Frequency stability of an optically pumped cesium beam frequency standard

Results obtained in an experimental optically pumped cesium beam frequency standard in which a single semiconductor laser is used for the state selection and the atom detection are reported. The separation between the two interaction regions is equal to 21 cm. This gives a 500 Hz linewidth which is observed with a signal to noise ratio equal to 10,000 in a 1 Hz noise bandwidth. A quartz crystal oscillator is frequency controlled by the atomic transition. The measured short term frequency stability is given by _y()=2×10^–12–1/2 for 1 s<500 s. Prospects for improvement of this frequency stability are discussed.     

Preparation and electrochemical behavior of ordered rh adlayers on Pt(100) electrodes

Rhodium adlayers on Pt(100) substrates have been prepared by electrodeposition from dilute Rh(III) acidic solutions. The initially disordered layer is electrochemically annealed by applying a polarization program consisting of high-sweep-rate multicycle sequences between 0.05 and 0.78 V(RHE) in 0.1 M H(2)SO(4). In this way, a pseudomorphic Rh monolayer can be prepared on Pt(100) substrates. The degree of order of the electrochemically annealed layer has been evidenced not only through voltammetric experiments but also by means of scanning tunneling microscopy with atomic resolution for iodine-protected adlayers, which show a c(2 x 2) structure. The electrochemically induced ordering of the Rh adlayer appears to be a consequence of the repeated cycles of adsorption/desorption of H and, especially, oxygenated species. Voltammetry in sulfuric acid solutions permits examination of the energetics of H/anions and OH/O adsorption as a function of the Rh coverage. The first monolayer adsorbs both hydrogen and oxygenated species more strongly than the second one. This can be explained through an electronic effect caused by the underlying Pt(100) substrate.     

Strong, low-energy two-photon absorption in extended amine-terminated cyano-substituted phenylenevinylene oligomers

Three quadrupolar oligophenylenevinylenes with five rings in the conjugated backbone, terminal donor groups, and various acceptors and/or donors along the backbone were synthesized and their two-photon spectroscopic properties investigated. These chromophores exhibit large two-photon absorption cross sections over a wide wavelength range and two distinct peaks, the strongest of which (deltamax > 3600 GM) is observed at 960-970 nm, a wavelength close to twice the value of the linear absorption maximum (2lambda(1)max). The findings on these chromophores are compared with those for analogous molecules with shorter conjugation length, for which the main two-photon band is at significantly shorter wavelength than 2lambda(1)max.     

Synthesis,crystal structures and electrochemical properties of group 6 metal carbonyl anion complexes

A series of [M(CO)₅I]⁻ and [M₂(CO)₁₀I]⁻ anion complexes have been synthesized by the photochemical reaction of PhCH₂N(CH₃)₃I or FcCH₂N(CH₃)₃I (Fc=ferrocenyl) with M(CO)₆ (M = Cr, Mo or W), and characterized by elemental analyses, i.r., ¹H-n.m.r. and ¹³C-n.m.r. spectra in the case of the molybdenum and tungsten complexes. These complexes exhibit considerably different electrochemical behavior, when investigated by cyclic voltammetry. The crystal structures of [PhCH₂N(CH₃)₃][Cr(CO)₅I] and [FcCH₂N(CH₃)₃][W₂(CO)₁₀I] have been determined by X-ray diffraction, indicating that only weak contacts maybe exist between anions and cations by the I...H bond in the former, and there are no direct interactions between anions and cations in the latter.     

Transition metal complexes with Girard reagent-based ligands

The ligand, salicylaldehyde Girard-T hydrazonium chloride, [H₂SalGT]Cl (1), and two complexes [Cu(HSalGT)X₂]·H₂O (X = Br(2); Cl(3)) were synthesized and their crystal structures were determined by single-crystal X-ray analysis. In the two isostructural complexes, the Cu(II) is located in a square-pyramidal environment, with the chelating ligand and one halogen atom in the basal plane and the second halogen in the apical position. The most apparent structural difference between the 1 and its complexes 2 and 3 is the orientation of the N(CH₃)₃ group: in 1, it is practically coplanar to the rest of the molecule, while in 2 and 3 it is oriented to the side of the axially bonded halogen, which can be explained by the C–H…X intramolecular interactions. The compounds were characterized by elemental analysis, molar conductivity, magnetic susceptibility and electronic absorption spectra.     

Maskierung mit Kaliumjodid bei direkten Titrationen von Kupfer(II) mit Äthylendiamintetraessigsäure

Zusammenfassung Die Jodidoxydation durch Cu(II) wird in Anwesenheit von Ammoniak, Natriumtartrat, citrat oder -oxalat infolge Komplexbildung verhindert. Daher kann man z. B. Quecksüber(II) in Anwesenheit der genannten Komplexbildner mit Kaliumjodid maskieren und Kupfer(II) durch direkte Titration mit ÄDTA gegen Murexid, Brenzcatechinviolett, Chromazurol S, PAN oder PAR als Indikator bestimmen.

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Masking with potassium iodide in direct titrations of copper(II) with ethylenediamine tetraacetic acid

Summary The formation of a complex prevents the iodide oxidation by Cu(II) in the presence of ammonia, sodium tartrate, citrate, or oxalate. Hence mercury(II) for instance, can be masked against potassium iodide in the presence of these complex formers, and Cu(II) can be determined by direct titration with EDTA in the presence of murexide, pyrocatechol violet, chromazurol, PAN or PAR as indicator.

     

Synthesis,crystal structure and ferro-magnetic interaction of a binuclear copper(II) complex with the 1,4-dinitro-2,3,5,6-tetracarboxylatobenzenic anion as bridging ligand

A binuclear complex [Cu₂(DTB)(DMF)₄(H₂O)]·2DMF (DTB = 1,4-dinitro-2,3,5,6-tetracarboxylatobenzenic anion; DMF = N,N-dimethylformamide) has been synthesized and its crystal structure determined by X-ray crystallography. In the complex Cu ion is located in a distorted square pyramidal environment with two oxygen atoms O(1) and O(3) from two carboxylate groups, another two oxygen atoms O(7) and O(8) from terminal ligands of two DMF molecules, and a fifth coordinated oxygen atom O(9) from the terminal ligand of one H₂O molecule, in which the O(8) atom is situated in the apex of the pyramid. DTB as bridging ligand coordinates two Cu ions through its four carboxylate groups. The variable-temperature magnetic susceptibility of the complex was measured in the 5–300 K range. The magnetic coupling parameter is consistent with a ferromagnetic exchange between the two copper(II) centers and the data fit a binuclear magnetic exchange model based on the Hamiltonian operator ( = -2J₁₂, ₁ = ₂= 1/2), giving the ferromagnetic coupling parameter of 2J = 1.80 cm^{- 1}. This is the first example of a tetracarboxylatobenzenic bridging complex exhibiting ferromagnetic interaction.     

Reactions of sulfur(IV) with an octahedral manganese(IV) complex of 1,8-bis(2-hydroxy benzamido)-3,6-diaza octane: the role of phenolate–amide–amine coordination

The Mn^IV complex of tetra-deprotonated 1,8-bis(2-hydroxybenzamide)-3,6-diazaoctane (Mn^IVL) engrossed in phenolate-amido-amine coordination is reduced by HSO₃⁻ and SO₃²⁻ in the pH range 3.15–7.3 displaying biphasic kinetics, the Mn^IIIL⁻ being the reactive intermediate. The Mn^IIIL⁻ species has been characterized by u.v.–vis. spectra {λ _max, (ε, dm³ mol⁻¹ cm⁻¹): 285(15 570), 330 sh (7570), 469(6472), 520 sh (5665), pH=5.42}. SO₄²⁻ was the major oxidation product of S^IV; dithionate is also formed (18 ± 2% of [Mn^IV]_T) which suggests that dimerisation of SO₃^−• is competitive with its fast oxidation by Mn^IV/III. The rates and activation parameters for Mn^IVL + HSO₃⁻ (SO₃²⁻) → Mn^IIIL⁻; Mn^IIIL⁻ + HSO₃⁻ (SO₃²⁻) → Mn^IIL²⁻ are reported at 28.5–45.0 °C (I=0.3 mol dm⁻³, 10% (v/v) MeOH + H₂O). Reduction by SO₃²⁻ is ca. eight times faster than by HSO₃⁻ both for Mn^IVL and Mn^IIIL⁻. There was no evidence of HSO₃⁻/SO₃²⁻ coordination to the Mn centre indicating an outer sphere (ET) mechanism which is further supported by an isokinetic relationship. The self exchange rate constant (k₂₂) for the redox couple, Mn^IIIL⁻/Mn^IVL (1.5 × 10⁶ dm³ mol⁻¹ s⁻¹ at 25 °C) is reported.