Hydrogen-bond-directed self-assembly of D-(+)-dibenzoyltartaric acid and 4-aminopyridine: optical nonlinearities and stoichiometry-dependent novel structural features

Attempts to prepare hydrogen-bond-directed nonlinear optical materials from a 1:1 molar mixture Of D-(+)-dibenzoyltartaric acid (DBT, I) and 4-aminopyridine (4-AP, II) resulted in two salts of different stoichiometry. One of them crystallizes in an unusual 1.5:1 (acid:base) monohydrate salt form III while the other one crystallizes as 1:1 (acid:base) salt IV. Crystal structures of both of the salts were determined from single-crystal X-ray diffraction data. The salt III crystallizes in a monoclinic space group C2 with a = 30.339(l), b = 7.881(2), c = 14.355(1) angstrom, beta = 97.48(1)degrees, V = 3403.1(9) angstrom3, Z = 4, R(w) = 0.058, R(w)= 0.058. The salt IV also crystallizes in a monoclinic space group P2(1) with a = 7.500(1), b = 14.968(2), c = 10.370(1) angstrom, beta = 102.67(1)degrees, V = 1135.9(2) angstrom3, Z = 2, R = 0.043, R(w) = 0.043. Interestingly, two DBT molecules with distinctly different conformation are present in the same crystal lattice of salt III. Extensive hydrogen-bonding interactions are found in both of the salts, and both of them show SHG intensity 1.4-1.6 times that of urea.

Levitation Effect: Role of Symmetry and Dependence of Diffusivity on the Bond Length of Homonuclear and Heteronuclear Diatomic Species

Molecular dynamics investigation of model diatomic species confined to the alpha-cages of zeolite NaY is reported. The dependence of self-diffusivity on the bond length of the diatomic species has been investigated. Three different sets of runs have been carried out. In the first set, the two atoms of the diatomic molecule interact with the zeolite atoms with equal strength (example, O-2, the symmetric case). In the second and third sets which correspond to asymmetric cases, the two atoms of the diatomic molecule interact with unequal strengths (example, CO). The result for the symmetric case exhibits a well-defined maximum in self-diffusivity for an intermediate bond length. In contrast to this, the intermediate asymmetry leads to a less pronounced maximum. For the large asymmetric case, the maximum is completely absent. These findings are analyzed by computing a number of related properties. These results provide a direct confirmation at the microscopic level of the suggestion by Derouane that the supermobility observed experimentally by Kemball has its origin in the mutual cancellation of forces. The maximum in diffusivity from molecular dynamics is seen at the value predicted by the levitation effect. Further, these findings suggest a role for symmetry in the existence of a diffusivity maximum as a function of diameter of the diffusant often referred to as the levitation effect. The nature of the required symmetry for the existence of anomalous diffusivity is interaction symmetry which is different from that normally encountered in crystallography.

Reflections on the Hydrogen Bond in Crystal Engineering

Comments on aspects of the new definition of the hydrogen bond specific to crystal engineering are given.

ATP hydrolysis is required for DNA cleavage by EcoPI restriction enzyme

The type III restriction endonuclease EcoPI, coded by bacteriophage Fl, cleaves unmodified DNA in the presence of ATP and magnesium ions. We show that purified EcoPI restriction enzyme fails to cleave DNA in the presence of non-hydrolyzable ATP analogs. More importantly, this study demonstrates that EcoPI restriction enzyme has an inherent ATPase activity, and ATP hydrolysis is necessary for DNA cleavage. Furthermore, we show that the progress curve of the reaction with Eco PI restriction enzyme exhibits a lag which is dependent on the enzyme concentration. Kinetic analysis of the progress curves of the reaction suggest slow transitions that can occur during the reaction, characteristic of hysteretic enzymes. The role of ATP in the cleavage mechanism of type III restriction enzymes is discussed.

Diagrammatic valence bond studies on hemocyanin

The Diagrammatic Valence Bond studies on the active sites of hemocyanin, consisting of two Cu(I) ions and an oxygen molecule, are performed to find out the stable geometrical pattern and electronic structure. Different parameters used in this theoretical approach are taken from existing literature on high T-c superconductors. Attempts have been made to find out the differences in electronic structure of [Cu2O2](+2) and [Cu2O2N4](+2) as it is observed that coordination of nitrogen ligand do affect electronic structure i.e. spin excitation gaps and charge and spin density distribution. A comparison of our results with earlier theoretical results are also presented.

DNA cleavage by intercalatable cobalt-bispicolylamine complexes activated by visible light

Two intercalatable Co-II-complexes of anthryl or anthraquinone attached bispicolylamine derivatives cleave plasmid pTZ19R DNA spontaneously upon exposure to visible light under ambient conditions.

Flexural bond strength of masonry using various blocks and mortars

This paper deals with an experimental study on flexural bond strength of masonry using various blocks in combination with different mortars. Flexural bond strength of masonry has been determined by testing stack-bonded prisms using a modified bond wrench test set-up. The effect of mortar composition and strength on the masonry's. flexural bond strength using three types of masonry units (stabilized mud blocks, stabilized soil-sand blocks and burnt brick) has been examined. The effect of the masonry unit's moisture content on flexural bond strength has also been studied. Increases in mortar strength lead to increased flexural bond strength for cement mortar, irrespective of the type of masonry unit. It has been found that combination mortars, such as soil-cement mortar and cement-lime mortar, lead to better bond strength compared to cement mortars. The moisture content of the masonry unit at the time of casting has displayed significant influence on the flexural bond strength of the masonry. It has been found that for each type of masonry unit, an optimum moisture content exists, beyond which the flexural bond strength falls off quickly.

Brownian dynamics simulation of dense binary colloidal mixtures. II. Translational and bond-orientational order

We report the Brownian dynamics simulation results on the translational and bond-angle-orientational correlations for charged colloidal binary suspensions as the interparticle interactions are increased to form a crystalline (for a volume fraction phi = 0.2) or a glassy (phi = 0.3) state. The translational order is quantified in terms of the two- and four-point density autocorrelation functions whose comparisons show that there is no growing correlation length near the glass transition. The nearest-neighbor orientational order is determined in terms of the quadratic rotational invariant Q(l) and the bond-orientational correlation functions g(l)(t). The l dependence of Q(l) indicates that icosahedral (l = 6) order predominates at the cost of the cubic order (l = 4) near the glass as well as the crystal transition. The density and orientational correlation functions for a supercooled liquid freezing towards a glass fit well to the streched-exponential form exp[-(t/tau)(beta)]. The average relaxation times extracted from the fitted stretched-exponential functions as a function of effective temperatures T* obey the Arrhenius law for liquids freezing to a crystal whereas these obey the Vogel-Tamman-Fulcher law exp[AT(0)*/(T* - T-0*)] for supercooled Liquids tending towards a glassy state. The value of the parameter A suggests that the colloidal suspensions are ''fragile'' glass formers like the organic and molecular liquids.

Mechanism of ?-cleavage in the excited states of keto sulfides

The beta-cleavage process in photoexcited ketones of structure RCOCH2X (X = CH2CH3, OCH3, SCH3; R = CH3, Ph) has been studied using the configuration interaction procedure within the framework of MINDO/3. The results explain qualitatively why the beta-cleavage process is faster than the alpha-cleavage process in keto sulfides while in keto ethers the reverse is true.

Organometallic chemistry of diphosphazanes .13. Palladium complexes of unsymmetrical diphosphazanes Ph(2)PN(Pr-i)PYY'

Unsymmetrical diphosphazanes Ph(2)PN(Pr-i)PYY' [YY' = O2C12H8 (L(1)), O2C20H12 (L(2)); Y = Ph and Y' = OC6H4Br-4 (L(3)), OC(6)H(4)Me-4 (L(4)), OC(6)H(3)Me(2)-3,5 (L(5)), N(2)C(3)HMe(2)-3,5 (L(6))] react with cis-[PdCl2(COD)] (COD = cycloocta-1,5-diene) giving the chelate complexes of the type cis-[PdCl2{eta(2)-Ph(2)PN(Pr-i)PYY'}] [YY' = O2C12H8 (1), O2C20H12 (2), Y = Ph and Y' = OC6H4Br-4 (3), OC(6)H(4)Me-4 (4), OC(6)H(3)Me(2)-3,5 (5), N(2)C(3)HMe(2)-3,5 (6)]. The P-N bond in 3 and 5 undergoes a facile cleavage in methanol solution to give cis-[PdCl2{eta(1)Ph(2)P(OMe)}{eta(1)-PhP(NHPri)(Y')}] [Y' = OC6H4Br-4 (7), OC(6)H(3)Me(2)-3,5 (8)]. Reactions of Pd-2(dba)(3) . CHCl3 (dba = dibenzylideneacetone) with the diphosphazanes Ph(2)PN(Pr-i)PPhY' [Y' = OC(6)H(4)Me-4 (L(4)), N(2)C(3)HMe(2)-3,5 (L(6)), N2C3H3 (L(7))] in the presence of MeI yields cis-[PdI2{eta(2)-Ph(2)PN(Pr-i)PPhMe}] (9); the P-O or P-N(pyrazolyl) bond of the starting ligands is cleaved and a p-C(Me) bond is formed. An analogous oxidative addition reaction in the presence of Ph(2)PN(Pr-i)PPh(2) (L(8)) yields cis-[PdI(Me)(eta(2)-L(8))] (10) and cis-[PdI2(eta 2-L(8))] (11). The structures of 8 and 9 have been determined by X-ray diffraction. Copyright (C) 1996 Elsevier Science Ltd

Primary pyrolysis products of thiokol liquid polysulfide

An analysis of the primary degradation products of the widely used commercial polysulfide polymer Thiokol LP-33 by direct pyrolysis-mass spectrometry (DP-MS) is reported. The mechanism of degradation is through a radical process involving the random cleavage of a formal C-O bond followed by backbiting to form the cyclic products.

Effect of Strain Rate on Ductile-to-Brittle Transition Temperature of a Free-Standing Pt-Aluminide Bond Coat

The ductile-to-brittle transition temperature (DBTT) of a free-standing Pt-aluminide (PtAl) bondcoat was determined using the microtensile testing method and the effect of strain rate variation, in the range 10(-5) to 10(-1) s(-1), on the DBTT studied. The DBTT increased appreciably with the increase in strain rate. The activation energy determined for brittle-to-ductile transition, suggested that such transition is most likely associated with vacancy diffusion. Climb of aOE (c) 100 > dislocations observed in analysis of dislocation structure using a transmission electron microscope (TEM) supported the preceding mechanism.

ChemInform Abstract: Simple and Regioselective Reductive Cleavage of Tetrahydropyranyl Ethers to Alcohols

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Synthesis of new transition metal nitrides, MWN2 (Mtriple bond; length as m-dashMn, Co, Ni)

We report the synthesis of ternary transition metal nitrides of the formula MWN(2) for M=Mn, Co, Ni by reaction of the corresponding MWO(4) with NH3 gas at 600-700 degrees C. MnWN2 is isostructural with the already-known FeWN2, crystallizing in a hexagonal structure (a=2.901(2), b=16.48(5) Angstrom) related to LiMoN2. CoWN2 and NiWN2 (which are isostructural amongst themselves) adopt a different hexagonal structure with a smaller c parameter. While the Mn and Fe nitrides are semiconducting, the Co and Ni nitrides are semimetallic.

Synthesis, structure and properties of monomeric complexes obtained from the cleavage of a (?-oxo)bis(?-carboxylato) diruthenium(III) core

Reaction of [Ru2O(O(2)CR)(2)(MeCN)(4)(PPh(3))(2)](ClO4)(2) (1) with 1,2-diaminoethane (en) in MeOH-H2O yielded a mixture of products from which a diamagnetic ruthenium(II) complex [Ru(MeCN)(en)(2)(PPh(3))](ClO4)(2) (2) and a paramagnetic ruthenium(III) species [Ru(O(2)CR)(en)(2)(PPh(3))](BPh(4))(2) (3) (R = Ph, a; C6H4-p-Me, b; C6H4-p-OMe, c) were isolated and characterized. The crystal structure of complex 2, obtained by X-ray diffraction analysis, shows a cis arrangement of the unidentate ligands in this octahedral complex. Complex 3 displays an axial EPR spectrum. Complex 2 undergoes two successive irreversible metal-centred one-electron oxidation processes at 1.13 and 1.33 V vs SCE in MeCN-0.1 M [NBu(4)(n)]ClO4 at 50 mV s(-1). The mechanistic aspects of the core cleavage reactions in 1 are discussed.