The mechanism of the recrystallization process in epitaxial GaN under dynamic stress field: atomistic origin of planar defect formation

The mechanism of the recrystallization in epitaxial (0001) GaN film, introduced by the indentation technique, is probed by lattice dynamic studies using Raman spectroscopy. The recrystallized region is identified by micro‐Raman area mapping. ‘Pop‐in’ bursts in loading lines indicate nucleation of dislocations and climb of dislocations. These processes set in plastic motion of lattice atoms under stress field at the center of indentation for the initiation of the recrystallization process. A planar defect migration mechanism is evolved. A pivotal role of vacancy migration is noted, for the first time, as the rate‐limiting factor for the dislocation dynamics initiating the recrystallization process in GaN. Copyright © 2009 John Wiley & Sons, Ltd.     

Reactions of 2-(arylazo)aniline with ruthenium substrates: Isolation, characterizations and reactivities of delocalized diazoketiminato and orthometallated Ru(II) chelates

Reactions of 2-(arylazo)aniline, HL-NH₂ [H represents the dissociable protons upon complexation and HL-NH₂ is p-RC₆H₄NNC₆H₄-NH₂; R = H for HL¹-NH₂; CH₃ for HL²-NH₂ and Cl for HL³-NH₂] with Ru(H)(CO)(PPh₃)₃Cl and Ru(CO)₃(PPh₃)₂ afforded products of compositions [(HL-NH)Ru(CO)Cl(PPh₃)₂] and [(L-NH)Ru(PPh₃)₂(CO)], respectively. All the complexes were characterized unequivocally. The X-ray structures of the complexes 4c and 5c have been determined. The cyclic volatammograms exhibited one reversible oxidative response in the range of 0.56–0.16 V versus SCE for [(L-NH)Ru(PPh₃)₂(CO)] and a quasi reversible oxidative response within 0.56–0.70 V versus SCE for [(HL-NH)Ru(CO)Cl(PPh₃)₂]. The conversion of ketones to corresponding alcohols has been studied in presence of newly synthesized ruthenium complexes.     

Density functional studies on chromium catalyzed ethylene trimerization

Mechanism of ethylene trimerization using chromium catalyst is investigated using density functional methods. Recent experimental results indicate Cr-based homogeneous catalysts to follow metallacycle pathway in ethylene tri-, teta- and oligomerization reactions. Given the importance of chlorinated Cr-based active catalysts in these reactions, we have used “bare” minimal ligands like Cl^? and considered catalytic cycles with neutral or cationic intermediates starting with [Cr(II)Cl₂(ethylene)₂] and [Cr(II)Cl(ethylene)₂]⁺, respectively. We have compared both ‘Cossee’ and the ‘metallacycle’ mechanisms on these model systems utilizing density functional computations at B3LYP/LANL2DZ(d,p) level. The metallacycle mechanism with cationic Cr(II)–Cr(IV) intermediates is found to be the most favored path, with oxidative coupling of two coordinated ethylene to form the chromacyclopentane being the rate determining step (RDS). We also found that with neutral intermediates the Cossee pathway rather than the metallacycle mechanism is followed. Thus in spite of the simplicity of using just Cl^? as ligand in the model catalytic intermediates, our computational results match remarkably well with many recent and important experimental findings.     

Sulfur bridging interactions of cis-planar NiII-S2N2 coordination units with nickel(II), copper(I,II), zinc(II), and mercury(II): a library of bridging modes, including NiII(micro2-SR)2MI,II rhombs

Sulfur bridging interactions between three cis-planar NiII-S2N2 complexes and NiII, CuI,II, ZnII, and HgII reactants were investigated by synthesis and X-ray crystal structures of some 24 complexes. This work was stimulated by recent crystallographic structures of the A-cluster of carbon monoxide dehydrogenase/acetylcoenzyme A synthase. This bridged biological assembly has the minimal formulation [Fe4S4]-(micro2-SCys)-[M((micro2-SCys)2Gly)Ni] with M = NiII, CuI, and ZnII at sites distal and proximal, respectively, to the iron-sulfur cluster. Bridges supported by representations of the distal nickel site were sought by reactions of the complexes [NiII(LH-S2N2)]2- and [NiII(LR-S2N2)], with 5-5-5 chelate ring patterns. Reaction products implicate the bridges Ni-(micro2-S)1,2-M in a variety of molecular structures, some with previously unknown connectivities of bridge atoms. The most frequently encountered bridge units are the nonplanar rhombs Ni(2-S)2M involving both sulfur atoms of a given complex. Those with M = NiII are biologically relevant inasmuch as the catalytic metal at the proximal site is nickel. The complex [Ni(L-655)]2-, containing the 6-5-5 ring pattern and coordination sphere of the distal nickel site, was prepared and structurally characterized. It was shown to sustain Ni2(micro2-S)2 rhombic interactions in the form of trinuclear [[Ni(L-655)]2Ni]2- and [[Ni(L-655)]Ni(R2PCH2CH2PR2)] (R = Et, Ph) in which the second NiII simulates the proximal site. Bridging interactions of NiII-S2N2 complexes are summarized, and geometrical features of Ni2(2-S)2 rhombs in these complexes, as dependent on ring patterns, are considered (LH-S2N2 = N,N'-ethylenebis(2-mercaptoisobutyramide)(4-); LR-S2N2 = trans-rac-N,N'-bis(2-mercapto-2-methylprop-1-yl)-1,2-cyclohexanediamine(2-); L-655 = N-(2-mercaptopropyl)-N'-(2'-mercaptoethyl)glycinamide(4-)).     

Novel syntheses of 3-cyano-3,4-dihydroquinolin-2(1H)-one and derivatives of 2,3-dicarbomethoxy-2-hydroxycyclopenta[b]quinoline

A facile solid phase conversion of 2-chloro-3-cyano-4-substituted-1,4-dihydroquinolines to 3-cyano-4-substituted-3,4-dihydroquinolin-2(1H)-ones in almost quantitative yields and a novel synthesis of 2,3-dicarbomethoxy-2-hydroxycyclopenta[b]quinoline are described.     

Fragmentation of certain substituted azadispiro(5.1.5.2)pentadec-9-ene and azadispiro(4.1.4.2)tridec-8-ene derivatives under electron ionization

A study of the electron ionization mass spectra of certain azadispiro(5.1.5.2)pentadec-9-ene-7,15-diones and azadispiro(4.1.4.2)tridec-8-ene-6,13-diones and their derivatives has revealed that these molecules undergo fragmentation primarily by two routes, viz. loss of CO and elimination of the substituent on the pyrrolidine nitrogen. Under positive ionization conditions loss of CO is the predominant process in the diones as it releases the ring strain, while in the 6- or 7-ols loss of the substituent on nitrogen is the favoured pathway. The further decomposition pathways of these primary fragments [M ? CO]⁺˙ and [M ? OR³]⁺ have been delineated with the help of high-resolution mass measurements, D₂O exchange and metastable spectra, These compounds give very simple negative ion spectra showing only [M ? OR³]^? and [NCO]^? ions except the N-hydroxy compounds which show [M ? H]^? ions as well.     

Preliminary communicationCatalytic olefin hydrogenation with nonacarbonyltris(triphenylphosphine)triruthenium and its derivatives. Crystal structure of [Ru2(CO)5(PPh3)(μ-PPh2)(μ-OCPh)]

The complexes [Ru₃(CO)₇(PPh₂)₂(C₆H₄)] and [Ru₂(CO)₅(PPh₃)(μ-PPh₂)(μ-OCPh)] were obtained by pyrolysis of [Ru₃(CO)₉(PPh₃)₃] and tested as catalysts for the hydrogenation of cyclohexene and 2-cyclohexen-1-one. The structure of [Ru₂(CO)₅(PPh₃)(μ-PPh₂)(μ-OCPh)] was established by a single crystal X-ray diffraction study.     

Salicylhydroxamic acid as an analytical reagent

Summary Salicylhydroxamic acid has been used as a colorimetric reagent for the estimation of uranium, vanadium, molybdenum and iron. It permits the direct estimation of vanadium in presence of molybdenum and uranium, though iron interferes, while the estimation of uranium or molybdenum is not possible in presence of each other or of vanadium. The vanadium complex can be removed from solution by extraction with ethyl acetate and estimated colorimetrically between p_H 0.8 to 3.5. This permits its determination in steels after removal of iron. Sensitivity: U 0.1 g, V 0.017 g, Mo 0.015 g, Fe 0.0125 g.     

Bridging nitrate groups in [Mn(4)O(3)(NO(3))(O(2)CMe)(3)(R(2)dbm)(3)] (R = H, Et) and [Mn(4)O(2)(NO(3))(O(2)CEt)(6)(bpy)(2)](ClO(4)): acidolysis routes to tetranuclear manganese carboxylate complexes

New synthesis procedures are described to tetranuclear manganese carboxylate complexes containing the [Mn(4)O(2)](8+) or [Mn(4)O(3)X](6+) (X(-) = MeCO(2)(-), F(-), Cl(-), Br(-), NO(3)(-)) core. These involve acidolysis reactions of [Mn(4)O(3)(O(2)CMe)(4)(dbm)(3)] (1; dbm is the anion of dibenzoylmethane) or [Mn(4)O(2)(O(2)CEt)(6)(dbm)(2)] (8) with HX (X(-) = F(-), Cl(-), Br(-), NO(3)(-)); high-yield routes to 1 and 8 are also described. The X(-) = NO(3)(-) complexes [Mn(4)O(3)(NO(3))(O(2)CR)(3)(R'(2)dbm)(3)] (R = Me, R' = H (6); R = Me, R' = Et (7); R = Et, R' = H (12)) represent the first synthesis of the [Mn(4)O(3)(NO(3))](6+) core, which contains an unusual eta(1):mu(3)-NO(3)(-) group. Treatment of known [Mn(4)O(2)(O(2)CEt)(7)(bpy)(2)](ClO(4)) with HNO(3) gives [Mn(4)O(2)(NO(3))(O(2)CEt)(6)(bpy)(2)](ClO(4)) (15) containing a eta(1):eta(1):mu-NO(3)(-) group bridging the two body Mn(III) ions of the [Mn(4)O(2)](8+) butterfly core. Complex 7 x 4CH(2)Cl(2) crystallizes in space group P2(1)2(1)2(1) with (at -168 degrees C) a = 21.110(3) A, b = 22.183(3) A, c = 15.958(2) A, Z = 4, and V = 7472.4(3) A(3). Complex 15 x (3)/(2)CH(2)Cl(2) crystallizes in space group P2(1)/c with (at -165 degrees C) a = 26.025(4) A, b = 13.488(2) A, c = 32.102(6) A, beta = 97.27(1) degrees, Z = 8, and V = 11178(5) A(3). Complex 7 contains a [Mn(4)(mu(3)-O)(3)(mu(3)-NO(3))](6+) core (3Mn(III), Mn(IV)) as seen for previous [Mn(4)O(3)X](6+) complexes. Complex 15 contains a butterfly [Mn(4)(mu(3)-O)(2)](8+) core. (1)H NMR spectra have been recorded for all complexes reported in this work and the various resonances assigned. All complexes retain their structural integrity on dissolution in chloroform and dichloromethane. Magnetic susceptibility (chi(M)) data were collected on 12 in the 5-300 K range in a 10.0 kG (1 T) field. Fitting of the data to the theoretical chi(M) vs T expression appropriate for a [Mn(4)O(3)X](6+) complex of C(3)(v)() symmetry gave J(34) = -23.9 cm(-)(1), J(33) = 4.9 cm(-)(1), and g = 1.98, where J(34) and J(33) refer to the Mn(III)Mn(IV) and Mn(III)Mn(III) pairwise exchange interactions, respectively. The ground state of the molecule is S = 9/2, as found previously for other [Mn(4)O(3)X](6+) complexes. This was confirmed by magnetization data collected at various fields and temperatures. Fitting of the data gave S = 9/2, D = -0.45 cm(-1), and g = 1.96, where D is the axial zero-field splitting parameter.     

Rapid solid-phase syntheses of a peptidic-aminoglycoside library

A library of mono- and di-amino acid peptidic-aminoglycosides (PAs), with kanamycin and neomycin as the model aminoglycosides, was systematically and rapidly synthesized via solid phase peptide synthesis. Aminoglycosides were first converted into N-Boc protected carboxylic acids and fifteen l-amino acids were then used in the diversification of the full library. The approach outlined describes a rapid synthetic procedure where >200?PA compounds can be synthesized in a few months with 85–95% purity. UV thermal denaturation assessed the binding stabilization by PAs to model human and bacterial A-site rRNA sequences. Significant differences were found in thermal melting profiles among PAs that were attributed to specific amino acid sequences. Neomycin PAs lead to a much larger variation in the stabilization of A-site rRNA sequences (ΔT_m?=?2.6–17.1?°C) as compared to kanamycin PAs (ΔTm?=?0.4–4.3?°C). Kanamycin PAs had little activity against Gram-negative and Gram-positive bacteria as compared with neomycin PAs that had significant antibacterial activity with MIC ranging from 2 to 16?μM.