Use of mobile phase 18-crown-6 to improve peak resolution between mono- and divalent metal and amine cations in ion chromatography

It is difficult to quantify NH4+ by ion chromatography in the presence of high concentrations of Na+ due to peak overlap. The Dionex IonPac CS15 column, which contains phosphonate, carboxylate, and 18-crown-6 functional groups, was originally developed to overcome this problem. We have found that the addition of 18-crown-6 to the eluent promotes improved peak resolution between Na+ and NH4+ even at concentrations as high as 60,000 to 1 using this column. Its use also improves the separation of alkali and alkaline earth metal and amine cations. Mobile phase 18-crown-6 increased the retention times of CH3NH3+, NH4+, and K+, and decreased the retention time of Sr2+. The retention times of Li+, Na+, Mg2+, Ca2+, (CH3)2NH2+, and (CH3)3NH+ were not affected. This method makes possible the direct analysis of ammonia from nitrogenase, the enzyme responsible for biological nitrogen fixation. The resolution of the NH4+ peak from the Na+ and Mg2+ peaks improved from zero resolution to values of 6.19 and 5.65, respectively. This technique considerably reduces the analysis time of NH4+ in the presence of high concentrations of Mg2+ and Na+ over traditional indophenol measurements.     

Syntheses and characterization of two dioxygen-reactive dinuclear macrocyclic schiff-base copper(I) complexes

The dinuclear copper(I) complex [Cu(2)L(1)(CH(3)CN)(2)](ClO(4))(2) (1, L(1) = 3,6,9,17,20,23-hexaazatricyclo[23.3.1.1]triaconta-1(29),2,9,11(30),12(13),14,16,23,25,27-decaene) has been structurally characterized. As previously described, intramolecular ligand hydroxylation (at the aromatic ring) was observed when 1 was reacted with dioxygen. A stopped-flow analysis of the reaction of 1 with dioxygen under different conditions did not allow a "dioxygen intermediate" to be spectroscopically detected. Detailed NMR and electrochemical data on 1 are also presented and evaluated for the first time. No copper(II) complexes of L(1) could be characterized due to hydrolysis of the compounds. In contrast, complex 2-differing from 1 only in an increase in the size of the chelate rings-did not undergo intramolecular hydroxylation when it was oxidized. The crystal structure of 2 is also described.     

High spin d5 complexes of tris(6-hydroxymethyl-2-pyridylmethyl)amine (H3L): hepta-coordinated [Mn(H3L)]Cl2 and linear trinuclear [Fe3L2](ClO4)3

Reaction of MnCl(2).4H(2)O with H(3)L (H(3)L = tris(6-hydroxymethyl-2-pyridylmethyl)amine) in methanol gives hepta-coordinated [Mn(H(3)L)]Cl(2) involving attachment of Mn(II) to all four nitrogens and three hydroxymethyl arms. Reaction of H(3)L with Fe(ClO(4))(2).6H(2)O in CH(3)CN in the presence of NaO(2)CC(6)H(5) in an attempt to make [Fe(III)OH(H(3)L)(O(2)CC(6)H(5))](ClO(4)), a putative model for soybean lipoxygenase-1, instead gave rise to the linear triiron(III) complex [Fe(3)L(2)](ClO(4))(3) with all three hydroxymethyl arms deprotonated and forming three alkoxide bridges between each Fe(III) centre. The central Fe(III) is hexa-coordinated to only the alkoxide bridges and flanked by two hepta-coordinated iron(III) centres analogous to the Mn(ii) complex. [Fe(3)L(2)](ClO(4))(3) exhibits two reversible 1e(-) reductions to mixed-valence [Fe(3)L(2)](2+) and [Fe(3)L(2)](+) forms. Structure data and magnetochemistry on [Fe(3)L(2)](ClO(4))(3) reveals the tightest Fe-O-Fe angle (87.4 degrees ) and shortest Fe...Fe distance (2.834 A) yet found for any weakly antiferromagnetically-coupled high spin alkoxide-bridged di- or triiron(iii) system and challenges current theories involved in correlating the extent/nature of magnetic interactions in such systems based on Fe-O(bridge) distances and Fe-O-Fe angles. The central hexa-alkoxide coordinated Fe(III) is novel and shows a remarkable resistance towards reduction to Fe(II).     

Sol–gel processing of IrO<Subscript>2</Subscript>–TiO<Subscript>2</Subscript> mixed metal oxides based on an iridium acetate precursor

Mixed IrO₂–TiO₂ oxides were prepared by the sol–gel method by adding an aqueous solution of an iridium(III) acetate precursor [Ir₃O(OAc)₆ (HOAc)₃]OAc, to titanium tetraethoxide in ethanol. By using an acetylacetonate modifier to stabilize the hydrolysed titanium alkoxide and by omitting a catalyst, gels were produced in all cases. Transmission electron microscopy and EDX analysis confirmed the high dispersion of iridium in the dried gel material on the nanometre scale. The images also show spherical cage features up to 20 nm in diameter. High mass losses of the gels in the TGA scans suggested low degrees of hydrolysis of the acetate precursor, but calcination gave a crystalline, mixed oxide (Ti_xIr_1–xO₂) solid solution. The precursor is also soluble in ethanol, which provides a slightly modified route to similar materials.     

Distinct water-exchange mechanisms for trinuclear transition-metal clusters

Mechanisms for water exchange from the bioxo-capped M-M-bonded trinuclear clusters, [M3(mu3-O)2(mu-O2CCH3)6(OH2)3]2+ [M = Mo(IV) and W(IV)], were investigated using high-pressure 17O NMR and compared to our previous work on a similar Rh(III) trimer. Reaction rates decrease by more than a factor of 2 when pressure is increased from 6 to 250 MPa for the Mo(IV) trimer, while exchange rates increase by less than a factor of 1.2 (10-229 MPa) for the W(IV) trimer. From the pressure dependence of the reaction rate, activation volumes (DeltaV()) were calculated to be DeltaV() = +8.0 (+/-0.4) cm(3) mol(-1) and DeltaV = -1.9 (+/-0.2) cm(3) mol(-1) for the Mo(IV) cluster and W(IV) cluster, respectively, which is the largest difference ( approximately 10 cm(3) mol(-1)) in activation volumes for any pair of 4d-5d (and 3d-4d) transition metal species located within the same group of the periodic table. If we interpret these activation volumes in terms of Swaddle's semiempirical model, which he established for simple octahedral monomers (Associative (A) = DeltaV approximately -13; Interchange (I) = DeltaV approximately 0; or Dissociative (D) = DeltaV approximately +13), our results suggest that water exchange follows a dissociative-interchange (Id) mechanism for the Mo(IV) cluster and an associative-interchange (Ia) activation mode for the W(IV) trimer. These volumes exhibit a unique changeover in the water-exchange mechanism despite considerable similarities in molecular structure and reactivity. This changeover could provide a standard for computational simulations of ligand-exchange pathways in molecules that are more complicated than monomers.     

Stability of a microwave heated fluid Layer

When a time harmonic electromagnetic wave impinges on a slaba certain portion of the wave creates heat within the slab throughdipolar and ohmic heating. The electrical and thermal propertiesof the material dictate the dynamical nature of the heatingprocess, as well as the steady-state temperature profile. Thematerial considered here is a slab of fluid. We consider thecase where the fluid is bounded by thin rigid layers of transparentmaterial. The steady-state heating profile governs the typesof convective motions that can occur and also affects the stabilitycharacteristics of temperature, pressure and velocity perturbationsintroduced in the slab. The main objective here is to examinesuch stability characteristics, initially in the linear regime.Both rigid-rigid and rigid-free configurations are considered.