Condensed phase photoelectron asymmetry

Large directional asymmetries were observed in photoelectron distributions from core and valence levels in metals and adsorbate atoms. Intensities varied by factors up to 25 as the angle between A and p was varied. The energy dependence of asymmetry showed atomic behavior similar to that expected for free atoms, modified for the tendency p∥A.     

Transparent conducting oxides: texture and microstructure effects on charge carrier mobility in MOCVD-derived CdO thin films grown with a thermally stable, low-melting precursor

A series of low-melting, thermally stable cadmium metal-organic chemical vapor deposition (MOCVD) precursors have been synthesized, structurally and spectroscopically characterized, and implemented in growth of highly conductive and transparent CdO thin films. One member of the series, bis(1,1,1,5,5,5-hexafluoro-2,4-pentanedionato)(N,N-diethyl-N',N'-dimethyl-ethylenediamine)cadmium(II), Cd(hfa)(2)()(N,N-DE-N',N'-DMEDA), represents a particularly significant improvement over previously available Cd precursors, owing to the low melting point and robust thermal stability. High-quality CdO films were grown by MOCVD on glass and single-crystal MgO(100) between 300 and 412 degrees C. Film growth parameters and substrate surface have large effects on microstructure and electron carrier transport properties. Enhanced mobilities observed for highly biaxially textured films grown on MgO(100) vs glass are attributed, on the basis of DC charge transport and microstructure analysis, to a reduction in neutral impurity scattering and/or to a more densely packed grain microstructure. Although single-grained films grown on MgO(100) exhibit greater mobilities than analogues with discrete approximately 100 nm grains and similar texture, this effect is attributed, on the basis of charge transport and Hall effect measurements as well as optical reflectivity analysis, to differences in carrier concentration rather than to reduced grain boundary scattering. Unprecedented conductivities and mobilities as high as 11,000 S/cm and 307 cm(2)/V.s, respectively, are obtained for epitaxial single-grained films (X-ray diffraction parameters: fwhm(omega) = 0.30 degrees, fwhm(phi) = 0.27 degrees ) grown in situ on MgO(100) at a relatively low temperature (400 degrees C).     

The synthesis and characterisation of ferromagnetic CaMn2O4 nanowires.

The synthesis of marokite CaMn(2)O(4) nanowires using a hydrothermal method is reported. Transmission electron microscopy and electron diffraction measurements show that the nanowires are polycrystalline in nature with diameters between 10 and 20 nm and lengths ranging from approximately 100 to 500 nm. Most interestingly, in contrast with the bulk material, magnetization measurements show that these nanowires exhibit ferromagnetic ordering with a Curie temperature (T(C)) of approximately 40 K.     

Enantioreversal in the sharpless asymmetric epoxidation reaction controlled by the molecular weight of a covalently appended achiral polymer

Polymers such as poly(ethylene glycol) (PEG) have proven use in a variety of applications including organic synthesis. We now disclose our investigations into the recently disputed report that PEG tartrate esters can reverse the enantioselectivity of the Sharpless asymmetric epoxidation reaction. The results presented herein have clarified that the enantioselectivity of this reaction can be reproducibly reversed solely as a function of the molecular weight of the appended PEG. By preparing a range of tartrate ligands with varying PEG chains lengths, the reversal was found to occur within a molecular weight change of only 800. As the PEG chain did not affect the inherent chirality of the ligand, the enantioreversal was proposed to occur as a result of two Ti-ligand complexes which differ in their molecularity of ligand, one monomeric in ligand and the other dimeric. Support for this hypothesis was given through equilibrium measurements which revealed that the predominant species in Ti/PEG tartrate ester mixtures is a distinct 2:1 Ti-ligand complex, as opposed to the 2:2 Ti-ligand complex of traditional Sharpless asymmetric epoxidations. In total, these data represent an unrecognized property of PEG-supported catalysts that could open up new venues in the control of asymmetric reactions by means of achiral appended polymers.     

The formation of propylene fumarate oligomers for use in bioerodible bone cement composites

Poly(propylene fumarate) (PPF) oligomers were synthesized by step polymerization using bis(2-hydroxypropyl fumarate) or propylene bis(hydrogen maleate) as starting materials. Oligomers possessing identical degrees of polymerization (DP), but varying in their end group character (either hydroxyl or carboxyl) were first prepared and characterized, then used as part of a bone cement preparation consisting of oligomer, tricalcium phosphate, calcium carbonate, and methyl methacrylate. Compressive strength of the resulting composite appeared to be dependent on both the degree of polymerization of the PPF, and the nature of the oligomers' end groups.     

High electron mobility in solution-cast and vapor-deposited phenacyl-quaterthiophene-based field-effect transistors: toward N-type polythiophenes

New carbonyl-functionalized quaterthiophenes, 5,5' '-diperfluorophenylcarbonyl-2,2':5',2' ':5' ',2' '-quaterthiophene [DFCO-4T], 5,5' '-diphenyl-2,2':5',2' ':5' ',2' '-quaterthiophene [DPCO-4T], and a polymer having the same basic motif as DFCO-4T, poly{1,4-bis[(3'-n-octyl-2,2'-dithiophene)carbonyl]-2,3,5,6-tetrafluorobenzene} [P(COFCO-4T)], have been synthesized, characterized, and the crystal structures of the molecules determined. Field-effect transistors fabricated with vapor-deposited and solution-cast films of DFCO-4T exhibit very high Ion:Ioff current ratios (up to 108) and electron mobilities up to approximately 0.51 and approximately 0.25 cm2.V-1.s-1, respectively. Solution-cast blends of P(COFCO-4T) and DFCO-4T (1:1 weight ratio) exhibit an electron mobility of approximately 0.01 cm2.V-1.s-1 (Ion:Ioff = 104).     

Frustrated beta-chloride elimination. Selective arene alkylation by alpha-chloronorbornene catalyzed by electrophilic metallocenium ion pairs

Single-site polymerization catalysts generated in situ via activation of Cp*MMe(3) (Cp* = C(5)Me(5); M = Ti, Zr), (CGC)MMe(2) (CGC = C(5)Me(4)SiMe(2)NBu(t)(); M = Ti, Zr), and Cp(2)ZrMe(2) with Ph(3)C(+)B(C(6)F(5))(4)(-) catalyze alkylation of aromatic molecules (benzene, toluene) with alpha-chloronorbornene at room temperature, to regioselectively afford the 1:1 addition products exo-1-chloro-2-arylnorbornane (aryl = C(6)H(5) (1a), C(6)H(4)CH(3) (1b)) in good yields. Analogous deuterium-labeled products exo-1-chloro-2-aryl-d(n)-norbornane-7-d(1) (aryl-d(n) = C(6)D(5) (1a-d(6)), C(6)D(4)CD(3) (1b-d(8))) are obtained via catalytic arylation of alpha-chloronorbornene in either benzene-d(6) or toluene-d(8). Isolated ion-pair complexes such as (CGC)ZrMe(toluene)(+)B(C(6)F(5))(4)(-) and Cp(2)ThMe(+)B(C(6)F(5))(4)(-) also catalyze the reaction of alpha-chloronorbornene in toluene-d(8) to give 1b-d(8) in good yields, respectively. Small quantities of the corresponding bis(1-chloronorbornyl)aromatics 2 are also obtained from preparative-scale reactions. These reactions exhibit turnover frequencies exceeding 120 h(-1) (for the Cp*TiMe(3)/Ph(3)C(+)B(C(6)F(5))(4)(-)-catalyzed system), and chlorine-free products are not observed. Compounds 1 and 2 were characterized by (1)H, (2)H, (13)C, and 2D NMR, GC-MS, and elemental analysis. The aryl group exo-stereochemistry in 1a and 1b is established using (1)H-(1)H COSY, (1)H-(13)C HMBC, and (1)H-(1)H NOESY NMR, and is further corroborated by X-ray analysis of the product 1,4-bis(exo-1-chloro-2-norbornyl)benzene (2a). Control experiments and reactivity studies on each component step suggest a mechanism involving participitation of the metal electrophiles in the catalytic cycle.     

C2-symmetric bis(oxazolinato)lanthanide catalysts for enantioselective intramolecular hydroamination/cyclization

C(2)-symmetric bis(oxazolinato)lanthanide complexes of the type [(4R,5S)-Ph(2)Box]La[N(TMS)(2)](2), [(4S,5R)-Ar(2)Box]La[N(TMS)(2)](2), and [(4S)-Ph-5,5-Me(2)Box]La[N(TMS)(2)](2) (Box = 2,2'-bis(2-oxazoline)methylenyl; Ar = 4-tert-butylphenyl, 1-naphthyl; TMS = SiMe(3)) serve as precatalysts for the efficient enantioselective intramolecular hydroamination/cyclization of aminoalkenes and aminodienes. These new catalyst systems are conveniently generated in situ from the known metal precursors Ln[N(TMS)(2)](3) or Ln[CH(TMS)(2)](3) (Ln = La, Nd, Sm, Y, Lu) and 1.2 equiv of commercially available or readily prepared bis(oxazoline) ligands such as (4R,5S)-Ph(2)BoxH, (4S,5R)-Ar(2)BoxH, and (4S)-Ph-5,5-Me(2)BoxH. The X-ray crystal structure of [(4S)-(t)BuBox]Lu[CH(TMS)(2)](2) provides insight into the structure of the in situ generated precatalyst species. Lanthanides having the largest ionic radii exhibit the highest turnover frequencies as well as enantioselectivities. Reaction rates maximize near 1:1 BoxH:Ln ratio (ligand acceleration); however, increasing the ratio to 2:1 BoxH:Ln decreases the reaction rate, while affording enantiomeric excesses similar to the 1:1 BoxH:Ln case. A screening study of bis(oxazoline) ligands reveals that aryl stereodirecting groups at the oxazoline ring 4 position and additional substitution (geminal dimethyl or aryl) at the 5 position are crucial for high turnover frequencies and good enantioselectivities. The optimized precatalyst, in situ generated [(4R,5S)-Ph(2)Box]La[N(TMS)(2)](2), exhibits good rates and enantioselectivities, comparable to or greater than those achieved with chiral C(1)-symmetric organolanthanocene catalysts, even for poorly responsive substrates (up to 67% ee at 23 degrees C). Kinetic studies reveal that hydroamination rates are zero order in [amine substrate] and first order in [catalyst], implicating the same general mechanism for organolanthanide-catalyzed hydroamination/cyclizations (intramolecular turnover-limiting olefin insertion followed by the rapid protonolysis of an Ln-C bond by amine substrate) and implying that the active catalytic species is monomeric.