The Missing Link in Low Nuclearity Pure Polyoxovanadate Clusters: Preliminary Synthesis and Structural Analysis of a New {V16} Cluster and Related Products

Two new metastable polyoxovanadate-based cluster compounds have been isolated and crystallographically characterized with nuclearities of {V₁₆} (1) and {V₁₈} (2). The {V₁₆} cluster represents a new framework type and incorporates two protons into the cluster framework whereas the {V₁₈} framework has been previously characterised, and the oxidation states of the {V₁₈} cluster can be assigned as {V^IV ₁₅V^V ₃}. Compound K₁₀[H₂V₁₆O₃₈]13H₂O (1) crystallizes in the monoclinic space group P2₁/c, a=12.12820(10), b=38.2302(3), c=12.35400(10) Å, =115.0470(10)°, V=5189.43(7) ų, D _c=2.624 gcm^–3. 10086 unique reflection and 702 refined parameters were used in structure refinement. R1=0.039, R2=0.109 (all data). From the same preparation the new compound K₁₁[V₁₈O₄₂(SO₄)]20H₂O (2) was also isolated and crystallographically characterized. Compound 2 crystallizes in the monoclinic space group P2₁/n, a=12.7854(3), b=20.2812(5), c=13.2386(4) Å, =115.3400(10)°, V=3102.53(14) ų, D _c=2.650 gcm^–3. 7115 unique reflections and 462 refined parameters were used in structure refinement. R1=0.046, R2=0.121 (all data).     

Electronic structure of tubular aromatic molecules derived from the metallic (5,5) armchair single wall carbon nanotube

All-electron static and time-dependent DFT electronic calculations, with complete geometrical optimization, are performed on tubular molecules up to C(210)H(20) that are finite sections of the (5,5) metallic single wall carbon nanotube with hydrogen termination at the open ends. We find pronounced C-C bond reconstruction at the tube ends; this initiates bond alternation that propagates into the tube centers. For the especially low band gap molecules C(120)H(20), C(150)H(20), and C(180)H(20), alternation increases, and a second nearly isoenergic structural isomer of different alternation is found. A small residual C-C bond alternation and band gap may be present in the infinite tube. The van Hove band gap forms quickly with length, while the metallic Fermi point (at the crossing of linear bands) forms very slowly with length. There are no end-localized states at energies near the Fermi energy. The HOMO-LUMO gap and the lowest singlet excited state, whose energies show a periodicity with length as previously calculated, are optically forbidden. However, each molecule shows an intense visible "charge transfer" transition, not present in the infinite tube, whose energy varies smoothly with length; this transition should be an identifying signature for these molecules. The static axial polarizability per unit length increases rapidly with N as the "charge transfer" transition moves into the infrared; this indicates increasing metallic character. However, the ionization potential, electron affinity, chemical hardness, and relative energetic stability all show the length periodicity seen in the HOMO-LUMO gap, in contrast to the optical "charge transfer" transition and the static axial polarizability. These periodicities, due to a one-dimensional quantum size effect as originally modeled by Coulson in 1938, nevertheless cancel in the calculated Fermi energy, which varies smoothly toward a predicted bulk work function near 3.9 eV. A detailed study of C(190)H(20) with up to eight extra electrons or holes shows the total energy is closely fit by a simple classical charging model, as is commonly applied to metallic clusters.     

Development of screen-printed carbon electrodes,chemically modified with cobalt phthalocyanine,for electrochemical sensor applications

A method is described for the production of screen-printed graphite electrodes and also for similar electrodes chemically modified with the electrocatalyst cobalt phthalocyanine. Using cyclic voltammetry, the electrochemical behaviour of these electrodes towards ascorbic acid, reduced glutatione and coenzyme A (CoA-SH) was investigated. The modified electrodes were found to give significant decreases in the over-potential required for the oxidation of these species at carbon electrodes. The useful electrochemical window for the unmodified carbon film electrodes was ?1.08 V to +0.85 V vs. SCE, using 1 μA background current cut-off points.Amperometry in stirred solutions was used to investigate the hydrodynamic behaviour of the electrodes and their calibration performance. The limits of detection for ascorbic acid and reduced glutathione at the modified films were 5 × 10^?8 and 1 × 10^?7 M, respectively. The calibration graphs were also linear up to 2 mM concentrations of both analytes. Using differential-pulse voltammetry, linear calibration graphs were obtained for both species up to 2.5 mM. This technique was also used to assess the reproducibility of the electrode manufacture; the coefficient of variation was 2.8% for 1.49 mM ascorbic acid and 6.9% for 0.92 mM reduced glutathione.     

Formal synthesis of (+/-)-guanacastepene A: a tandem ring-closing metathesis approach

A concise route to a key intermediate in the total synthesis of guanacastepene A is described. The main features include the simultaneous construction of the seven- and six-membered rings, using a tandem ring-closing metathesis and a stereoselective introduction of the oxygenated function at the C5 position. [reaction: see text]     

Sur l'acylation des mäthylfluorànes V. Formylation selon Rieche du mäthy1-1-fluoráne

The formylation of 1-methylfluorene according Rieche yields 1-methyl-2-fluorene-carbaldehyde, the constitution of which being proved by three independent ways.     

Streaming current measurements in zirconia-coated capillaries

The electroosmotic flow created in zirconia-modified capillaries has been previously investigated. In this paper, we compared the electroosmotic data set with streaming current measurements and we related all these data through zeta-potential. Streaming current measurements give an excellent indication on the direction and the value of the electroosmotic mobility of an electrolyte/capillary system for a large set of experimental conditions: 2 < pH < 12, 0 < ACN < 80 %, 10(-4) M < [SO(2- )4 ] < 4 x 10(-2) M. A good correlation between zeta-potential from streaming current measurements and zeta-potential from electroosmotic mobility measurements was observed (r2 = 0.95). However, the values obtained from streaming current were always slightly lower than the one calculated from electroosmotic mobility (slope = 0.86, sigma = 0.06). In zirconia-coated capillaries the zeta-potential can be tuned from -50 to +100 mV depending on the composition of the electrolyte.     

Fluorénacènes et fluorénaphènes Synthèses dans la série des indéno-fluorènes,XVIII. Le diméthyl-5, 6-dihydro-7, 12-indéno[1, 2-a]fluorène

Starting from 3, 4-dimethylfluorene the title compound, 5, 6-dimethyl-7, 12-dihydro-indeno[1,2-a]fluorene (IV), is synthesized in four steps (overall yield 9%). The same hydrocarbon is also obtained in four steps starting from the phthalaconecarboxylic acid of Gabriel. This corroborates the structure of IV.     

Cationic sigma-phenylplatinum(II) complexes with carboxylic acid functionality: pK(a) determinations and x-ray structures

The preparation and characterization of a novel series of cationic sigma-phenylplatinum(II) complexes of the type trans-[Pt(sigma-C(6)H(5))(L)(2)A]OTf (A = picolinic acid, L = PPh(3) (4) and PMePh(2) (7); A = nicotinic acid, L = PPh(3) (5) and PMePh(2) (8); A = isonicotinic acid, L = PPh(3) (6), PMePh(2) (9), and PEt(3) (10)) are described. The pK(a) value for the carboxylic acid functionality in selected complexes was found to follow the order 7 (pK(a) = 5.23 +/- 0.09) > 8 (4.85 +/- 0.10) > 9 (3.51 +/- 0.08) > 6 (3.26 +/- 0.07) approximately 10 (3.21 +/- 0.08) by means of potentiometric titration experiments in 50% (v/v) EtOH/H(2)O solution at 295 K. The X-ray crystal structures of 9 and 10 were also determined. The asymmetric unit of each of 9 and 10 comprises a univalent complex cation, a triflate anion, and a solvent CH(2)Cl(2) molecule of crystallization. Centrosymmetrically related pairs of complex cations in 9 associate via the familiar carboxylic acid dimer motif, whereas with 10, the carboxylic acid dimer motif is absent. Instead, the carboxylic acid residue forms both donor and acceptor interactions to the triflate anion and CH(2)Cl(2) solvent of crystallization, respectively, to afford a 10-membered ring structure. Possible reasons for the observed differences in the solid-state structures of 9 and 10 are presented.     

Vibrational lifetime measurements of matrix isolated molecules by an optical “double resonance” method

An optical-optical “double-resonance” experiment has been developed and successfully applied to the study of vibrational lifetimes. This pulsed dual laser technique should be applicable to all species possessing resolvable allowed vibronic transitions and it permits the direct measurement of vibrational lifetimes which, at present, may be as short as 20 μs. Application of this technique to the υ″ = 1 and υ″ = 2 states of matrix isolated C₂^? yields half lifes of 0.2 and 1.2 ms, respectively, in an argon matrix at 16K, and 0.3 and 1.3 ms in an N₂ matrix at 14K. No significant temperature variation of these rate constants has been found in the range 14 to 24 K. This is the first direct measurement of the vibrational lifetime of a homonuclear diatomic molecule isolated in a matrix environment.