CdSe and CdSe/CdS nanorod solids

We demonstrate the self-organization of CdSe nanorods into nematic, smectic, and crystalline solids. Layered colloidal crystals of CdSe nanorods grow by slow destabilization of a nanocrystal solution upon allowing the diffusion of a nonsolvent into the colloidal solution of nanocrystals. The colloidal crystals of nanorods show characteristic birefringence, which we assign to specific spherulite-like texture of each nanorod assembly. To demonstrate the general character of nanorod self-assembly technique, CdSe/CdS heterostructure nanorods were organized into highly luminescent superlattices.     

Enantioselective aziridination using copper complexes of biaryl Schiff bases

Racemic 2,2'-diamino-6,6'-dimethylbiphenyl is resolved using simulated moving bed chromatography, and the absolute configuration of the enantiomers is confirmed via the X-ray crystal structure of a derivative. The diamine is condensed with a range of aldehydes to give bidentate aldimine proligands L. Molecular structures of the complexes formed between L and Cu(I) fall into two classes; bimetallic double helices ([Cu(2)L(2)](2+)) and monometallic ([CuL](+)). The latter are strikingly more efficient in the aziridination of alkenes than are the former in terms of rate, turnover, and enantioselection. In particular, the imine ligand formed from the diamine and 2,6-dichlorobenzaldehyde gives, in combination with Cu(I) or Cu(II), up to 99% ee in the aziridination of 6-acyl-2,2-dimethylchromene and 88-98% ee for a range of cinnamate esters. Styrenic and other alkenes are converted with lower selectivities (5-54%). The catalytic system shows a linear response in product ee to catalyst ee, and the product ee does not vary significantly during the reaction. UV spectrophotometric investigations indicate that conversion of Cu(I) to Cu(II) is not essential for catalysis but that Cu(II) is probably also a competent system.     

Effect of modified virtual orbitals on local Coulomb correlation holes in FCN

Local Coulomb correlation hole distribution functions may be used to assess the extent to which electron correlation effects are present in large scale SCF + CI wave functions. From a set of modified virtual orbitals, ordered according to their interaction with the SCF configuration, we have constructed a limited SCF + CI wave function with improved convergence characteristics with respect to that formed from the canonical virtual orbital set. These wave functions, of the same size yet with different energies, have been used to examine the range and depth of local Coulomb correlation holes in FCN. In all cases, the depth of the local Coulomb hole is no more than 10% or so of that of the corresponding Fermi hole, and the range Fermi correlation is generally less than that of Fermi correlation. This is particularly marked in the high density regions around the nuclei. The significance of our results is discussed in relation to a recent proposal for the incorporation of Coulomb correlation into the local exchange method.     

Ruthenium bipyridyl compounds with two terminal alkynyl ligands

Compounds of the form Ru(X2bipy)(PPh3)2(-C triple bond CC6H4NO2-p)2(X2bipy = 4,4'-X(2)-2,2'-bipyridine, X = Me 3a, Br 3b, I 3c) have been synthesised from the mono-alkynyl precursors Ru(X2bipy)(PPh3)2(-C triple bond CC6H4NO2-p)Cl (X = Me 2a, Br 2b, I 2c); the former are the first ruthenium bis-alkynyl compounds that also contain a bipyridyl ligand. Spectroelectrochemical investigation of 3a shows that the metal is readily oxidised to form the ruthenium(III) compound 3a+, and will also undergo a single-electron reduction at each nitro group to form 3a2-. ESR and UV/visible spectra of these redox congeners are presented. We also report the synthesis of [Ru(Me2bipy)(PPh3)2(-C triple bond CBut)(N triple bond N)][PF6] during the attempted synthesis of Ru(Me2bipy)(PPh3)2(-C triple bond CBut)2, and report its X-ray crystal structure and IR spectrum. X-Ray crystal structures of 3b and 3c(as two different solvates) are presented, and the nature of the intermolecular interactions seen therein is discussed. Z-Scan measurements on Ru(Me2bipy)(PPh3)2(-C triple bond CR)Cl (R = C6H4NO2-p2a, But, Ph, C6H4Me) are also reported, and show that Ru(Me2bipy)(PPh3)2(-C triple bond CR)Cl (R = C6H4NO2-p2a, Ph) exhibit moderate third-order non-linearities.     

A concise and regioselective synthesis of 1-alkyl-4-imidazolecarboxylates

Reaction between ethyl 3-N,N-(dimethylamino)-2-isocyanoacrylate (1) and a primary amine (2) regioselectively affords 1-alkyl-4-imidazolecarboxylates (3) in good yields (52-89%). The method is applicable to unhindered and hindered amine substrates, as well as those containing reactive functionality such as alcohols and secondary and tertiary amines. [reaction: see text]     

A glow discharge ion source with fourier transform ion cyclotron resonance mass spectrometric detection

A glow discharge (CD) ion source has been coupled to a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer using a four-element electrostatic lens to accelerate and focus ions generated external to the instrument’s high magnetic field into its analyzer cell. Like other CD mass spectrometers, GD-FT-ICR can provide a quantitative measure of bulk analyte concentration with good precision and accuracy. Although detection limits currently attainable are several orders of magnitude higher than the commercially available magnetic sector-based instrument, CD-FT-ICR holds promise for ultrahigh resolving power elemental mass analysis. Several schemes are proposed to lower the detection limits of the technique while still providing high enough resolution to resolve isobaric interferences.     

Photochemical synthesis of a palladium dichloromethyl complex, {(hexyl)HC(N-methyl-imidazol-2-yl)2}Pd(CHCl2)Cl.: X-ray molecular structures of {(hexyl)HC(N-methylimidazolyl-2-yl)2}Pd(X)Cl, X=Cl, and CHCl2

The reaction of (hexyl)HC(mim)₂ (1, mim=N-methyl-imidazol-2-yl) with (cod)PdMeCl in C₆H₆ yields {(hexyl)HC(mim)₂}Pd(Me)Cl (3). The photochemical reaction of 3 with CH₂Cl₂ at 23 °C in ambient room light yields {(hexyl)HC(mim)₂}Pd(CHCl₂)Cl (4). It is proposed that this reaction proceeds by homolytic scission of the PdMe bond of 3.     

Germanium-oxygen interactions in acyl-substituted germyleneazines L2GeNNC(COX)(COY)

MNDO calculations have been carried out on the reactions of the electron-rich germylene L₂Ge [L=(H₃Si)₂N] with diazo compounds, as models for the experimentally observed reactions of L₂Ge [L=(Me₃Si)₂N]. The most stable form of the 11 adduct of L₂Ge with N₂C(COOMe)₂ is found to have a cyclic configuration resulting from a strong intramolecular interaction between the oxygen of one of the carbonyl groups and the germanium atom. Protonation of this cyclic adduct occurs at nitrogen, giving an intermediate, addition to which of nucleophiles X^– provides acyclic L₂Ge(X)NHN(COOMe)₂, as observed experimentally. Two similar cyclic adducts are formed between L₂Ge and N₂C(COCH₃)(COOCH₃), the most stable of which provides, after a proton shift, the observed 1,3,4,2-oxadiazagermine system . Adduct formation between Me₂Si=NSiMe₃ and simple Lewis bases (H₂O, NH₃, THF, H₂CO) is calculated to be strong, but the corresponding adducts of Me₂Ge=NSiMe₃ are very weak: much stronger adducts are predicted for L₂GeNNC(COOMe)₂.     

Probing the intrinsic electronic structure of the cubane [4Fe-4S] cluster: nature's favorite cluster for electron transfer and storage

The cubane [4Fe-4S] is the most common multinuclear metal center in nature for electron transfer and storage. Using electrospray, we produced a series of gaseous doubly charged cubane-type complexes, [Fe4S4L4]2- (L = -SC2H5, -SH, -Cl, -Br, -I) and the Se-analogues [Fe4Se4L4]2- (L = -SC2H5, -Cl), and probed their electronic structures with photoelectron spectroscopy and density functional calculations. The photoelectron spectral features are similar among all the seven species investigated, revealing a weak threshold feature due to the minority spins on the Fe centers and confirming the low-spin two-layer model for the [4Fe-4S](2+) core and its "inverted level scheme". The measured adiabatic detachment energies, which are sensitive to the terminal ligand substitution, provide the intrinsic oxidation potentials of the [Fe4S4L4]2- complexes. The calculations revealed a simple correlation between the electron donor property of the terminal thiolate as well as the bridging sulfide with the variation of the intrinsic redox potentials. Our data provide intrinsic electronic structure information of the [4Fe-4S] cluster and the molecular basis for understanding the protein and solvent effects on the redox properties of the [4Fe-4S] active sites.     

Cathodic stripping voltammetry of trace Mn(II) at carbon film electrodes

A sensitive voltammetric method is presented for the determination of tract levels of Mn (II) using carbon film electrodes fabricated from carbon resistors of 2 Ω. Determination of manganese was made by square wave cathodic stripping voltammetry (CSV), with deposition of manganese as manganese dioxide. Chronoamperometric experiments were made to study MnO₂ nucleation and growth. As a result, it was found to be necessary to perform electrode conditioning at a more positive potential to initiate MnO₂ nucleation. Under optimised conditions the detection limit obtained was 4 nM and the relative standard deviation for eight measurements of 0.22 nM was 5.3%. Interferences from various metal ions on the response CSV of Mn(II) were investigated, namely Cd(II), Ni(II), Cu(II), Cr(VI), Pb(II), Zn(II) and Fe(II). Application to environmental samples was demonstrated.