Crystal And Molecular-Structure Of 8-Acetoxy-6-(2,4-Dimethoxy-5-Bromophenyl)-3-Methyl-Tricyclo[5,2,1,03,8 ]Decan-2-One

The crystal and molecular structure has been determined by the heavy-atom method and refined by the least-squares procedure to R= 8"3 % for 2033 photographically observed reflexions. The compound crystallizes in the space group P]" with two molecules in a unit cell of dimensions a = 11"68 + 0-02, b = 12"91 +0"02, c= 10"43+0"02/~, e= 114"7+ 1, fl=90-2+ 1 and 7,= 118.3+ 1 °. The unit cell also contains one molecule of the solvent, benzene. The 'cage' part of the molecule exhibits a large number of elongated bonds and strained internal valency angles. The bridgehead angle in the bicyclic heptane ring system is 89 °. The acetate group at C(16) and the methyl group at C(15) are cis to each other.

Metallacarboranes: Toward Promising Hydrogen Storage Metal Organic Frameworks

Using first principles calculations, we show the high hydrogen storage capacity of metallacarboranes, where the transition metal (TM) atoms can bind up to 5 H-2-molecules. The average binding energy of similar to 0.3 eV/H favorably lies within the reversible adsorption range. Among the first row TM atoms, Sc and Ti are found to be the optimum in maximizing the H-2 storage (similar to 8 wt %) on the metallacarborane cluster. Being an integral part of the cage, TMs do not suffer from the aggregation problem, which has been the biggest hurdle for the success of TM-decorated graphitic materials for hydrogen storage. Furthermore, the presence of carbon atom in the cages permits linking the metallacarboranes to form metal organic frameworks, which are thus able to adsorb hydrogen via Kubas interaction, in addition to van der Waals physisorption.

Temperature and concentration dependence of adsorption properties of methane in NaY: a molecular dynamics study

Molecular dynamics calculations on methane sorbed in NaY (Si/Al = 3.0) employing realistic methane-methane and methane-zeolite intermolecular potential functions at different temperatures (50, 150, 220, and 300 K) and concentrations (2, 4, 6, and 8 molecules/cage) are reported. The thermodynamic results are in agreement with the available experimental data. Guest-guest and guest-host radial distribution functions (rdfs), energy distribution functions, distribution of cage occupancy, center-of-cage-center-of-mass (coc-com) rdfs, velocity autocorrelation functions for com and angular motion and the Fourier transformed power spectra, and diffusion coefficients are presented as a function of temperature and concentration. At 50 K, methane is localized near the adsorption site. Site-site migration and essentially free rotational motion are observed at 150 K. Molecules preferentially occupy the region near the inner surface of the alpha-cage. The vibrational frequencies for the com of methane shift toward higher values with decreasing temperature and increasing adsorbate concentration. The observed frequencies for com motion are 36, 53, and 85 cm-1 and for rotational motion at 50 K, 95 and 150 cm-1 in agreement with neutron scattering data. The diffusion coefficients show a type I behavior as a function of loading in agreement with NMR measurements. Cage-to-cage diffusion is found to be always mediated by the surface.

Models for spin-state transitions in solids

The model for spin-state transitions described by Bari and Sivardiere (1972) is static and can be solved exactly even when the dynamics of the lattice are included; the dynamic model does not, however, show any phase transition. A coupling between the octahedra, on the other hand, leads to a phase transition in the dynamical two-sublattice displacement model. A coupling of the spin states to the cube of the sublattice displacement leads to a first-order phase transition. The most reasonable model appears to be a two-phonon model in which an ion-cage mode mixes the spin states, while a breathing mode couples to the spin states without mixing. This model explains the non-zero population of high-spin states at low temperatures, temperature-dependent variations in the inverse susceptibility and the spin-state population ratio, as well as the structural phase transitions accompanying spin-state transitions found in some systems.

A novel phenomenon of burst of oxygen uptake during decavanadate-dependent oxidation of NADH

Oxidation of NADH by decavanadate, a polymeric form vanadate with a cage-like structure, in presence of rat liver microsomes followed a biphasic pattern. An initial slow phase involved a small rate of oxygen uptake and reduction of 3 of the 10 vanadium atoms. This was followed by a second rapid phase in which the rates of NADH oxidation and oxygen uptake increased several-fold with a stoichiometry of NADH: O2 of 1ratio1. The burst of NADH oxidation and oxygen uptake which occurs in phosphate, but not in Tris buffer, was prevented by SOD, catalase, histidine, EDTA, MnCl2 and CuSO4, but not by the hydroxyl radical quenchers, ethanol, methanol, formate and mannitol. The burst reaction is of a novel type that requires the polymeric structure of decavanadate for reduction of vanadium which, in presence of traces of H2O2, provides a reactive intermediate that promotes transfer of electrons from NADH to oxygen.

Influence of non-geometrical factors on intracrystalline diffusion -- Role of sorbate-zeolite interactions

Molecular dynamics simulations on Xe in NaY and Ar in NaCaA zeolite are reported. Rates of cage-to-cage crossovers in the two zeolites exhibit trends which are contrary to that expected from geometrical considerations. The results suggest the important role of the sorbate-zeolite interactions in determining the molecular sieve properties of zeolites for small sized sorbates. The results are explained in terms of the barrier height for cage-to-cage crossover in the two zeolites.

Coordination-Driven Self-Assembly of M3L2 Trigonal Cages from Preorganized Metalloligands Incorporating Octahedral Metal Centers and Fluorescent Detection of Nitroaromatics

The design and preparation of novel M3L2 trigonal cages via the coordination-driven self-assembly of preorganized metalloligands containing octahedral aluminum(III), gallium(III), or ruthenium(II) centers is described. When tritopic or dinuclear linear metalloligands and appropriate complementary subunits are employed, M3L2 trigonal-bipyramidal and trigonal-prismatic cages are self-assembled under mild conditions. These three-dimensional cages were characterized with multinuclear NMR spectroscopy (H-1 and P-31) and high-resolution electrospray ionization mass spectrometry. The structure of one such trigonal-prismatic cage, self-assembled from an arene ruthenium metalloligand, was confirmed via single-crystal X-ray crystallography. The fluorescent nature of these prisms, due to the presence of their electron-rich ethynyl functionalities, prompted photophysical studies, which revealed that electron-deficient nitroaromatics are effective quenchers of the cages' emission. Excited-state charge transfer from the prisms to the nitroaromatic substrates can be used as the basis for the development of selective and discriminatory turn-off fluorescent sensors for nitroaromatics.

Parallel Tempering Simulations of Liquid-Phase Adsorption of n-Alkane Mixtures in Zeolite LTA-5A

Adsorption of n-alkane mixtures in the zeolite LTA-5A under liquid-phase conditions has been studied using grand canonical Monte Carlo (GCMC) simulations combined with parallel tempering. Normal GCMC techniques fail for some of these systems due to the preference of linear molecules to coil within a single cage in the zeolite. The narrow zeolite windows severerly restrict interactions of the molecules, making it difficult to simulate cooperative rearrangements necessary to explore configuration space. Because of these reasons, normal GCMC simulations results show poor reproducibility in some cases. These problems were overcome with parallel tempering techniques. Even with parallel tempering, these are very challenging systems for molecular simulation. Similar problems may arise for other zeolites such as CHA, AFX, ERI, KFI, and RHO having cages connected by narrow windows. The simulations capture the complex selectivity behavior observed in experiments such as selectivity inversion and azeotrope formation.

Diffusion of sorbates in zeolites Y and A novel dependence on sorbate size and strength of sor ba t e-Zeoli t e interaction

A systematic investigation of monatomic spherical sorbates in the supercages of zeolites Y and A by molecular dynamics technique is presented. Rates of intercage diffusion, rates of cage visits, and the diffusion coefficients have been calculated as a function of the sorbate-zeolite interaction strength. These properties exhibit markedly different dependences on interaction strength for the two zeolites. The observed behavior is shown to be a consequence of the two principal mechanisms of intercage diffusion and the energetic barrier associated with them. The diffusion coefficient and other properties associated with intercage diffusion are found to be directly proportional to the reciprocal of the square of the sorbate diameter when the sorbate diameter is significantly smaller than the window diameter. As the sorbate diameter increases, a peak is observed in all the transport properties investigated including the diffusion coefficient. We call this surprising effect as the ring or levitation effect and it explains several anomalous results reported in the literature and suggests a breakdown of the geometrical criterion for diffusion of sorbates. It shows that under certain conditions nongeometrical factors play a major role and geometrical factors become secondary in the determination of the molecular sieve property. A generalized parameter has been proposed which suggests conditions under which one can expect the ring or levitation effect in any porous medium. Inverse size selectivity becomes operative under these conditions.

Formation of rearrangement product in photolysis of α,α-dimethydeoxybenzoin upon cyclodextrin complexation

Solid-state irradiation of cyclodextrin complexes of alpha,alpha-dimethyldeoxybenzoin results in the formation of a significant amount of rearrangement product, 4-isopropylbenzophenone, in addition to cage products. This behavior is not observed in the photolysis in solution or in micellar media.

Thermodynamic and structural consequences of changing a sulfur atom to a methylene group in the M13Nle mutation in ribonuclease-S

Two fragments of pancreatic ribonuclease A, a truncated version of S-peptide (residues 1-15) and S-protein (residues 21-124), combine to give a catalytically active complex. We have substituted the wild-type residue at position 13, methionine (Met), with norleucine (Nle), where the only covalent change is the replacement of the sulfur atom with a methylene group. The thermodynamic parameters associated with the binding of this variant to S-protein, determined by titration calorimetry in the temperature range 10-40 degrees C, are reported and compared to values previously reported [Varadarajan, R., Connelly, P. R., Sturtevant, J. M., & Richards, F. M. (1992) Biochemistry 31, 1421-1426] for other position 13 analogs. The differences in the free energy and enthalpy of binding between the Met and Nle peptides are 0.6 and 7.9 kcal/mol at 25 degrees C, respectively. These differences are slightly larger than, but comparable to, the differences in the values for the Met/Ile and Met/Leu pairs. The structure of the mutant complex was determined to 1.85 Angstrom resolution and refined to an R-factor of 17.4% The structures of mutant and wild-type complexes are practically identical although the Nle side chain has a significantly higher average B-factor than the corresponding Met side chain. In contrast, the B-factors of the atoms of the cage of residues surrounding position 13 are all somewhat lower in the Nle variant than in the Met wild-type. Thus, the large differences in the binding enthalpy appear to reside entirely in the difference in chemical properties or dynamic behavior of the -S- and -CH2- groups and not in differences in the geometry of the side chains or the internal cavity surface. In addition, a novel method of obtaining protein stability data by means of isothermal titration calorimetry is introduced.

Molecular dynamics investigation of sorption of argon in NaCaA zeolite

A molecular dynamics calculation of argon in NaCaA zeolite at 393 K and 1 atom per cage is reported. Equilibrium properties such as guest-host interaction energy, guest-guest dimerization and bonding energy, various guest-host and guest-guest radial distribution functions and dynamical properties such as the mean-square displacement, power spectra and diffusion coefficient have been obtained.

H-1 MAS NMR study of protonic conduction in layered HNbWO6 center dot xH(2)O (x=1.5, 0.5)

H-1 Magic Angle Spinning (MAS) NMR of layered HNbWO6 . xH(2)O (x = 1.5, 0.5) is carried out at room temperature and at various spinning speeds (1-12 kHz). Results on the fully hydrated sample (x = 1.5) are consistent with the model of diffusion of H3O+ ions within the layers. In the partially dehydrated sample (x = 0.5) an exchange between the distinctly present cage protons and H3O+ protons leads to protonic conduction.

Structures, Stabilities and Strain in C60 and C70 Derivatives Fullerenes, Nanotubes and Carbon Nanostructures

The stabilities of a number of small adducts as well as larger hydrides of C-60 and C-70 are reported using semiempirical MO methods. The data are shown to be consistent with the nature of bond alternation in the parent fullerenes and strain effects in the cage systems.

Interfacial Regions Governing Internal Cavities of Dendrimers. Studies of Poly(alkyl aryl ether) Dendrimers Constituted with Linkers of Varying Alkyl Chain Length

This report deals with a study of the properties of internal cavities of dendritic macromolecules that are capable Of encapsulating and mediating photoreactions of guest molecules. The internal cavity structures of dendrimers are determined by the interfacial regions between the aqueous exterior and hydrocarbon like interior constituted by the linkers that connect symmetrically sited branch points constituting the dendrimer and head groups that cap the dendrimers. Phloroglucinol-based poly(alkyl aryl ether) dendrimers constituted with a homologous series of alkyl linkers were undertaken for the current study. Twelve dendrimers within first, second, and third generations, having ethyl, n-propyl, n-butyl, and n-pentyl groups as the linkers and hydroxyl groups at peripheries in each generation, were synthesized. Encapsulation of pyrene and coumarins by aqueous basic solutions of dendrimers were monitored-by UV-vis and fluorescence spectroscopies, which showed that a lower generation dendrimer with an optimal alkyl linker presented better encapsulation abilities than a higher generation dendrimer. Norrish type I photoreaction of dibenzyl ketone was carried out within the above: series of dendrimers to probe their abilities to hold guests and reactive inthermediate radical pairs within themselves. The extent of cage effect from the series of third generation dendrimers was observed to be higher with dendrimers having an n-pentyl group as the linker.