A Six-Coordinate Tris(3,5-dimethyl-1-pyrazolyl)methane-Thallium(I) Complex with a Stereochemically Inactive Lone Pair: Syntheses and Solid State Structures of {[HC(3,5-Me(2)pz)(3)](2)Tl}PF(6) and {[HC(3,5-Me(2)pz)(3)]Tl}PF(6) (pz = Pyrazolyl)

The addition of the tris(pyrazolyl)methane ligand HC(3,5-Me(2)pz)(3) (pz = pyrazolyl ring) to a THF solution of TlPF(6) results in the immediate precipitation of {[HC(3,5-Me(2)pz)(3)](2)Tl}PF(6). The structure has been determined crystallographically. The arrangement of the nitrogen donor atoms about the thallium is best described as a trigonally distorted octahedron. The thallium atom sits on a crystallographic center of inversion; thus the planes formed by the three nitrogen donor atoms of each ligand are parallel. The Tl-N bond distances range from 2.891(5) to 2.929(5) ? (average = 2.92) ?. The lone pair on thallium is clearly stereochemically inactive and does not appear to influence the structure. The pyrazolyl rings are planar, but are tilted with respect to the thallium atom so as to open up the N.N intraligand bite distances. The thallium(I) complex with a ligand to metal ratio of 1/1, {[HC(3,5-Me(2)pz)(3)]Tl}PF(6), is prepared in acetone by the reaction of equimolar amounts of HC(3,5-Me(2)pz)(3) and TlPF(6). The structure of the cation is a trigonal pyramid, with Tl-N bond distances that range from 2.64(1) to 2.70(1) ? (average = 2.67) ?. Pyrazolyl ring tilting is also observed in this complex, but the degree of tilting is smaller. Crystal data for {[HC(3,5-Me(2)pz)(3)](2)Tl}PF(6): monoclinic, P2(1)/c, a = 9.210(6) ?, b = 13.36(1) ?, c = 16.067(8) ?, beta = 92.48(5) degrees, V = 1975(2) ?(3), Z = 2, R = 0.029. For {[HC(3,5-Me(2)pz)(3)]Tl}PF(6): monoclinic, P2(1)/n, a = 10.685(2) ?, b = 16.200(5) ?, c = 13.028(3) ?, beta = 94.02(2) degrees, V = 2249.6(8) ?(3), Z = 4, R = 0.042.     

Speciation analysis of mercury by solid-phase microextraction and multicapillary gas chromatography hyphenated to inductively coupled plasma-time-of-flight-mass spectrometry

This paper reports the development of an analytical approach for speciation analysis of mercury at ultra-trace levels on the basis of solid-phase microextraction and multicapillary gas chromatography hyphenated to inductively coupled plasma-time-of-flight mass spectrometry. Headspace solid-phase microextraction with a carboxen/polydimethylsyloxane fiber is used for extraction/preconcentration of mercury species after derivatization with sodium tetraethylborate and subsequent volatilization. Isothermal separation of methylmercury (MeHg), inorganic mercury (Hg2+) and propylmercury (PrHg) used as internal standard is achieved within a chromatographic run below 45 s without the introduction of spectral skew. Method detection limits (3 x standard deviation criteria) calculated for 10 successive injections of the analytical reagent blank are 0.027 pg g(-1) (as metal) for MeHg and 0.27 pg g(-1) for Hg2+. The repeatability (R.S.D., %) is 3.3% for MeHg and 3.8% for Hg2+ for 10 successive injections of a standard mixture of 10pg. The method accuracy for MeHg and total mercury is validated through the analysis of marine and estuarine sediment reference materials. A comparison of the sediment data with those obtained by a purge-and-trap injection (PTI) method is also addressed. The analytical procedure is illustrated with some results for the ultra-trace level analysis of ice from Antarctica for which the accuracy is assessed by spike recovery experiments.     

Determination of trace impurities in tin by neutron activation analysis

Two methods are described to determine indium and managenese in high-purity tin. In the first method indium and manganese are separated from the tin and antimony matrix activities on Dowex 1X8 anion exchanger. Tin and antimony are adsorbed in 10M HF while indium and manganese are eluted. In the second method the incident γ-ray intensity due to the tin matrix is reduced by placing a lead absorber between the sample and the detector. The reproducibility and the sensitivity of both methods are of the order of 10 ppb for manganese and of 1 ppb for indium for 1 g samples and a neutron flux of 10¹¹ n·cm⁻²·sec⁻¹. Aspirant of the N. F. W. O.     

Fluorescence of cis-1-amino-2-(3-indolyl)cyclohexane-1-carboxylic acid: a single tryptophan chi(1) rotamer model

A constrained derivative, cis-1-amino-2-(3-indolyl)cyclohexane-1-carboxylic acid, cis-W3, was designed to test the rotamer model of tryptophan photophysics. The conformational constraint enforces a single chi(1) conformation, analogous to the chi(1) = 60 degrees rotamer of tryptophan. The side-chain torsion angles in the X-ray structure of cis-W3 were chi(1) = 58.5 degrees and chi(2) = -88.7 degrees. Molecular mechanics calculations suggested two chi(2) rotamers for cis-W3 in solution, -100 degrees and 80 degrees, analogous to the chi(2) = +/-90 degrees rotamers of tryptophan. The fluorescence decay of the cis-W3 zwitterion was biexponential with lifetimes of 3.1 and 0.3 ns at 25 degrees C. The relative amplitudes of the lifetime components match the chi(2) rotamer populations predicted by molecular mechanics. The longer lifetime represents the major chi(2) = -100 degrees rotamer. The shorter lifetime represents the minor chi(2) = 80 degrees rotamer having the ammonium group closer to C4 of the indole ring (labeled C5 in the cis-W3 X-ray structure). Intramolecular excited-state proton transfer occurs at indole C4 in the tryptophan zwitterion (Saito, I.; Sugiyama, H.; Yamamoto, A.; Muramatsu, S.; Matsuura,T. J. Am. Chem. Soc. 1984, 106, 4286-4287). Photochemical isotope exchange experiments showed that H-D exchange occurs exclusively at C5 in the cis-W3 zwitterion, consistent with the presence of the chi(2) = 80 degrees rotamer in solution. The rates of two nonradiative processes, excited-state proton and electron transfer, were measured for individual chi(2) rotamers. The excited-state proton-transfer rate was determined from H-D exchange and fluorescence lifetime data. The excited-state electron-transfer rate was determined from the temperature dependence of the fluorescence lifetime. The major quenching process in the -100 degrees rotamer is electron transfer from the excited indole to carboxylate. Electron transfer also occurs in the 80 degrees rotamer, but the major quenching process is intramolecular proton transfer. Both quenching processes are suppressed by deprotonation of the amino group. The results for cis-W3 provide compelling evidence that the complex fluorescence decay of the tryptophan zwitterion originates in ground-state heterogeneity with the different lifetimes primarily reflecting different intramolecular excited-state proton- and electron-transfer rates in various rotamers.     

Gamma-oxidation of linear low density polyethylenes: Ethylene–butene and ethylene–hexene copolymers

Different linear low density polyethylenes (LLDPEs) based on hexene and butene comonomer were irradiated with γ-rays under air. The oxidation products have been quantified using NO and SF₄ treatments by IR spectroscopy. An FTIR examination displayed unexpected products such as free hydroperoxides and trans-vinylenes. It was found that the type of α-olefin used to process LLDPE films may influence the mechanism of oxidation. Parameters such as the melt index and the density seem to affect the γ-oxidation rate. © 1993 John Wiley & Sons, Inc.     

Flow injection on-line sorption preconcentration of platinum in a knotted reactor coupled with electrothermal atomic absorption spectrometry

A flow injection on-line sorption preconcentration method for the electrothermal AAS determination of platinum has been developed. The pyrrolidine dithiocarbamate complexes of either Pt⁴⁺ or Pt²⁺, formed in 0.7 mol L^–1 HNO₃, are on-line adsorbed on the inner walls of a PTFE knotted reactor and subsequently eluted with methanol. An enhancement factor of 112 and a detection limit (3 σ) of 10 ng L^–1 along with a sampling frequency of 21 h^–1 are achieved with a 90 s preconcentration time at a sample flow rate of 8.8 mL min^–1. The relative standard deviation is 2.5% for 0.4 μg L^–1 Pt. The method has been applied to the determination of platinum in blood samples. Received: 6 October 1997 / Revised: 26 November 1997 / Accepted: 3 December 1997     

Rhodium cluster complexes containing bridging phenylgermanium ligands

The reaction of Rh(4)(CO)(12) with Ph(3)GeH at 97 degrees C has yielded the first rhodium cluster complexes containing bridging germylene and germylyne ligands: Rh(8)(CO)(12)(mu(4)-GePh)(6), 9, and Rh(3)(CO)(5)(GePh(3))(mu-GePh(2))(3)(mu(3)-GePh)(mu-H), 10. When the reaction is performed under hydrogen, the yield of 9 is increased to 42% and no 10 is formed. Compound 9 contains a cluster of eight rhodium atoms arranged in the form of a distorted cube. There are six mu(4)-GePh groups bridging each face of this distorted cube. Four of the rhodium atoms have two terminal carbonyl ligands, while the remaining four rhodium atoms have only one carbonyl ligand. Compound 10 contains a triangular cluster of three rhodium atoms with one terminal GePh(3) ligand, three bridging GePh(2) ligands, and one triply bridging GePh ligand. There is also one hydrido ligand that is believed to bridge one of the Rh-Ge bonds. Compound 9 reacted with PPhMe(2) at 25 degrees C to give the tetraphosphine derivative Rh(8)(CO)(8)(PPhMe(2))(4)(mu(4)-GePh)(6), 11. The structure of 11 is similar to 9 except that a PPhMe(2) ligand has replaced a carbonyl ligand on each the four Rh(CO)(2) groups. Compound 10 reacted with CO at 68 degrees C to give the complex Rh(3)(CO)(6)(mu-GePh(2))(3)(mu(3)-GePh), 12. Compound 12 is formed by the loss of the hydrido ligand and the terminal GePh(3) ligand from 10 and the addition of one carbonyl ligand. All compounds were fully characterized by IR, NMR, elemental, and single-crystal X-ray diffraction analyses.     

Synthesis, Structure, and Superconductivity in Be1.09B3

The compound Be_1.09B₃ was prepared by arc-melting of the elemental constituents. The structure of single crystals taken from the arc-melted boule was determined from single-crystal X-ray data (T=120 K) and is hexagonal, having space group P6/mmm, and lattice parameters a=9.7738(7) Å and c=9.5467(6) Å, R=0.047. The structure consists of a hexagonal array of boronicosahedra, nonicosahedral B₁₂ cages, and B₁₈ cages. Stacked hexagonal layers of boron atoms, hexagons formed by B and Be, and equilateral triangles of boron atoms disordered by a 60° rotation exist along a 6-fold axis down the [001] direction. A superconducting transition at 0.72 K is clearly indicated by resistivity measurements.     

Gold-Stickstoff-Heterocyclen. Synthese,Eigenschaften und Struktur von [(CH3)2AuNH2]4 und [(CH3)2AuN(CH3)2]2

Gold Nitrogen Heterocycles. Synthesis, Properties, and Structure of [(CH₃)₂AuNH₂]₄ and [(CH₃)₂AuN(CH₃)₂]₂ Dimethyl gold iodide reacts with alkali metal amides to form Au-N heterocycles. KNH₂ yields the eight-membered ring [(CH₃)₂AuNH₂]₄, whereas with LiN(CH₃)₂ the four-membered ring [(CH₃)₂AuN(CH₃)₂]₂ is obtained. The light sensitive, cyclic gold amides are stable against hydrolysis and do not react with Lewis bases. [(CH₃)₂AuN(CH₃)₂]₂ crystallizes monoclinic in the space group P2₁/c with Z = 2. The molecules exhibit the symmetry D_2h. Symmetrical amido bridges form a planar Au-N heterocyclus with distances Au-N = 214 pm.     

Application of graphical methods of spin algebras to limited CI approaches. I. Closed shell case

The time independent diagrammatic technique based on the mathematical methods of quantum electrodynamics (second quantization, Wick's theorem, Feynman-like diagrams) is combined with graphical techniques of spin algebras to derive general expressions for the matrix elements of spin independent one- and two-particle operators between spin symmetry adapted ground, mono- and bi-excited configurations of a closed shell system. Two coupling schemes are considered for bi-excited states and their relative merits are discussed. Finally, the results are used to derive compact expressions for the coupling coefficients of the direct configuration interaction from molecular integrals (CIMI ) method.