Syntheses, spectroscopy, structures, and conformations of .lambda.3-cyclotriphosphazanes: role of negative hyperconjugation

The reactions of As-chlorocyclotriphosphazane [EtNPCl], with phenols or trifluoroethanol yield the respective aryloxy- or trifluoroethoxy-containingX 3-cyclotriphosphazanes [EtNP(OR)]3 (R = C6H4Br-4 (2),C 6H5 (3C,6 H3-Mez-3,5 (4), C6H3Mez-2,6 (5), CH2CF3 (6)) as their cis-transisomericmixtures. The products have beencharacterized by IRand NMRspectroscopy. Thecrystalstructuresofboth thecis (2a) and trans(2b) isomer_softhep-bromophenoxy derivative have been determined by X-ray diffraction. Crystal data for 2a: triclinic, P1, a = 9.872(4) A, b = 13.438(6) A, c = 13.548(8) A, CY = 117.02(5)', 0 = 96.00(6)', y = 105.38(4)O, Z = 2, final R = 0.080. Crystal data for 2b: monoclinic, P21/n, a = 12.721(6) A, b = 13.468(7) A, c = 17.882(5) A, /3 = 101.62(3)O, Z = 4, final R = 0.066. The cis isomer exhibits a chair-triaxial conformation and the trans isomer a boat-triaxial conformation. Conformational preferences of X3-cyclotriphosphazanes have been probed by both MNDO and ab initio calculations on model systems [HNPXIp (X = H, F). In addition to vicinal lone pair repulsions, negative hyperconjugative interactions involving the nitrogen lone pairs and adjacent P-X Q* orbitals are found to be important (especially when X is an electronegative substituent) in determining the conformational preferences of X3-cyclotriphosphazanes. The calculations also show that the axial - equatorial conversion at phosphorus has a large activation barrier in these systems

Synthesis of Novel Oxide Pyrochlores, A2BB?O7 (A = La, Nd; BB? = Pb, Sn, Bi), by Alkali Melt Route

Oxide pyrochlores of the formula A2BB? O7 (A = La, Nd; BB? = Pb, Sn, Bi) have been synthesized by a low-temperature ambient-pressure route employing KOH melts. All the compositions, including La2Bi2O7 and its strontium-substituted derivatives, La2-xSrxBi2O7-?, are deeply colored insulators, confirming that a metallic ground state is not achieved for Pb(IV) and Bi(IV/V) oxides with the pyrochlore structure.

An Approach Toward Homocalystegines and Silyl-homocalystegines. Acid-Mediated Migrations of Acetates in Seven-Membered Ring Systems

A short access to homocalystegine analogues silylated at C7 is described. The synthesis involves the desymmetrization of a (phenyldimethylsilyl)methylcycloheptatriene using osmium-mediated dihydroxylation, followed by the diol protection and a cycloaddition involving the remaining diene moiety and an acylnitroso reagent. Additions of the osmium and acylnitroso reagents were shown, through X-ray diffraction studies of the resulting major isomers, to occur anti and syn, respectively, relative to the SiCH2 substituent. N-O bond cleavage on the resulting cycloadduct then produces the aminopolyol having a silylmethyl substituent. Oxidation of the C-Si bond also afforded an access to unusual amino-heptitols having five contiguous stereogenic centers. In the course of this work, we finally observed a unusual rearrangement taking place on cycloheptanone 18 substituted by two acetyl groups and a neighboring Boc-protected amine. A profound reorganization of the substituents on the seven-membered ring effectively took place under acidic conditions (TFA) leading to the thermodynamically more stable homocalystegine-type compound., DFT calculations of the conformational energy of isomeric silyl homocalystegines indicated that the product observed upon the acid-mediated rearrangement was the most stable of a series of analogues with various distributions of substituents along the seven-membered ring backbone. A tentative mechanism is proposed to rationalize the acetate migrations and inversions of the stereochemistry at various stereocenters.

Geometry and quadratic nonlinearity of charge transfer complexes in solution using depolarized hyper-Rayleigh scattering

We report large quadratic nonlinearity in a series of 1:1 molecular complexes between methyl substituted benzene donors and quinone acceptors in solution. The first hyperpolarizability, beta(HRS), which is very small for the individual components, becomes large by intermolecular charge transfer (CT) interaction between the donor and the acceptor in the complex. In addition, we have investigated the geometry of these CT complexes in solution using polarization resolved hyper-Rayleigh scattering (HRS). Using linearly (electric field vector along X direction) and circularly polarized incident light, respectively, we have measured two macroscopic depolarization ratios D = I-2 omega,I-X,I-X/I-2 omega,I-Z,I-X and D' = I-2 omega,I-X,I-C/I-2 omega,I-Z,I-C in the laboratory fixed XYZ frame by detecting the second harmonic scattered light in a polarization resolved fashion. The experimentally obtained first hyperpolarizability, beta(HRS), and the value of macroscopic depolarization ratios, D and D', are then matched with the theoretically deduced values from single and double configuration interaction calculations performed using the Zerner's intermediate neglect of differential overlap self-consistent reaction field technique. In solution, since several geometries are possible, we have carried out calculations by rotating the acceptor moiety around three different axes keeping the donor molecule fixed at an optimized geometry. These rotations give us the theoretical beta(HRS), D and D' values as a function of the geometry of the complex. The calculated beta(HRS), D, and D' values that closely match with the experimental values, give the dominant equilibrium geometry in solution. All the CT complexes between methyl benzenes and chloranil or 1,2-dichloro-4,5-dicyano-p-benzoquinone investigated here are found to have a slipped parallel stacking of the donors and the acceptors. Furthermore, the geometries are staggered and in some pairs, a twist angle as high as 30 degrees is observed. Thus, we have demonstrated in this paper that the polarization resolved HRS technique along with theoretical calculations can unravel the geometry of CT complexes in solution. (C) 2011 American Institute of Physics. doi:10.1063/1.3514922]

Geometry and quadratic nonlinearity of charge transfer complexes in solution: A theoretical study

In this paper, we have computed the quadratic nonlinear optical (NLO) properties of a class of weak charge transfer (CT) complexes. These weak complexes are formed when the methyl substituted benzenes (donors) are added to strong acceptors like chloranil (CHL) or di-chloro-di-cyano benzoquinone (DDQ) in chloroform or in dichloromethane. The formation of such complexes is manifested by the presence of a broad absorption maximum in the visible range of the spectrum where neither the donor nor the acceptor absorbs. The appearance of this visible band is due to CT interactions, which result in strong NLO responses. We have employed the semiempirical intermediate neglect of differential overlap (INDO/S) Hamiltonian to calculate the energy levels of these CT complexes using single and double configuration interaction (SDCI). The solvent effects are taken into account by using the self-consistent reaction field (SCRF) scheme. The geometry of the complex is obtained by exploring different relative molecular geometries by rotating the acceptor with respect to the fixed donor about three different axes. The theoretical geometry that best fits the experimental energy gaps, beta(HRS) and macroscopic depolarization ratios is taken to be the most probable geometry of the complex. Our studies show that the most probable geometry of these complexes in solution is the parallel displaced structure with a significant twist in some cases. (C) 2011 American Institute of Physics. doi:10.1063/1.3526748]

2-(4-Methylsulfanylphenyl)-1H-benzimidazol-3-ium bromide

In the cation of the title compound, C14H13N2S+center dot Br-, the essentially planar benzimidazole system (r.m.s. deviation = 0.0082 angstrom) is substituted with a 4-methylsulfanylphenyl ring. The dihedral angle between the benzimidazole system and the 4-methylsulfanylphenyl ring is 2.133 (2)degrees. The crystal structure is characterized by strong and highly directional intermolecular N-H center dot center dot center dot Br hydrogen bonds involving the bromide ion. Moreover, C-H center dot center dot center dot S interactions result in chains of molecules along the c axis. The supramolecular assembly is further stabilized by pi-pi stacking interactions between the benzimidazole system and 4-methylsulfanylphenyl rings centroid-centroid distance = 3.477 (4) angstrom].

Second harmonic generation in poled molecularly doped polymer films : Push-pull butadienes in poly(methyl methacrylate) and polystyrene

We report second harmonic generation in a new class of organic materials, namely donor-acceptor substituted all-trans butadienes doped in poly(methyl methacrylate) or polystyrene and oriented by corona poling at elevated temperatures. Second harmonic measurements were made at room temperature. The observed d33 coefficients are greater than those of potassium dihydrogen phosphate or 4-dimethylamino-4'-nitrostilbene doped in similar polymer matrices. Rotational diffusion coefficients estimated from the decay characteristics of the second harmonic intensity in the polymer films indicate that the polymer matrix plays a major role in stabilizing the dopants in a nonlinear optics conducive environment.

5-Fluorouracil induces structural changes in rat liver tRNA

5-fluorouracil (FUra) has been shown to modulate the aminoacylation function of rat liver tRNA. The present study was aimed at studying the structure-function relationship of FUra-substituted tRNA. Male Wistar rats (2-3 month old) were given a single i.p. injection of FUra at 50, 250, or 500 mg/kg body wt. and FUra-substituted total liver tRNA, i.e. tRNA(FUra50, 250, and 500, respectively, were isolated 3 h later. Normal tRNA (tRNA(N)) was isolated from saline-treated control rats. Thermal denaturation studies showed higher melting temperatures for tRNA(FUra) compared to tRNA(N). Heat denaturation followed by renaturation of total tRNA did not affect the activity of tRNA(N) and tRNA(FUra50), where as tRNA(FUra250 and 500) lost 35% and 72% of activity, respectively, compared to the corresponding group of non-denatured tRNA. Antibodies specific to rat liver tRNA recognized normal and FUra-substituted tRNA in the order of tRNA(N) > tRNA(FUra50) > or = tRNA(FUra250) > tRNA(FUra500) in an avidin-biotin micro-enzyme linked immunosorbant assay. tRNA(N) or tRNA(FUra50) preincubated with tRNA antiserum showed 74% and 59% of aminoacylation activity, respectively, compared to that of corresponding tRNA preincubated with normal rabbit IgG. However, activities of similarly treated tRNA(FUra250 and 500) were not affected. The observations of possible changes in the secondary structure of rat liver tRNA upon incorporation of FUra are discussed.

AB-initio study of alpha-silyl and beta-silyl substituent effects on vinyl radicals

The geometries of alpha- and beta-silyl substituted vinyl radicals and of alpha,beta-disilylvinyl radical have been optimised with the STO-3G and the STO-3G* basis sets. The relative stabilities of various conformers have been determined at the UMP2/6-31G* level. The stabilisation of vinyl radicals through alpha-silyl substitution is larger than that due to corresponding alkyl groups. The presence of an alpha-silyl group also leads to a tendency towards linearisation of the vinyl radical centre and a corresponding reduction in the inversion barrier. In marked contrast, the beta-silyl effect is negligible. The geometric, conformational and energetic consequences are insignificant. Overall, the silyl substituent effect at vinyl radicals is very different from that computed earlier for the vinyl cations, but qualitatively similar to that found in carbanions.

A salt of a protonated (amino)spirocyclic cyclotriphosphazene

The title compound 4,4,6,6-tetrakis(dimethylamino)-2 lambda(5),4 lambda(5),6 lambda(5)-cyclotriphosphaza- 1,3,5-trien-1-ium-2-spiro-2'-(2'-phospha-1',3'-diazacyclohexane) tetrachloroplatinate, [HN3P3(NMe(2))(4)(NHCH2CH2CH2NH)](2)-[PtCl4], contains a cyclophosphazenium cation in which the hetero-substituted (amino)spirocyclic cyclotriphosphazene is protonated. The protonation has occurred on one of the P3N3 ring N atoms adjacent to the spiro P atom. The protonation results in lengthening of the ring P-N bonds and puckering of the phosphazene ring. In the crystal lattice, 2n cyclophosphazenium cations are connected by n [PtCl4](2-) anions mediated by N-H...Cl hydrogen bonds to form a linear polymeric structure.

A regiospecific radical annulation strategy to functionalised chiral bicyclo[3.3.l]nonanes

A radical annulation, i.e. an intermolecular radical Michael addition followed by an intramolecular Michael addition of the resultant radical (radical cyclisation) has been employed for the construction of chiral functionalised bicyclo[3.3,1]-nonanes. Thus reaction of carvone hydrohalides 7 with (n)Bu(3)SnH and AIBN in the presence of excess of radicophiles 4 furnished, regiospecifically bicyclo[3.3.1]nonanes 8-14, introducing three new chiral centres in a stereoselective manner. Analogously the bromide 18 generated the bridgehead substituted bicyclo[3.3.1]-nonanes 19-21.

Synthesis, structure and ionic conductivity in scheelite type Li(0.5)Ce(0.5-x)Ln(x)MoO(4) (x=0 and 0.25, Ln = Pr, Sm)

Scheelite type solid electrolytes, Li(0.5)Ce(0.5-x)Ln(x)MoO(4) (x = 0 and 0.25, Ln = Pr, Sm) have been synthesized using a solid state method. Their structure and ionic conductivity (a) were obtained by single crystal X-ray diffraction and ac-impedance spectroscopy, respectively. X-ray diffraction studies reveal a space group of I4(1)/a for Li(0.5)Ce(0.5-x)Ln(x)MoO(4) (x = 0 and 0.25, Ln = Pr, Sm) scheelite compounds. The unsubstituted Li0.5Ce0.5MoO4 showed lithium ion conductivity similar to 10(-5)-10(-3) Omega(-1)cm(-1) in the temperature range of 300-700 degrees C (sigma = 2.5 x 10(-3) Omega(-1) cm(-1) at 700 degrees C). The substituted compounds show lower conductivity compared to the unsubstituted compound, with the magnitude of ionic conductivity being two (in the high temperature regime) to one order (in the low temperature regime) lower than the unsubstituted compound. Since these scheelite type structures show significant conductivity, the series of compounds could serve in high temperature lithium battery operations.

Ce0.98Pd0.02O2-delta: Recyclable, ligand free palladium(II) catalyst for Heck reaction

Palladium substituted in cerium dioxide in the form of a solid solution, Ce-0.98 Pd-0.02 O-1.98 is a new heterogeneous catalyst which exhibits high activity and 100% trans-selectivity for the Heck reactions of aryl bromides including heteroaryls with olefins. The catalytic reactions work without any ligand. Nano-crystalline Ce-0.98 Pd-0.02 O-1.98 is prepared by solution combustion method and Pd is in +2 state. The catalyst can be separated, recovered and reused without significant loss in activity.

Interaction of ecoP15I DNA methyltransferase with oligonucleotides containing the asymmetric sequence 5?-CAGCAG-3

EcoP15I DNA methyltransferase (Mtase) recognizes the asymmeteric sequence CAGCAG and catalyzes the transfer of a methyl group from S-adenosyl-L-methionine to the second adenine residue. We have investigated the DNA binding properties of EcoP15I DNA Mtase using gel mobility shift assays. EcoP15I DNA Mtase binds approximately threefold more tightly to DNA containing its recognition sequence, CAGCAG, than to non-specific sequences in the absence or presence of cofactors. Interestingly, in the presence of ATP the discrimination between specific and non-specific sequences increases significantly. These results suggest for the first time a role for ATP in DNA recognition by type III restriction-modification enzymes. In addition, we have shown that bromodeoxyuridine-containing oligonucleotides form complexes with EcoP15I DNA Mtase that are crosslinked upon irradiation. More importantly, we have shown that the crosslink site is at the site of DNA binding, since it can be suppressed by an excess of unmodified oligonucleotide. EcoP15I DNA Mtase exhibited Michaelis-Menten kinetics with both unmodified and bromodeoxyuridine-substituted DNA, with a higher specificity constant for the latter. Furthermore, gel mobility shift assays showed that proteolyzed EcoP15I DNA Mtase formed a specific complex with DNA, which had similar mobility as the native protein-DNA complex. Taken together these results form the basis fora detailed structure-function analysis of EcoP15I DNA Mtase.

Reactivity and catalytic activity of layered YBA2CU3O7-delta (123) type defect perovskites

The chemical modifications of structure, reactivity and catalytic properties of layered triple perovskite oxides, related to the YBa2Cu3O7-delta (123) system, have been briefly reviewed. These oxides form a versatile family of materials with wide-ranging chemical and physical properties. The multiple sites available for chemical doping, and the ability to reversibly intercalate oxygen at the defect sites have rendered these oxides important model systems in the area of oxide catalysis. An attempt has been made to comprehend the hitherto known catalytic reactions and correlate them to various factors like structure, oxygen diffusional limitations, different geometries adopted by various substituents, oxidative non-stoichiometry and activation energy for oxygen desorption. In particular, results on the enhanced catalytic activity of cobalt-substituted 123 oxide systems towards the selective catalytic oxidation of ammonia to nitric oxide and carbon monoxide to carbon dioxide are presented.