bluuebunny/crossref_metadata_2025 huggingface dataset:
Fabrication of lithographically defined optical coupling facets for silicon-on-insulator waveguides by inductively coupled plasma etching
Authors: Yap K. P. , Lamontagne B. , Delâge A. , Janz S. , Bogdanov A. , Picard M. , Post E. , Chow-Chong P. , Malloy M. , Roth D. , Marshall P. , Liu K. Y. , Syrett B.
Published: May 2006

We present a technique to lithographically define and fabricate all required optical facets on a silicon-on-insulator photonic integrated circuit by an inductively coupled plasma etch process. This technique offers 1μm positioning accuracy of the facets at any location within the chip and eliminates the need of polishing. Facet fabrication consists of two separate steps to ensure sidewall verticality and minimize attack on the end surfaces of the waveguides. Protection of the waveguides by a thermally evaporated aluminum layer before the 40–70μm deep optical facet etching has been proven essential in assuring the facet smoothness and integrity. Both scanning electron microscopy analysis and optical measurement results show that the quality of the facets prepared by this technique is comparable to the conventional facets prepared by polishing.
Conductance measurement of a conical tube and calculation of the pressure distribution
Authors: Mercier B.
Published: May 2006

In recent articles [J. Gomez-Goni, and P. J. Lobo J. Vac. Sci. Technol. A 21, 1452 (2003); P. Swemin and M. Niewinski, Vacuum 67, 359 (2002)] the conductance of conical tube in the molecular flow regime has been calculated using the Monte Carlo method or by the resolution of the Clausing integral equation, reformulated by Iczkowski et al. [J. Phys. Chem. 67, 229 (1963)], for the case of a cone. The comparison between the analytical values and different simulations allows one to determine a correction factor k to apply to the intrinsic conductance of the cones. This coefficient depends on the propagation direction of flow and increases considerably for larger conic angles. For a cone half-angle of 40° and a length ten times greater than smallest entrance radii, the correction factor is approximately 5.3 for a circulating flow from the smallest to the largest orifice. Our experimental device measured the conical conductance by a dynamic method. In order to do this, it was necessary to determine the surface pressure distribution. The extension of the Oatley method, with the addition of several components of various transmission probabilities, permits one to establish this distribution for a vacuum system and thus to give the pressure measured by a gauge situated along the wall of the duct. This method provides a good approximation for tubes and cones and can be used for engineering practice. The determination of this distribution is all the more critical when the conductance and the pumping speed are large and can thus have a great influence on the vacuum metrology.
Characterization of the NiFe sputter etch process in a rf plasma
Authors: Kropewnicki Thomas J. , Paterson Alex M. , Panagopoulos Theodoros , Holland John P.
Published: May 2006

The sputter etching of NiFe thin films by Ar ions in a rf plasma has been studied and characterized with the use of a Langmuir probe. The NiFe sputter etch rate was found to depend strongly on incident ion energy, with the highest NiFe etch rates occurring at high rf bias power, low pressure, and moderate rf source power. NiFe etch rates initially increased with increasing rf source power, then saturated at higher rf source powers. Pressure had the weakest effect on NiFe etch rates. Empirically determined sputter yields based on the NiFe etch rates and ion current densities were calculated, and these compared favorably to sputter yields determined using the sputtering model proposed by Sigmund [Phys. Rev. 184, 383 (1969)].
Switching-speed calculations for Schottky-barrier carbon nanotube field-effect transistors
Authors: John D. L. , Pulfrey D. L.
Published: May 2006

The switching times and on/off-current ratios are computed for Schottky-barrier carbon nanotube field-effect transistors with different tube diameters and insulator thicknesses. It is indicated that it may be difficult to obtain a device exhibiting both high speed and low leakage current. A small-diameter nanotube with a thin insulator may offer the best compromise. It is also demonstrated that interelectrode capacitances can be large, thereby calling into question the usefulness of the intrinsic switching time as a figure of merit for transistors intended for digital-logic applications. The extrinsic switching time is a more appropriate metric and it is shown here that considerable optimization of the carbon nanotube field-effect transistor will be required to achieve figures better than for modern Si complementary metal-oxide-semiconductor transistors over a reasonable range of on/off-current ratio.
The synthesis, structure, and electrochemical properties of Fe(C≡CC≡N)(dppe)Cp and related compounds
Authors: Smith Mark E , Cordiner Richard L , Albesa-Jové David , Yufit Dmitri S , Hartl František , Howard Judith AK , Low Paul J
Published: May 2006

The cyanoacetylide complex Fe(C≡CC≡N)(dppe)Cp (3) is readily obtained from sequential reaction of Fe(C≡CSiMe3)(dppe)Cp with methyllithium and phenyl cyanate. Complex 3 is a good metalloligand, and coordination to the metal fragments [RhCl(CO)2], [Ru(PPh3)2Cp]+, and [Ru(dppe)Cp*]+ affords the corresponding cyanoaceylide-bridged heterobimetallic complexes. In the case of the 36-electron complexes [Cp(dppe)Fe-C≡CC≡N-MLn]n+, spectroscopic and structural data are consistent with a degree of charge transfer from the iron centre to the rhodium or ruthenium centre via the C3N bridge, giving rise to a polarized ground state. Electrochemical and spectroelectrochemical methods reveal significant interactions between the metal centres in the oxidized (35 electron) derivatives, [Cp(dppe)Fe-C≡CC≡N-MLn](n+1)+. Key words: cyanide, cyanoacetylide, crystal structure.
(–)-Sparteine-mediated stereoselective directed ortho metalation of ferrocene diamides
Authors: Laufer Radoslaw , Veith Ulrich , Taylor Nicholas J , Snieckus Victor
Published: May 2006

The utility of (–)-sparteine-mediated directed ortho metalation (DoM) has been investigated in stereoselective preparation of planar chiral ferrocenes derived from 1,1′-N,N,N′,N′-tetraisopropylferrocenedicarboxamide (5). In the synthesis of C2-symmetric analogs of 5, the protocol (base, solvent, and two-step DoM) was found to be crucial for obtaining high enantio- and diastereo-selectivities of the products. A variety of highly enantioenriched mono and doubly functionalized derivatives of 5 have been synthesized. The synthetic applications of these compounds as chiral ligands in asymmetric alkylation of aldehydes and asymmetric palladium-catalyzed allylic substitutions have been demonstrated.Key words: directed ortho metalation, stereoselective deprotonation, ferrocene ligands, asymmetric catalysis.
Antimony(III) chloride impregnated on alumina — An efficient and economical Lewis acid catalyst for one-pot synthesis of dihydropyrimidinones under solvent-free conditions
Authors: Kapoor Kamal K , Ganai Bilal A , Kumar Satish , Andotra Charanjeet S
Published: May 2006

The antimony(III) chloride impregnated on alumina efficiently catalyses a one-pot, three-component condensation reaction among an aldehyde, a β-ketoester, and urea or thiourea to afford the corresponding dihydropyrimidinones in good to excellent yields. The reactions are probed in microwave (MW), ultrasonic, and thermal conditions and the best results are found using MW under solvent-free conditions.Key words: Biginelli dihydropyrimidinones synthesis, SbCl3–Al2O3, MW, sonication, solid supported.
QSAR modeling of neonicotinoid insecticides for their selective affinity towards Drosophila nicotinic receptors over mammalian α4β2receptors
Authors: Basu Anindya , Gayen Shovanlal , Samanta Soma , Panda Parthasarathi , Srikanth Kolloru , Jha Tarun
Published: May 2006

Neonicotinoids are emerging as a major class of insecticides with promising insecticidal activity having a specific affinity towards the nicotinic acetylcholine receptors (nAChR). A quantitative structureactivity relationship (QSAR) study was performed on some azidopyridinyl neonicotinoids for their selective insecticidal activity over mammalian toxicity. The result showed that increased surface area of the molecules may help to increase the binding affinity of the compounds towards the Drosophila receptor and the presence of the azido group on the other hand may be detrimental towards the affinity. Compounds having low polarity, increased probability of nucleophilic attack at the particular position (N-1), and a higher positive charge at the C-12 position can reduce the binding affinity of these compounds towards the mammalian receptor.Key words: QSAR, neonicotinoids, Drosophila nicotinic receptor, mammalian α4β2receptor, AM1.
Biosynthesis of 5-hydroxy-4-oxo-L-norvaline in Streptomyces akiyoshiensis
Authors: White Robert L. , Smith Kevin C. , DeMarco Alphonse C.
Published: May 2006

The biosynthesis of 5-hydroxy-4-oxo-L-norvaline (HON) in Streptomyces akiyoshiensis has been investigated using 13C-labelled substrates. Incorporations of 13C label from sodium [1-13C]-, [2-13C]-, and [1,2-13C2]acetate indicated that HON was formed from a four-carbon compound derived from the citric acid cycle and the methyl carbon of acetate. Feeding experiments using DL-[4-13C]- and DL-[2-13C,15N]aspartate demonstrated that aspartate served as the four-carbon precursor to HON. Both enantiomers of aspartate were metabolized by S. akiyoshiensis, but the D isomer was consumed at a slower rate. The distribution of 13C label in the intracellular L-glutamic acid isolated in these feeding experiments is consistent with the operation of the citric acid cycle in S. akiyoshiensis. A biosynthetic hypothesis that involves a condensation reaction between acetyl or malonyl CoA and the β-carboxyl group of aspartate, and subsequent oxidative decarboxylation, is proposed to account for the incorporation results. An analogous condensation step has been proposed for the biosynthesis of other natural products, including the carbapenem antibiotics. DL-[2-13C,15N]Aspartate was synthesized from [2-13C]diethylmalonate and potassium [15N]phthalimide via diethyl [2-13C,15N]phthalimidomalonate.
Electrochemical oxidation and kinetics of the decay of UV-absorbing intermediate of uric acid oxidation at pyrolytic graphite electrodes
Authors: Goyal Rajendra N. , Mittal Alok , Agarwal Divya
Published: May 2006

The electrochemical oxidation of uric acid has been studied in 0.50 M NaCl and phosphate buffers of ionic strength 0.05 to 0.5 M. The kinetics of the decay of UV-absorbing intermediate generated during electrooxidation of uric acid (7,9-dihydro-1H-purine-2,6,8-(3H)-trione) has also been studied. On the basis of pseudo first order rate constants observed in different media, it has been concluded that the diimine species formed from the 2e, 2H+ oxidation of uric acid is attacked by water to give diol, which then decomposes in a series of reactions to give allantoin as the major product at pH 7.2. The possibility of attack by phosphate on diimine has been ruled out.
Linkage isomerization and electrochemical behavior of two geometrical isomers of dichlorobis(dimethylsulfoxide)bis(t-butylpyridine)ruthenium(II)
Authors: Silva Denise O. , Toma Henrique E.
Published: May 2006

The reaction of cis-[RuCl2(dmso)4] (dmso = dimethylsulfoxide) with 4-tert-butylpyridine (tbpy) in chloroform leads to the all cis (ccc) and the cis,cis,trans (cct) geometrical isomers of [RuCl2(S-dmso)2(tbpy)2]. The cct isomer exhibits a reversible cyclic voltammogram, ascribed to the RuIII/II redox couple with E1/2 = 1.12 V versus SHE. The ccc isomer can be oxidized at E1/2 = 1.27 V; however, the oxidized species undergoes a relaxation equilibrium, (Ks→o = 1.59), involving the S-to-O linkage isomerization of a coordinated dmso ligand (ks→o = 1.9 s−1 and kO→S = 1.2 s−1). The isomerization reaction in the RuIIIccc isomer is discussed in terms of the competitive effects between the S-bound dmso and the tbpy ligands.
Does the Csp2—Csp3 barrier in ethylbenzene have a hyperconjugative component? An indirect experimental and theoretical approach
Authors: Schaefer Ted , Schurko Robert W. , Bernard Guy M.
Published: May 2006

The proton chemical shifts and the proton spin–spin coupling constants are reported for 1-phenyl-1-butyne and 1-phenyl-1-pentyne dissolved in CS2/C6D12 and acetone-d6. The long-range coupling constants between the methylene and ring protons are used to derive the twofold barriers to internal rotation in these molecules. They are 0.5 ± 0.1 kJ/mol; the perpendicular conformer is the most stable, the one in which the C(3)—C(4) bond of the side chain lies in a plane perpendicular to the phenyl group. This energetic preference is assigned to the difference between C—C and C—H hyperconjugative interactions with the aromatic π electron system, the C—C interaction being larger. Comparison of the twofold components of the internal rotational barriers in biphenyl and diphenylacetylene, that in the latter amounting to 40% of that in the former, implies that the hyperconjugative component of the internal barrier in ethylbenzene is 1.2 ± 0.3 kJ/mol, a minor component of the total magnitude. Molecular orbital computations of the conformational energies of 1 -phenyl-1-butyne all agree that the perpendicular conformer has the lowest energy but only to the extent of 0.1 kJ/mol at most.
One-pot synthesis of precocene I and II and a formal synthesis of robustadial A and B via 2-phenyl-4H-1,3,2-benzodioxaborin
Authors: Bissada Suzanne , Lau Cheuk K. , Bernstein Michael A. , Dufresne Claude
Published: May 2006

The reaction of a phenol, and α,β-unsaturated aldehyde, and phenylboronic acid yields a 2-phenyl-4H-1,3,2-benzodioxaborin. Upon heating, this compound decomposes to an orthoquinone-methide intermediate, which undergoes an electrocyclization reaction to a chromene ring system. This method has been applied to the synthesis of precocenes I and II and the robustadial A and B.
The syntheses of 2,9,16,23- and 1,8,15,22-tetrahydroxyphthalocyanines
Authors: Leznoff Clifford C. , Hu Mougang , McArthur Colin R. , Qin Yongnian , van Lier Johan E.
Published: May 2006

2,9,16,23-Tetra-p-n-butylbenzyloxy-, 2,9,16,23-tetradiphenylmethoxy-, 2,9,16,23-tetramethoxymethoxy-, and 1,8,15,22-tetra-p-n-butylbenzyloxyphthalocyanines were synthesized from the appropriate phthalonitriles. Metal-free phthalocyanines were converted to their zinc derivatives. Cleavage from the appropriate precursor with trifluoroacetic acid produced 2,9,16,23-tetrahydroxyphthalocyanine and 1,8,15,22-tetrahydroxyphthalocyanine and their zinc derivatives. NMR spectroscopy revealed that all the 2,9,16,23-tetrasubstituted phthalocyanines are the usual mixtures of the 2,9,16,23, 2,9,16,24, 2,9,17,24, and 2,10,16,24 isomers, but the two 1,8,15,22-tetrasubstituted phthalocyanines were formed as pure single isomers.
Synthèses et caractérisations structurales de complexes perchlorato et triflato anhydres de praseodyme(III)
Authors: Pascal Jean Louis , Haddad Musthapha El , Rieck Heicko , Favier Frederic
Published: May 2006

The syntheses of Pr(ClO4)3, Pr(OTf)3 (OTf = CF3SO3), Pr(ClO4)3•Cl2O6, and NO2Pr(ClO4)4 are described. Vibrational spectroscopy, X-ray powder diffraction, and EXAFS (for Pr(ClO4)3) are used to characterize the structure of the complexes. [ClO4] and [OTf] appear as tridentate in Pr(ClO4)3 and Pr(OTf)3 and Pr is 9-coordinated. In Pr(ClO4)3•Cl2O6, chlorine trioxide is included in the lattice of Pr(ClO4)3 and shows the ionic ↔ covalent equilibrium. In NO2Pr(ClO4)4, Pr(ClO4)4− retains the structure of Pr(ClO4)3, the extra [ClO4] is weakly bonded to Pr• Pr(ClO4)3 crystallizes in the monoclinic system (P21/m), a = 8.608; b = 11.827; c = 7.416 Å; β = 106.29°, and V = 724.79 Å3. Distances Pr—O = 2.51 Å and Pr—Cl = 3.90 Å are determined by EXAFS. Strong fluorescence phenomena are observed when Pr(ClO4)3 is excited with the 488 nm line of an ionized Ar laser.
Molecular and crystal structure of a mixed-phosphine ligand Tc(II) complex, Tc(PMe2Ph)2(DMPE)Cl2
Authors: Rochon Fernande D. , Melanson Robert , Kong Pi-Chang
Published: May 2006

The reaction of Tc(PMe2Ph)3Cl3 with an excess of bis(dimethylphosphino)ethane (DMPE) in ethanol solution produced a yellow compound, which was identified by X-ray diffraction methods as a mixed-ligand Tc(II) complex, Tc(PMe2Ph)2(DMPE)Cl2. The compound crystallizes in the P21/c space group with a = 12.899(6), b = 13.142(8), c = 19.088(9) Å, β = 121.13(3)°, V = 2770(2) Å3, Z = 4, R = 0.061, and Rw = 0.051. The geometry around the Tc atom is octahedral with the chloro ligands trans to each other, while the two PMe2Ph ligands are in cis positions. The Tc—Cl bond distances are 2.431 (5) and 2.431 (5) Å while the Tc—P bond lengths are 2.434(4) and 2.435(4) Å for PMe2Ph and 2.400(4) and 2.398(4) Å for the bidentate DMPE ligand. An increase of multiple bonding of the Tc—P bond, as the oxidation state of Tc decreases, is suggested.
Variable-pressure dynamic NMR studies: effects of paramagnetic metal ions on NMR parameters in nonexchanging systems
Authors: Takagi Hideo D. , Matsuda Kayoko , Aizawa Sen-Ichi , Funahashi Shigenobu , Kinrade Stephen D. , Swaddle Thomas W.
Published: May 2006

The possible effects of paramagnetic relaxation on the apparent volumes of activation for exchange reactions in solution, as measured by NMR at high pressures, are considered. Two model paramagnetic systems that do not undergo ligand exchange on the NMR time scale were examined: tri(acetylacetonato)chromium(III) in various perdeuterated solvents, and tris(ethylenediamine)nickel(II) ion in ethylenediamine solvent. No pressure dependence was discernible up to 200 MPa for the chemical shifts of 1H (exemplifying nuclei of spin 1/2) in the Cr(III) complex, or of solvent 14N (representing quadrupolar nuclei) in the Ni(II)–ethylenediamine case. The line widths Δv1/2, however, were significantly dependent on pressure. For 1H in the Cr complex, the increase of Δv1/2 with pressure was less than expected from the theory of scalar interactions, and was small enough to imply that any contribution from this source to the observed volume of activation in exchanging systems may be neglected. For 14N in liquid ethylenediamine, the increase of Δv1/2 with pressure was significantly greater when a paramagnetic solute was present. Thus, before the observed Δv1/2 at a pressure P (in MPa) measured by the NMR of a quadrupolar nucleus can be used to obtain a chemical exchange rate for a paramagnetic solute, it should be reduced by an amount Δv1/20 exp{θ(P − 0.1)(ΔVV‡/RT + κ)}, where ΔVV‡ is the activation volume for viscous flow and κ the compressibility of the solvent, Δv1/20 is the linewidth at 0.1 MPa in the absence of chemical exchange, and θ is a scaling factor between 0 and 1. The factors Δv1/20 and θ(ΔVV‡/RT + κ) are obtainable from measurements with a chemically equivalent, nonexchanging, paramagnetic solute.
1994 Syntex Award Lecture: Anionic Electrophiles, Protein Modification, and Artificial Blood
Authors: Kluger Ronald
Published: May 2006

Monomethyl esters of acyl phosphates (mixed anhydrides of carboxylic and phosphoric acids) are being developed as site-directed acylating agents for amino groups in proteins. In an illustrative application, these anionic materials are shown to bind to a positively charged region of hemoglobin where they convert amino groups to amides. Bifunctional acyl methyl phosphates cross-link hemoglobin to give a variety of products, some of which have the oxygen-binding properties anticipated for materials that can serve as an alternative to red cells in transfusions. Higher yields of desired products result from the use of a trifunctional analogue. Kinetic patterns of the reactions of a series of alkyl amines and methyl aroyl phosphates indicate that the transition state for formation of the amide involves almost complete development of positive charge on nitrogen.
An energy-resolved study of the fragmentation reactions of alkyl ammonium ions
Authors: Bosma Nancy L. , Harrison Alex. G.
Published: May 2006

The unimolecular metastable ion and low-energy collision-induced fragmentation reactions of alkyl ammonium ions RnH4−nN+ (n = 2 to 4, R = n-C3H7 and n-C4H9) have been studied and breakdown graphs expressing the % fragment ion abundances as a function of the centre-of-mass collision energy have been established. The dialkyl and trialkyl ammonium ions fragment primarily by olefin ([R − H]) elimination or by formation of the alkyl ion R+ (with rearrangement to the more stable secondary structure). By contrast the tetraalkyl ammonium ions show both elimination of [R − H] and elimination of an alkane, which is C5H12 for the propyl compound C7H16 for the butyl compound. The results are interpreted in terms of competition between heterolytic bond cleavage to form a [R+---NH4−nRn−1] complex and homolytic bond cleavage to form a [Rn−1H4−nN+•---•R] complex. The former complex fragments either to form R+ or undergoes internal proton transfer to form Rn−1H4−nNH+ + [R − H], while the latter complex fragments by alkane elimination resulting from attack of R• on the α-CC bond of the alkyl group. For the propyl ammonium ions plausible potential energy profiles are established which show that the former complex is favoured for n = 2, 3 while the two complexes have similar energies for n = 4.
Isotherme 75 °C, du système quaternaire réciproque H3PO4–K2SO4–H2SO4–K3PO4–H2O• Etude des nappes du liquidus
Authors: Bourguiba Néji , Jouini Tahar
Published: May 2006

The liquidus curves at 75 °C, 1 atm, of the system under consideration have been studied systematically by means of solubilities determined conductimetrically, and lines of constant water content have been deduced. The study used isoplethic sections chosen to complete data on previous measurements of limiting ternary systems and lines of double saturation. Four isoplethic sections: K2SO4–KH2PO4, K2SO4–K2HPO4, KHSO4–K2HPO4, and KHSO4–KH2PO4 were determined, the first two of which are quasi-ternary systems. A total of nine portions of the curves were measured to extend knowledge of the liquidus. The limiting curves in the field of KHSO4 and K8(H2PO4)1+X (HSO4)7−X show curvature, which reveals retrograde solubility as a function of water content.
Differential reactivity of carbohydrate hydroxyls in glycosylations. II. The likely role of intramolecular hydrogen bonding on glycosylation reactions. Galactosylation of nucleoside 5′-hydroxyls for the syntheses of novel potential anticancer agents
Authors: Whitfield Dennis M. , Douglas Stephen P. , Tang Ting-Hua , Csizmadia Imre G. , Pang Henrianna Y.S. , Moolten Frederick L. , Krepinsky Jiri J.
Published: May 2006

Contrary to expectations, many primary hydroxy groups are completely unreactive in glycosylation reactions, or give the desired glycosides in very low yields accompanied by products of many side reactions. Hydrogens of such primary hydroxyls are shown to be intramolecularly hydrogen bonded. Intermediates formed by nucleophilic attack by these hydroxyls on activated glycosylating agents may resist hydrogen abstraction. This resistance to proton loss is postulated to be the origin of the observed unreactivity. It is shown that successful glycosylations take place under acidic conditions under which such hydrogen bonds cease to exist. Accordingly, direct galactosylations of the normally unreactive 5′-hydroxyls of nucleosides were accomplished for the first time with a galactose trichloroacetimidate donor in chloroform under silver triflate promotion. It is noted that such galactosylated anticancer nucleosides may have improved biological specificity.
Speed of sound, viscosity, and refractive index of multicomponent systems: analysis of experimental data from the Bertrand–Acree–Burchfield equation
Authors: Pandey J. D. , Jain P. , Vyas V.
Published: May 2006

The speed of sound in and the viscosity and refractive index of two ternary mixtures (I) toluene + n-heptane + n-hexane and (II) cyclohexane + n-heptane + n-hexane, and six binary combinations of these constituents have been measured experimentally. The excess values of these properties are calculated and the results are analysed using the Bertrand–Acree–Burchfield (BAB) model. All the experimental and theoretical values are in fairly reasonable agreement.
Complexation of glucose by α- and β-cyclodextrins
Authors: Hirsch Warren , Muller Tamas , Pizer Richard , Ricatto P.J.
Published: May 2006

The concentration of glucose in aqueous solutions was monitored, using a blood glucose meter, as small weighed cyclodextrin portions were added. Equilibrium constants for the formation of inclusion complexes were calculated. A binding constant determined by this method has been verified by a technique involving competition of the guest with methyl orange indicator dye. ΔH0 values for the reactions were estimated using calorimetry. These values along with ΔG0 provided estimates of the standard entropies. The values calculated compare favorably with those from a previously determined empirical equation. The possibility of hydrogen bond formation is suggested by the large negative enthalpies as well as by molecular mechanics calculations. Keywords: α-, β-cyclodextrin, glucose, binding constants.
Stepwise synthesis of selenium coronands containing an odd number of selenium atoms: 1,5,9-triselenacyclododecane and 1,5,9,13,17-pentaselenacycloeicosane
Authors: Cordova-Reyes Ivan , VandenHoven Elizabeth , Mohammed Ali , Pinto B. Mario
Published: May 2006

The stepwise synthesis of two new selenium coronands containing an odd number of selenium atoms, starting from the fragments HO-(CH2)3-Cl, HO-(CH2)3-Br, HO-(CH2)3-SeCN, and NCSe-(CH2)3-SeCN, is described. The key diol containing one selenium atom, 4-selenaheptan-1,7-diol 4, was obtained in 97% yield from the reaction of 3-bromo-1-propanol 3 and the selonolate anion derived from 3-selenocyanato-1-propanol 2 by sodium metal reduction in liquid ammonia. The ditosylate 5, obtained from 4, was then treated with the bisselenolate anion derived from 1,3-bisselenocyanatopropane 6 to give the target selenium coronand, 1,5,9-triselenacyclododecane [12Se3] 7, in 96% yield. The ditosylate 5 was treated with two equivalents of the selenolate anion derived from 2 to give a second key acyclic diol 8 now containing three selenium atoms. The ditosylate 9, obtained from 8, was then treated with the bisselenolate anion derived from 6 to afford the second target coronand, 1,5,9,13,17-pentaselenacycloeicosane [20Se5] 11 in 44% yield. Keywords: selenium coronands, stepwise synthesis, macrocycles, heterocycles.
Adamantane-based nitrile sulfides: the generation of the synthetic equivalent of the first bis(nitrile sulfide)
Authors: Bridson John N. , Schriver Melbourne J. , Zhu Shuguang
Published: May 2006

The reaction of 1,3-adamantanedicarbonamide with chlorocarbonylsulfenyl chloride in hot toluene gives two new oxathiazolone derivatives that have had their structures confirmed crystallographically: 1,3-bis(1,3,4-oxathiazol-2-on-5-yl)-adamantane and 1 -cyano-3-(1,3,4-oxathiazol-2-on-5-yl)-adamantane. (Crystal data: C10H14(COC(O)SN)2: P21/m, a = 10.447(2), b = 7.119(3), c = 10.684(2) Å, β = 112.98(1)°, V = 731.5(3) Å3, z = 2, Dc = 1.536 g cm−3, R = 0.034, Rw = 0.029; (NC)C10H14(COC(O)SN): P21/c, a = 9.153(2), b = 19.134(2), c = 7.130(3) Å, β = 105.38(2)°, V = 1204(1) Å3, z = 4, Dc = 1.447 g cm−3, R = 0.038, Rw = 0.035.) The reaction of the precursor with dimethyl acetylenedicarboxylate (DMAD) in refluxing chlorobenzene gave a mixture that was resolved into pure compounds by Kugelrohr sublimation; these were shown to be sulfur, tetramethyl thiophenetetracarboxylate, 1,3-adamantanedinitrile, 1,3-bis(4,5-bis(methoxycarbonyl)-isothiazol-3-yl)-adamantane, and 1-cyano-3-(4,5-bis(methoxycarbonyl)-isothiazol-3-yl)-adamantane. The structures of the two new isothiazole derivatives were confirmed crystallographically. (Crystal data: C10H14(CNS•DMAD)2: Pca21, a = 15.857(4), b = 11.562(4), c = 13.602(4) Å, V = 2494(2) Å3, z = 4, Dc = 1.424 g cm−3, R = 0.057, Rw = 0.038; C10H14(CN)(CNS•DMAD): P21/c, a = 19.683(2), b = 11.566(3), c = 7.727 (3) Å, β = 93.30(2)°, V = 1756(1) Å3, z = 4, Dc = 1.363 g cm−3, R = 0.079, Rw = 0.076.) The pattern of reactivity indicates that the syntheticequivalent of the bis(nitrile sulfide) may be present in solution but has a very short lifetime and failed to react at all with 1,3-adamantanedinitrile. Keywords: oxathiazolone, nitrile sulfide, cycloaddition, isothiazole.
Order–disorder effects in clathrates. A crystallographic and 35Cl NQR spectroscopic study of CCl3X guest molecules (X=Cl, Br, CH3, CN, NO2, H) in hexakis(phenylthio)benzene
Authors: Pang L. , Brisse F. , Lucken E.A.C.
Published: May 2006

X-ray crystallography and NQR spectroscopy have been jointly used to study CCl3X-hexakis(phenylthio)benzene (HPTB) (X=Cl, Br, CH3, CN, NO2, H) clathrates. The crystal of the non-clathrated HPTB is triclinic, [Formula: see text] a = 9.5614(7), b = 10.2088(13), c = 10.6193(11) Å, α = 68.45(1)°, β = 76.98(1)°, γ = 65.62(1)°, Z = 1; crystals of the clathrates with CCl3CH3, CCl3Br, CCl3NO2, and CCl3CN are isostructural and trigonal, [Formula: see text] a = 14.18–14.32, c = 20.47–20.62 Å, Z = 3. The trigonal host cavity of HPTB clathrates contains two CCl3X guest molecules, and the X group occupies either the axial or one of the equatorial positions and thus undergoes a molecular order/disorder depending upon its nature. Dipole–dipole interactions between host and guest molecules are considered here to explain this order/disorder effect quantitatively. Keywords: clathrates, crystal structures, NQR, hexakis(phenylthio)benzene.
Sulfonyl esters 6. Elucidation of the first sequence in the Trithioorthoformate Reaction
Authors: Baum James Clayton , Black Bradley Eugene , Precedo Laura , Goehl John Edward , Langler Richard Francis
Published: May 2006

The one-pot conversion of a mercaptan and an aryl methanesulfonate into a trithioorthoformate is described. The intermediacy of a sulfide-sulfonate ester is firmly established. Experimental and computational results permit the formulation of a possible pathway for the formation of the intermediate sulfide-sulfonate ester. Keywords: single electron transfer, sulfonate esters, sulfide-sulfonate esters.
1H NMR study of the solvolysis of the paramagnetic tetrachloro-bis(imidazole)ruthenium(III) anion in water, methanol, and dimethyl sulfoxide
Authors: Anderson Craig , Beauchamp André L.
Published: May 2006

The1H NMR signals of the Ru(III) species present in solution are considerably broadened and shifted by paramagnetism, but they can be used to follow chloride displacement in the trans-[RuCl4Im2]−ion. This anion remains predominant for several hours at room temperature in D2O, but its signals are progressively replaced by those of a monoaqua [RuCl3(D2O)Im2] complex. Over a period of days, two new sets of peaks appear, corresponding to two isomers of [RuCl2(D2O)2Im2]+. The same behaviour is observed for the 1-methyl-and 4-methylimidazole analogues. These reactions can be driven backwards by addition of KCl, but [RuCl4Im2]−is not quantitatively regenerated in solution even for 6 M NaCl. Within several months, the [RuCl2(D2O)2Im2]+isomers further aquate to a single species [RuCl(D2O)3Im2]2+. In CD3OD, displacement of the first chloride of [RuCl4Im2]−takes place faster, over several hours, but substitution stops at the [RuCl3(CD3OD)Im2] stage. In DMSO, substitution occurs very slowly. The [RuCl3(DMSO)Im2]:[RuCl4Im2]−mixture (1:2) obtained after 12 days starts to show very slow reduction to two Ru(II) species, one of which precipitates as yellow crystals. From X-ray diffraction work (monoclinic, P21/n, a = 9.951, b = 8.564, c = 10.527 Å, β = 92.95°, R = 0.033), the compound was identified as [RuCl2(DMSO-d6)2Im2], where the metal has a trans-trans-trans coordination and the DMSO ligands are S-bonded. Keywords: paramagnetic ruthenium anion, solvolysis, chloro complexes.
Structure – activity relationships in antagonist and inverse agonist ligands for the benzodiazepine receptor
Authors: Codding Penelope W. , Roszak Aleksander W. , Szkaradzinska Maria B. , Cook James M. , Hagen Timothy J. , Allen Michael S.
Published: May 2006

The X-ray crystal and molecular structures of the three benzodiazepine (BZD) receptor ligands are presented and the electronic character of inverse agonist ligands is probed through molecular orbital calculations. Two of the ligands have a 6-benzylamino substituent: 6-benzylamino-β-carboline-3-carboxylic acid methyl ester, 1, which is a high affinity antagonist with IC50 = 10 nM, and 6-benzylamino-β-carboline, 2, which is a moderate affinity inverse agonist with IC50 = 106 nM. The third compound, 3-ethoxy-β-carboline hydrochloride, 3, displays partial inverse agonist activity with an IC50of 24 nM. Intermolecular interactions, including extensive hydrogen bonding involving both the pyridyl nitrogen atom and the indole N—H as well as π stacking of aromatic rings, are characteristic of β-carbolines and are found in these three structures. In addition, two of these compounds are protonated in the crystalline state, thereby providing a model for interactions in the absence of the 3-carboxylic acid ester function. Electronic calculations show that (1) the partial inverse agonist ligand has the highest charge on the N(2) atom and (2) high affinity β-carbolines possess two neighboring sites that have high electrostatic attraction for a hydrogen atom in an intermolecular interaction. These findings suggest that modifications to the 3-position side chain to enhance the charge on the pyridyl N atom and provide a hydrogen bond acceptor site will facilitate the development of partial inverse agonist ligands. Keywords: β-carbolines, benzodiazepine receptor, crystallography, structure–activity relationships.
Theoretical evidence for P—S and P—Npπ—pπ bonding within the heterocyclopentadienyl framework
Authors: MacLennan Martin T. , Darvesh Katherine Valenta
Published: May 2006

SCF and Møller–Plesset calculations have been carried out for systems containing P—S and P—N pπ—pπ bonds: namely, the 1,3,2-dithiaphospholium cation, the 1,3,2-diazaphospholium cation, and the 1,3,2-azathiaphospholium cation. The computed geometries are reported, along with Mulliken atomic charges and selected vibrational frequencies. ΔH0values for heterolytic P—H bond dissociation and for isodesmic bond separation reactions provide information on the energetics and stabilization of the P—S and P—N pπ—pπ bonds. Keywords: SCF and MP2 calculations, third period pπ—pπ bonding, bond separation reactions.
1994 Polanyi Award Lecture Concept of active electrons in chemistry
Authors: Huzinaga Sigeru
Published: May 2006

The notion of division between active and dormant electrons has been well received and widely used in the chemists' way of thinking. The core–valence separation in atoms is the best-known example. This paper describes a theoretical and computational method called the model potential method, which deals only with active electrons in molecular and solid state calculations. The method is capable of reaching computational accuracy of testing the validity of the separation of active and dormant electrons in individual cases. Keywords: separability of electrons, model potential method, valence orbitals, relativistic effects.
Density Functional calculations for some hydrogen-bonded systems. Influence of a dielectric surrounding
Authors: Jeanvoine Y. , Bohr F. , Ruiz-López M.F.
Published: May 2006

We present Density Functional calculations for hydrogen-bonded systems with small, medium, and large association energies, in a vacuum and in a dielectric environment. The results are in reasonably good agreement with accurate ab initio results and available experimental data. They confirm that DFT is very promising for modeling liquid state properties. Keywords: hydrogen bond, Density Functional, cooperative effect, liquid state.
A new synthesis of lactones from tertiary alkenylcarbinols by cobaltcatalyzed photocarbonylation under ambient conditions
Authors: Chow Yuan L. , Huang Yu-Jin , Dragojlovic Veljko
Published: May 2006

In the presence of a Co catalyst, tertiary vinyl-, propenyl-, and allylcarbinols were chelatively cycloadded to carbon monoxide to produce lactones by xanthone-sensitized photoreaction in tetrahydrofuran, under CO at 1 atmosphere and at room temperature. At lower temperatures the isomerization of alkenylcarbinols was suppressed and lactones were obtained with improved selectivity. The photoprocess was facilitated by addition of pyridine or hydrogen and retarded in presence of amines or phosphines. Mechanistic interpretations for the process and the accompanying effects are discussed. Keywords: chelative carbonylation, photocarbonylation, carbonylation of olefins, lactones synthesis from alkenylalcohols.
Enantioselective hydrosilation of ketones in the presence of an S,S-[1,2-bis(tetrahydroindenyl)ethane]-titanium catalyst
Authors: Xin Shixuan , Harrod John F.
Published: May 2006

S,S-[1,2-Bis(tetrahydroindenyl)ethane]titanium-based catalysts were used, in conjunction with either diphenyl- or methylphenylsilane, to effect the catalytic hydrosilation of a wide variety of dialkyl ketones. The highest enantioselectivity was observed with c-pentylethylketone (70%) and the selectivity was found to be very sensitive to the steric qualities of the alkyl groups, e.g., iPr-Me (15%), iPr-Et (65%), iPr-nPr (60%). Inclusion of a coordinating heteroatom in the alkyl group of the ketone substrate, or carrying out the reaction in a coordinating solvent such as THF, had no significant effect on the rate or selectivity of the reaction. Keywords: hydrosilation, enantioselective catalysis, ketones, ansa-metallocenes.
Preparations for 1,4,7,11,14,17-hexa-thiacycloeicosane, 1,4,7-trithiecan-9-ol, 1,11 -dioxa-4,8,14,18-tetrathia-cycloeicosane, 1 -oxa-4,8-dithiacyclo-decane, and some complexes of iron, cobalt, nickel, and palladium
Authors: Lucas C. Robert , Liu Shuang , Bridson John N.
Published: May 2006

Preparations are reported for the title ligands (L1–L4, respectively) and for [Co(L1)][ClO4]2, [Ni(L1)][ClO4]2, [{Pd(η3-C3H5)}2(L1)][ClO4]2, [Co(L2)2][ClO4]2, [Ni(L2)2][ClO4]2, [Co(L3)][ClO4]2, and [Ni(L3)2][ClO4]2. Clarification of earlier reports of L2 and some of its complexes has been achieved by X-ray crystallographic characterization. Crystal data for orthorhombic [Ni(L1)][ClO4]2: space group Pna21 (No. 33); a = 13.270(4), b = 16.564(4), c = 11.348(4) Å; Z = 4; R = 0.047, Rw = 0.040 for 1474 reflections. For monoclinic [Ni(L2)2][ClO4]2: space group P21/c (No. 14); a = 7.617(3), b = 18.034(3), c = 9.098(2) Å, β = 91.58(2)°; Z = 2; R = 0.046, Rw = 0.039 for 1487 reflections. For tetragonal [Co(L3)][ClO4]2: space group [Formula: see text] (No. 121); a = 13.840(5), c = 14.902(4) Å; Z = 4; R = 0.054, Rw = 0.058 for 974 reflections. The new complexes and previously reported [Fe(L1)][ClO4]2have been examined by cyclic voltammetry, NMR, ESR, electronic spectroscopy, and magnetic susceptibility measurements. The influence of L2's extra-cyclic hydroxyl, of changes in M—S lengths, and of O for S substitution on structural, magnetic, redox, and spectroscopic properties has been studied. Keywords: X-ray structures, electrochemistry, magnetic phenomena.
Electrocatalytic activity of a graphite electrode coated with hexadecachlorophthalocyanatoiron(II) toward sulfide oxidation, and its possible application in electroanalysis
Authors: Zhang Jiujun , Lever A.B.P. , Pietro W.J.
Published: May 2006

A graphite electrode coated with hexadecachlorophthalocyanatoiron(II), FeII[Cl16Pc(2-)], serves as an electrocatalyst for the oxidation of sulfide ion in basic solutions (pH range: 8–14). The surface electrochemical responses of this catalyst and corresponding electrocatalytic activity are discussed. The sulfide ion concentration dependences of the peak current in cyclic voltammograms are examined to show the possible application of this catalyst-coated electrode in amperometric sulfide sensors. Keywords: phthalocyanine, modified graphite electrode, sulfide oxidation, electroanalysis, electrocatalysis.
Preparation and characterization of a new series of Cr(II) hydroborates
Authors: Dionne Michel , Hao Shoukang , Gambarotta Sandro
Published: May 2006

The synthesis and characterization of a new series of mono-, di-, and trinuclear Cr(II) borohydride compounds is described. The reaction of CrCl2(TMEDA) with two equivalents of NaBH4 afforded the thermally unstable (TMEDA)Cr(BH4)2 (1), which was converted by treatment with pyridine into the octahedral monomeric (Py)4Cr(BH4)2 (2). The reaction proceeds via formation of an intermediate trinuclear complex {[(TMEDA)(Py)Cr(η2-BH4)]2[(Py)2Cr(η2-BH4)2]}(µ,η1-BH4)2 (3), which was isolated and characterized by X-ray crystallography. Reaction of 1 and 2 with both CO2 and RN=C=NR (R = Cy, iPr) afforded hydride insertion and formation of the corresponding diamagnetic lantern-type Cr(II) formate (HCO2)4Cr2Py2 (4) and formamidinate compounds [RNC(H)NR]2Cr2(µ-BH)4 (R = Cy (5a), iPr (5b)), respectively, with supershort Cr—Cr quadruple bonds. The structures of 1, 2, 3, and 5b were elucidated by X-ray analysis. Crystal data are as follows. 1: C6H24N2B2Cr, monoclinic, Cc, a = 8.517(2) Å, b = 15.921(5) Å, c = 9.624(2) Å, β = 115.59(1)°, Z = 4, R = 0.022, Rw = 0.029; 2: C28H44N4B2O2Cr, monoclinic, P21/n, a = 12.021(1) Å, b = 15.555(1) Å, c = 15.723(1) Å, β = 90.13(2)°, Z = 4, R = 0.074, Rw = 0.086; 3: C32H76N8B6Cr3, monoclinic, P21/n, a = 8.515(1) Å, b = 14.525(1) Å, c = 18.286(2) Å, β = 91.38(1)°, Z = 2, R = 0.051, Rw = 0.060; 5b: C21H49N6BCr2, monoclinic, C2/c, a = 17.000(1) Å, b = 9.033(1) Å, c = 19.160(1) Å, β = 105.579(9)°, Z = 4, R = 0.069, Rw = 0.078. Keywords: divalent chromium, borohydride, Cr—Cr quadruple bond.
Systematic substitution kinetics of the clusters Ru3(CO)11L and Ru3(CO)10L2 (L = P-donor ligands): The unimolecular path
Authors: Brodie Nancy M.J. , Poë Anthony J.
Published: May 2006

The kinetics have been studied of the substitution reactions of Ru3(CO)11L and Ru3(CO)10L2 (L = PCy3 (Cy = C6H11), P-n-Bu3, P(p-MeOC6H4)3, P(p-ClC6H4)3, P(OPh)3 and etpb (P(OCH2)3CEt) with AsPh3 or various P-donor nucleophiles, L′, in hexane. The reactions proceed through both [L′]- independent (CO dissociative) and [L′]-dependent (associative) paths. The values of the rate constants, k1, for the dissociative path for Ru3(CO)11L can be combined with other published data and are found to fit well to the equation: log k1 = α + βLδL + γLθL (βL and γL are constants that define how sensitive the values of log k1 are to Bodner's electronic parameter, δL, and to Tolman's steric parameter, θL, respectively.) Values of (βL and γL are both positive, increasing electron donicity and increasing size of L both leading to increased rates. The steric effect is more important and accounts for ca. 60% of the decrease in the free energies of activation. The effects, on the fit and on the values of βL and γL, of using different electronic (Bartik's χ values) and steric (Brown's ER values) parameters are examined and found to be small, and the question of which goodness-of-fit parameters should be used is considered. The kinetic effects of the substituents are only a small (ca. 20%) perturbation on the average free energies of activation, and the steric effect shows that use is made of only a few percent of the available ligand–ligand repulsion energy in accelerating the reactions. The second-order reactions of M3(CO)11L with Mc3NO (M = Ru or Os) also fit an equation analogous to that given above and the values of βL and γL obtained for these assisted CO displacement reactions are compared with those for the unimolecular, unassisted, CO dissociation. Unassisted reactions of the Ru3(CO)10L2 clusters show values of βL and γL that are closely similar to those for Ru3(CO)11L, but published data for CO dissociative reactions of some Ru3(CO)9L3 clusters lead to much larger positive values of βL and a large inverse steric effect, i.e., rates actually decrease substantially with increasing substituent size after electronic effects are allowed for. This effect is accounted for in terms of the different natures of the Ru3(CO)8L3 intermediates. Keywords: metal carbonyls, ruthenium, clusters, kinetics, substituent effects.
Organic and inorganic templates bearing CCo3(CO)9 cluster fragments: X-ray crystal structures of C(CH2OCOCCl3)4 and of CH3C(O)CHC(OH)MLn where MLn = CCo3(CO)9 or (C6H5)Cr(CO)3
Authors: Chao Lisa C.F. , Decken Andreas , Britten James F. , McGlinchey Michael J.
Published: May 2006

Pentaerythritol and trichloroacetyl chloride react to yield the tetrakis-trichloroacetate ester C(CH2-O2C-CCl3)4, 5, which crystallizes in the triclinic space group [Formula: see text] with a = 9.874(2) Å, b = 11.811(2) Å, c = 12.858(3) Å, α = 114.35(3)°, β = 95.63(3)°, γ = 99.11 (3)°, and V = 1326.5(5) Å3 for Z = 2. The molecule adopts a geometry in which the trichloroacetate chains fold to produce a flattened tetrahedron, giving a molecule of approximate D2d symmetry. Treatment of 5 with CO2(CO)8 gave, as the highest molecular weight product, the tris-cluster complex C[CH2-O2C-CCo3(CO)9]3(CH2-O2C-CCl3), 6, rather than the tetra-cluster C[CH2-O2C-CCo3(CO)9]4, 7. A space-filling model of 7, derived by replacing the CCl3 groups in 5 by CCo3(CO)9 moieties, exhibited considerable molecular crowding. Reaction of several 1,1,1-trichloroacetylacetonate salts with Co2(CO)8 gave the corresponding cluster complexes of Co(II), Co(III), or Al(III). The uncomplexed cluster ligand CH3C(O)CHC(OH)CCo3(CO)9, 9, crystallizes in the orthorhombic space group Pna21 with a = 16.226(2) Å, b = 11.6300(10) Å, c = 9.919(2) Å, V = 1871.7(4) Å3 for Z = 4. The direction of enolization of 9 has been studied by EHMO calculations. The tendency to enolize is also found in (benzoylacetone)Cr(CO)3, 17, which crystallizes in the tetragonal space group P43212 with a = 10.0840(10) Å, b = 10.0840(10) Å, c = 25.387(5) Å, V = 2581.5(6) Å3 for Z = 8. Keywords: inorganometallic clusters.
The syntheses and structures of the phenyl-di-μ-methylene-dirhodium complexes, [{(η5-C5Me5)Rh(μ-CH2)}2(Ph)(Br)] and [{(η5-C5Me5)Rh(μ-CH2)}2(Ph)2]
Authors: Wang Zhi-Qiang , Adams Harry , Bailey Neil A. , Turner Michael L. , Maitlis Peter M.
Published: May 2006

Reaction of PhMgBr with [{(η5-C5Me5)Rh(μ-CH2)2(Br)2] in a halide-specific reaction gave, first, the monophenyl complex 2a [{(η5-C5Me5)Rh(μ-CH2)}2(Ph)(Br)] and then the diphenyl complex 3[{(η5-C5Me5)Rh(μ-CH2)}2(Ph)2]. X-ray crystal structure determinations showed both molecules to have trans structures, [{(η5-C5Me5)Rh(μ-CH2)}2(Ph)(Br)] (a = 10.066(3), b = 11.065(3), c = 14.002(3) Å; α = 85.23(2)°, β = 70.10(2)°, γ = 65.22(2)°; U = 1328.2(6) Å3; Z = 2; space group [Formula: see text] final R, 0.0586) and [{(C5Me5)Rh(μ-CH2)}2(Ph)2] (a = 17.102(8), b = 10.396(4), c = 16.134(7) Å; U = 2869(2) Å3; Z = 4; spacegroup Pccn [Formula: see text] No. 56), final R = 0.0426). Keywords: X-ray structure determination, pentamethylcyclopentadienyl, di-μ-methylene, rhodium