed when X = F.     

Structures and relative stabilities of silicon-containing buckminsterfullerenes: An AM1 computational study

A computational study of 60-atom clusters having the general formula @C_60-n Si _n (n=1, 2, and 12) is presented. Based on total energies and calculated enthalpies of formation, the incorporation of silicon into buckminsterfullerene frameworks is destabilizing, but not prohibitively so. The synthesis of these silaballs by controlled pyrolysis of C₆H₆ mixed with compatible organosilicon compounds is considered. For the @C₅₈Si₂ system theortho (Si-Si bonded) isomer is less stable by about 40 kcal/mol than others in which the heteroatoms are separated.     

Theoretical comparison between structural and dynamical features of Dolastatins 11 and 12 antineoplastic depsipeptides

Molecular mechanics (MM) and dynamics (MD) calculations in vacuo and in water have been performed for the natural cyclodepsipeptides Dolastatins 11 and 12 isolated from the sea hare Dolabella auricularia. The analysis of the MD trajectories for the two systems can give useful insight on the backbone structural features, side-chain and peptide-water interactions as well as on the inter- and intra-molecular hydrogen bonds. A comparison between the selected and analysed lowest energy isomers shows the different conformational behaviour of the compounds. Finally, with the aim to ascertain a structure-activity relationship for the two peptides, the interactions of both Dolastatins with water, generic hydrophobic environment, magnesium and calcium ions have been investigated by means of the GRID program.     

A theorem for qualitative deductions in organic or inorganic chemistry regarding the relative stabilities,distortions and reactions of molecules

Regardless of symmetry, the qualitative behavior of molecules, individually or in classes, may be predicted vis-a-vis their relative stabilities, defor     

Density functional study of 8- and 11-vertex polyhedral borane structures: comparison with bare germanium clusters

Density functional theory (DFT) at the hybrid B3LYP level has been applied to the polyhedral boranes B(n)H(n)(z) (n = 8 and 11, z = -2, -4, and -6) for comparison with isoelectronic germanium clusters Ge(n)(z). The energy differences between the global minima and other higher energy borane structures are much larger relative to the case of the corresponding bare germanium clusters. Furthermore, for both B(8)H(8)(2-) and B(11)H(11)(2-), the lowest energy computed structures are the corresponding experimentally observed most spherical deltahedra predicted by the Wade-Mingos rules, namely the D(2)(d) bisdisphenoid and the C(2)(v) edge-coalesced icosahedron, respectively. Only in the case of B(8)H(8)(2-) is there a second structure close (+2.6 kcal/mol) to the D(2)(d) bisdisphenoid global minimum, namely the C(2)(v) bicapped trigonal prism corresponding to the "square" intermediate in a single diamond-square-diamond process that can lead to the experimentally observed room temperature fluxionality of B(8)H(8)(2-). Stable borane structures with 3-fold symmetry (e.g., D(3)(h), C(3)(v), etc.) are not found for boranes with 8- and 11-vertices, in contrast to the corresponding germanium clusters where stable structures derived from the D(3)(d) bicapped octahedron and D(3)(h) pentacapped trigonal prism are found for the 8- and 11-vertex systems, respectively. The lowest energy structures found for the electron-rich boranes B(8)H(8)(4-) and B(11)H(11)(4-) are nido polyhedra derived from a closo deltahedron by removal of a relatively high degree vertex, as predicted by the Wade-Mingos rules. They relate to isoelectronic species found experimentally, e.g., B(8)H(12) and R(4)C(4)B(4)H(4) for B(8)H(8)(4-) and C(2)B(9)H(11)(2-) for B(11)H(11)(4-). Three structures were found for B(11)H(11)(6-) with arachno type geometry having two open faces in accord with the Wade-Mingos rules.     

The estimation of the stabilities of bivalent transition metal complexes and deviations from the Irving-Williams order

The free energies of formation of a range of ligands with Mn^II Fe^II, Co^II and Zn^II have been related to ligand field stabilisations, and it has been found possible to predict on theoretical grounds deviations from the Irving-Williams order.     

Enolization of Delta9,10-Octal-1-one: relative stabilities of s-trans exocyclic and s-cis endocyclic dienolates

Delta9,10-Octal-1-one (3,4,5,6,7,8-hexahydro-1(2H)-naphthalenone, 1) can be deprotonated at either of its two gamma positions to form the isomeric exocyclic (1-exo) and endocyclic (1-endo) dienolates. The rate of deuterium exchange for the gammaexo protons of 1 (1.6(+/-0.1)x10(-4) M-1 s-1) is 12-fold faster than that for those at the gammaendo position (1.38(+/-0.08)x10(-5) M-1 s-1). Assuming a Br?nsted coefficient of 0.5, these results indicate that the exocyclic dienolate is 3.0 kcal/mol more stable than its endocyclic counterpart.     

The relative stabilities of cyclic dicationic derivatives of diphosphanes with three (3P) or four (4P) linked phosphorus atoms

Reaction of a diphosphane with a chlorophosphane in the presence of SnCl(2) or AlCl(3) leads to the formation of dicationic heterocycles with three (3P) or four (4P) linked phosphorus atoms. Some 3P derivatives with small alkyl substituents may also be prepared by direct alkylation of cyclic triphosphenium ions. Several new species were prepared in solution, some of which were isolated and characterised by single-crystal X-ray diffraction. Investigations into the factors favouring formation of 3P or 4P species are described.     

Synthetic and structural chemistry of groups 11 and 12 metal complexes of the zwitterionic ammonium thiolate ligands

The chemistry of metal complexes of the zwitterionic ammonium thiolates has expanded dramatically in the recent years. This review is intended to summarize the synthesis and crystal structures of groups 11 and 12 metal zwitterionic ammonium thiolate complexes. Seven methods for the synthesis of these metal complexes of the zwitterionic ammonium thiolates are outlined: proton transfer reaction, precursor reaction, ligand exchange reaction, oxidation–reduction reaction, solid-state reaction, electrochemical reaction and hydro(solvo)thermal reaction. These metal complexes of the zwitterionic ammonium thiolates are classified according to the number of metal atoms; their specific structures are briefly discussed.     

Dendritic nanostructures of silver: facile synthesis, structural characterizations, and sensing applications

Silver nanodendrites are synthesized by a simple surfactant-free method using a suspension of zinc microparticles as a heterogeneous reducing agent. Structural characterizations suggest the preferential growth along 100 and 111 directions by oriented attachment of silver nanocrystals in the diffusion limit, leading to the formation of silver nanodendrites 20-30 nm in stem and branch diameter and 5-50 microm in length. Surface-enhanced Raman scattering studies show that the silver nanodentrites give an intensive and enhanced Raman scattering when pyridine was used as a probing molecule. We have also demonstrated that the silver nanodendrites increase the sensitivity of an electrochemical glucose biosensor by as much as 1-2 orders of magnitude.     

A series of new copper iodobismuthates: structural relationships, optical band gaps affected by dimensionality, and distinct thermal stabilities

Three new copper iodobismuthates, red tetranuclear [n-Bu(4)N][Cu(2)(CH(3)CN)(2)Bi(2)I(10)] (1), dark-red infinite linear [Et(4)N](2n)[Cu(2)Bi(2)I(10)](n) (2), and black polymeric ladderlike [Cu(CH(3)CN)(4)](2n)[Cu(2)Bi(2)I(10)](n) (3), crystallize from solutions of BiI3 and CuI in the presence of different cations. A regular structural relationship from 0-D (1) to 1-D linear anion chains (2) to 1-D ladderlike anion chains (3) is observed. The self-assembly of the basic building unit Cu(2)Bi(2)I(10) as altered by different cations is proposed to be the driving force for their formation. The optical band gaps exhibit a structure-related decrease from 1 to 2/3, in agreement with their color changes and the density functional theory (DFT) calculation results. The electronic structures and the relationship with corresponding monobismuth analogues and the Ag-Bi isotypes are discussed on the basis of DFT calculations. In spite of their structural similarities, the compounds are distinctive thermally: 2 is stable to 230 degrees C, 1 undergoes a solvent loss at 85 degrees C to form a new phase that is thermally stable to 230 degrees C, and 3 releases a solvent molecule and decomposes at 80 degrees C into BiI(3) and CuI. The essential reasons for these differences are discussed.     

A theoretical study of porphyrin isomers and their core-modified analogues: cis-trans isomerism, tautomerism and relative stabilities

Semiempirical (AM1 and PM3) and density functional theory (DFT) calculations were performed on about 50 porphyrin isomers with 25 each of 1,2 (syn) and 1,3 (anti) tautomeric forms. The corresponding oxa-and thia-core-modified analogues were also computed. The variations of relative energies and stabilities of the core-modified analogues were compared with parent porphyrin1 and the corresponding oxa-and thia-analogues. The trends in relative energies are not significantly changed while going from parent system to oxa-and thia-core-modified porphyrins in case of bothsyn andanti tautomers. Isomers of types [2·2·0·0], [3·0·1·0], [3·1·0·0], and [4·0·0·0] are destabilized due to the absence of methine bridge, which results in angle strain for tetrapyrroles. Isomers having [2·1·1·0], [2·1·0·1], [2·0·2·0] and [2·2·0·0] connectivity, the Z isomers, are more stable compared to the correspondingE isomers in bothsyn andanti forms of parent and core-modified analogues.     

Comparison of the relative stabilities of polycyclic aryl nitrenium ions and arylmethyl cations: ab initio and semiempirical molecular orbital calculations

Ab initio HF/3-21G calculations predict the stabilities of a series of nitrenium ions to increase in the order, phenyl < 2-naphthyl < 1-naphthyl < 2-fluorenyl. Quantitatively similar results are obtained at the MP/6-31G(d) level for the phenyl and naphthyl systems. The relative stabilities of the nitrenium ions qualitatively parallel those of the corresponding isoconjugate arylmethyl cations but are significantly more sensitive to the nature of the aryl group. Semiempirical AM1 calculations of the same quantities are in reasonable agreement with the ab initio values and are used to extend the comparison to a further 20 polycyclic aromatic derivatives. The relative stabilities of the larger set of nitrenium ions are found to be very precisely linearly related to those of the corresponding arylmethyl cations if allowance is made for the electron donating ability of the aryl group as measured by the charge on the exocyclic carbon atom. A similar but approximate relationship can also be demonstrated in terms of simple perturbational molecular orbital theory.     

杂原子Ti-ZSM-12分子筛的合成与结构表征

以甲基三乙基溴化铵为模板剂,水热晶化法合成出杂原子Ti-ZSM-12型分子筛.X射线衍射,扫描电镜,IR及XPS等现代技术手段研究表明杂原子Ti取代骨架Si原子而存在于ZSM-12分子筛骨架上.     

Double-cuvette ISES: in situ estimation of enantioselectivity and relative rate for catalyst screening

Described is a new method for the screening of an array of catalysts, in situ, to estimate enantioselectivity and relative rates. We term this approach "double-cuvette ISES (in situ enzymatic screening)". The Co(III)-salen mediated hydrolytic kinetic resolution (HKR) of (+/-)-propylene oxide is used as a model reaction to demonstrate proof of principle. In two parallel cuvettes, a lower CHCl3-based organic layer is loaded with the epoxide and the chiral salen catalyst. Aqueous reporting layers, containing distinct "reporting enzymes" and their nicotinamide cofactors, are layered above the organic layers. The 1,2-propanediol enantiomers formed by the chiral catalyst diffuse into the aqueous layer and are oxidized there by the reporting enzymes at rates dependent upon the diol concentration, the R:S ratio of the diol, and the enantioselectivity of the reporting enzymes. A focused chiral salen library was constructed from seven chiral 1,2-diamines, derived from amino acid, terpenoid, and carbohydrates skeletons, and seven salicylaldehyde derivatives. Double-cuvette ISES identified a couple of interesting combinatorial hits in this salen array, wherein either the sense or magnitude of enantioselection for a given chiral diamine depends significantly upon the choice of "salicylaldehyde" partner. A comparison of predicted ee's and relative rates using this new screening tool with those independently measured is provided.     

An investigation of the relative stabilities of the isomers of CF2N2: Comparison of ab initio and MNDO calculations

Ab initio SCF calculations at the HF/3-21G level and semi-empirical MNDO calculations have been used to locate the stationary points on the CF₂N₂ energy surface. Perfluorodiazomethane is predicted to be most stable isomer, but perfluorodiazirine is predicted to lie only ca 41 kJ higher in energy at the SCF level. There are significant differences between the ab initio and MNDO results for the ordering of some of the isomers. Frequency calculations give results in good agreement with the limited experimental data on these molecules.