"Click" synthesis and properties of carborane-appended large dendrimers

Large dendrimers, noted G(n)-3(n+2)cage, containing 3(n+2) o-carborane cluster cages MeC(2)B(10)H(10) at their peripheries (n = number of generation noted G(n)) have been synthesized by Huisgen-type azide alkyne Cu(I)-catalyzed dipolar "click" cycloaddition reactions (CuAAC) between an o-carborane monomeric cluster containing an ethynyl group and arene-centered azido-terminated dendrimers G(n)-3(n+2)N(3) of generations 0, 1, and 2. Attempts to synthesize higher-generation dendrimers of this family yielded insoluble materials. The carborane dendrimers G(0)-9cage, G(1)-27cage, and G(2)-81cage have been characterized by (1)H, (13)C, (11)B NMR, elemental analysis, matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectroscopy, and size exclusion chromatography (SEC) showing low polydispersities, dynamic light scattering (DLS) showing hydrodynamic diameters of 5.7 nm for the G(1)-27cage and the 12.9 nm for the G(2)-81cage. These dendrimers are extremely robust thermally, with 10% mass loss temperatures of 411 °C for the G(0)-9cage, 371 °C for the G(1)-27cage, and 392 °C for the G(2)-81cage. They all showed a strong absorption in the UV region peaking at 258 nm, whereas emission spectra of low intensities were observed between 280 and 480 nm.     

N–H···π Interactions in Two Isomers of an Amino Group Containing <Emphasis Type="Italic">bis</Emphasis>-Phenol

Abstract  

Crystal structures of two bis-phenols namely bis-(3,5-dimethyl-2-hydroxyphenyl)(3-amino phenyl)methane 1 and bis-(3,5-dimethyl-4-hydroxyphenyl)(3-aminophenyl) methane 2 are determined. The compound 1 crystallises in monoclinic P2₁/c with a = 12.2579(16) ?, b = 16.0906(19) ?, c = 10.6664(13) ?, β = 115.417(7)°, V = 1900.2(4) ?³ whereas 2 crystallizes in monoclinic C2/c, a = 9.2538(2) ?, b = 18.6579(4) ?, c = 23.2725(5) ?, β = 98.796(2)°, V = 3970.89(15) ?³. The crystal lattice of both the compounds shows presence of N–H···π interactions but no O–H···π interactions.     

Azonium-Ammonium Tautomerism and Inclusion Complexation of 1-(2,4-diamino phenylazo) Naphthalene and 4-aminoazobenzene

Spectral characteristics of 1-(2,4-diamino phenylazo) naphthalene (FBRR, fat brown RR) and 4-aminoazobenzene (AAB) have been studied in various solvents, varying hydrogen ion concentrations and in β-cyclodextrin (β-CD). The inclusion complex of FBRR and AAB with β-CD were analysed by UV-visible, fluorometry and CAche-DFT methods. Solvent study reveals that only azo tautomer is present in both compounds and the large red shifted absorption and emission maxima of FBRR indicate naphthalene ring effectively increases the π-π* transition. Unusual red shift is observed in acid solutions suggests azonium-ammonium tautomer is present in both molecules. In β-CD solutions, the large hypsochromic shift is observed in S₀ and S₁ states indicates ortho amino group of FBRR molecule is entrapped in the β-CD cavity and the large bathochromic shift for AAB in the S₁ state indicates 1:1 inclusion complex is formed.     

On the directed Full Degree Spanning Tree problem

We study the parameterized complexity of a directed analog of the Full Degree Spanning Tree problem where, given a digraph $D$ and a nonnegative integer $k$, the goal is to construct a spanning out-tree $T$ of $D$ such that at least $k$ vertices in $T$ have the same out-degree as in $D$. We show that this problem is W[1]-hard even on the class of directed acyclic graphs. In the dual version, called Reduced Degree Spanning Tree, one is required to construct a spanning out-tree $T$ such that at most $k$ vertices in $T$ have out-degrees that are different from that in $D$. We show that this problem is fixed-parameter tractable and that it admits a problem kernel with at most $8k$ vertices on strongly connected digraphs and $O\left(k^2\right)$ vertices on general digraphs. We also give an algorithm for this problem on general digraphs with running time $O(5.942^k?n^{O\left(1\right)})$, where $n$ is the number of vertices in the input digraph.     

Synthesis, characterization, and controlled aggregation of biotemplated polystyrene nanodisks

Cross-linked polystyrene nanodisks were prepared by controlled polymerization of styrene and divinylbenzene in the interior of bicelles, discoidal lipid aggregates. Aggregation behavior of polymer nanodisks was studied in water, organic solvents, and solid phase. Nanodisks form stable dispersions in aqueous solutions of surfactants, such as sodium dodecyl sulfate (SDS). Varying SDS/nanodisk ratio allowed us to control the size of nanodisk aggregates. Nanodisks are readily solubilized in nonpolar organic solvents, such as toluene and carbon tetrachloride, to yield stable monodisperse suspensions. These findings open opportunities for creating nanodisk-based nanocomposite materials. Stable nanodisk suspension in toluene enabled small angle neutron scattering (SANS) measurements. SANS data confirmed the nanodisk diameter and allowed accurate measurement of nanodisk thickness (19.5 ± 1.0 Å). In solid phase, nanodisks aggregate in sub-micron platelets.     

Spectral Characteristics of Sulfa Drugs: Effect of Solvents,pH and <Emphasis Type="Italic">β</Emphasis>-Cyclodextrin

The absorption and fluorescence spectra of sulfamethoxazole (SMO), sulfisoxazole (SFO), sulfathiazole (STO) and sulfanilamide (SAM) in different solvents, pH and β-cyclodextrin (β-CD) have been analyzed. The inclusion complexes of the above sulfa drugs with β-CD were investigated by UV-visible spectroscopy, fluorometry, DFT, SEM, FT-IR and ¹H NMR. The solvent study indicates that the position of the substituent (oxazole or thiazole group) in the SAM molecule (R–SO₂–NH-group) is not the key factor to change the absorption and emission behavior of these sulpha drug molecules. In aqueous solution, a single fluorescence band (340 nm) was observed whereas in solutions of β-CD dual emission (430 nm) was noticed in sulpha drug compounds. Formation of the inclusion complex in SMO, SFO and STO should result dual emission which is due to a Twisted Intramolecular Charge Transfer band (TICT). The β-CD study indicates that (i) sulpha drugs form 1:1 inclusion complexes with β-CD and (ii) the red shift and the presence of TICT in the β-CD medium confirms heterocyclic ring encapsulated in the β-CD cavity with the aniline ring present on the out side of the β-CD cavity.     

Approximation for the mean value performance of locking algorithms for distributed database systems: a partitioned database

Concurrent access to databases must be synchronized for correct execution of transactions and preservation of data consistency. This is usually achieved through use of concurrency control algorithms, amongst which locking algorithms are the most popular both in the literature and in practice. Several analytic methods have been developed for predicting the performance of centralized database systems employing locking algorithms for concurrency control, but very few exist for distributed database systems.This paper proposes a method to approximate the mean value of various performance parameters in distributed database systems using locking for concurrency control. The main contribution of this approach is its ability to model the interaction between resource and data contention and the resulting effect on system performance. System performance is evaluated at a point where the interaction between these two factors is in equilibrium (stable state) and both the data and resource contention equations are simultaneously satisfied.The model involves the solution of a set of simultaneous polynomial equations whose order is dependent on several problem parameters such as the number of nodes and number of locks requested per transaction. These equations are solved by an iterative procedure to evaluate approximate values of relative throughput, utilization of servers and transaction response time. The small computational requirements of the analytical model permit sensitivity analysis on network parameters, and can thus be effectively used by system designers to evaluate choices of communication line speeds, processor capacity, database sizes, etc.The analytic approximations have been extensively verified against simulations for networks with up to 20 nodes. The input traffic was varied from light loads (about 5% utilization of the channels and processors) to heavy loads (about 65% utilization of the processors and channels). The discrepancies between the analytic approximation and the simulation were quite small (2–8%).This work was done while the authors were at Drexel University, Philadelphia.     

Can CH⋅⋅⋅π Interactions Be Used To Design Single‐Chain Magnets?

This theoretical study suggests that CH???π stacking interactions between monomeric units can be used to design novel single‐chain magnets (SCMs), as the sign of coupling is predictable and such chains inherently yield negative axial anisotropy, a condition often difficult to achieve in conventional SCMs.