Structural studies in main group chemistry : XIX. Organotin(IV) derivatives of tetracyanoethylene, 7,7,8,8-tetracyanoquinodimethane and 2,3,5,6-tetrachlorobenzoquinone. The isolation of stable organotin-substituted radicals

The reaction of organotin chlorides with the lithium salt of 7,7,8,8-tetracyanoquinodimethane (TCNQ) or hexaalkylditins with TCNQ yield stable organotin-substituted free radicals of the types R₃SnTCNQ^. (R = Me, n-Pr, n-Bu) and Me₂Sn(TCNQ^.)₂. The reaction of hexaphenylditin with TCNQ yields a (σ → π) charge transfer complex of stoichiometry (Ph₃SnSnPh₃)·TCNQ, whilst [Me₂SnCl(terpyridyl)⁺](TCNQ^-·) was isolated from the reaction of [Me₂SnCl(terpyridlyl)⁺][Me₂SnCl₃^-] and LiTCNQ. The oxidation of hexaalkylditins by tetracyanoethylene (TCNE) yields stable free radicals of the type R₃SnTCNE·, but treatment with 2,3,5,6-tetrachlorobenzoquinone yields either R₃SnOC₆Cl₄O·-p (R = Me) or R₃SnOC₆Cl₄OSnR₃-p (R = n-Bu, Ph). Tin-119 Mössbauer spectroscopy shows that the derivatives R₃SnTCNQ· and R₃TCNE· have trigonally-bipyramidally coordinated tin with planar [SnC₃] skeletons and bridging [TCNQ·] and [TCNE·] groups forming infinite one-dimensional chain structures. Me₃SnOC₆Cl₄O·-p was inferred to possess a similar structure but with oxy bridges forming chains with a Sn---O---Sn---O backbone. Me₂Sn(TCNQ·)₂ has a structure intermediate between tetrahedral and octahedral with a non-linear MeSnMe unit and anisobidentate chelation by two TCNQ groups. The TCNQ derivatives were of two types: (i) “green” or “brown”, indicative of delocalisation of the Ione electron over the cyanoquinone ligand, and (ii) a “blue” form in which spin-pairing of the Ione electron between adjacent organic groups takes place. Me₃SnTCNQ· may exist in both forms depending upon the mode of preparation.     

β-Substituted ketene thioacetals as β-lithioacrylate equivalents. The synthesis of (±)-eldanolide

Lithiation of 1,1-bis(phenylthio)-3-phenylthio-1-propene 3 and reaction with a range of electrophiles gave exclusively the γ-substituted product 4. This reagent has been used in a short synthesis of the pheromone, (±)-eldanolide.     

Synthesis and Characterization of the Isocyanide Ligated Centered Zirconium Halide Cluster [(Zr6Be)Cl12(CNXyl)6]

The first isocyanide ligated hexanuclear zirconium halide cluster is reported. The unoxidized [(Zr₆Be)Cl₁₂(CNXyl)₆] (CNXyl = 2,6-dimethylphenyl isocyanide) was obtained from the solid state precursor K₃Zr₆Cl₁₅Be by dissolution in CH₃CN in the presence of CNXyl. The CNXyl ligands occupy all the axial positions on the cluster. The compound was recrystallized from CH₂Cl₂ and Et₂O. [(Zr₆Be)Cl₁₂(CNXyl)₆].2CH₂Cl₂ crystallizes in the space group (#2) with a = 12.092(5) Å, b=12.728(5) Å, c = 14.102(8) Å, = 104.98(4)°, =107.11°, = 100.94°, V = 1919(2) ų, Z = l, R = 11.3% and R _W = 27.0%. For the bound isocyanide ligands, v _CN increases to 2140 cm^–1.     

Evolution of the interface and metal film morphology in the vapor deposition of Ti on hexadecanethiolate hydrocarbon monolayers on Au

The combination of in situ X-ray photoelectron spectroscopy, infrared reflection spectroscopy, atomic force microscopy, and time-of-flight secondary ion mass spectrometry are used to probe the nature of the evolving interface chemistry and metal morphology arising from Ti vapor deposition onto the surface of a CH(3)(CH(2))(15)S/Au{111} self-assembled monolayer (SAM) at ambient temperature. The results show that for a deposition rate of approximately 0.15 Ti atom.nm(-2).s(-1) a highly nonuniform Ti overlayer is produced via a process in which a large fraction of impinging Ti atoms do not stick to the bare SAM surface. The adsorbed atoms form isolated Ti clusters and react with CH(3) groups to form carbide products at the cluster-SAM interfaces. Further growth of Ti clusters appears to be concentrated at these scattered reaction centers. The SAM molecules in the local vicinity are subsequently degraded to inorganic products, progressing deeper into the monolayer as the deposition proceeds to give an inorganic/organic nanocomposite. A continuous overlayer does not form until metal coverage approaches approximately 50 Ti atoms per SAM molecule. These data indicate that for applications such as molecular device contacts the use of Ti may be highly problematic, suffering from both a highly nonuniform contact area and the presence of extensive inorganic products such as nonstoichiometric carbides and hydrides.     

Strategy for the study of paramagnetic proteins with slow electronic relaxation rates by nmr spectroscopy: application to oxidized human [2Fe-2S] ferredoxin

NMR studies of paramagnetic proteins are hampered by the rapid relaxation of nuclei near the paramagnetic center, which prevents the application of conventional methods to investigations of the most interesting regions of such molecules. This problem is particularly acute in systems with slow electronic relaxation rates. We present a strategy that can be used with a protein with slow electronic relaxation to identify and assign resonances from nuclei near the paramagnetic center. Oxidized human [2Fe-2S] ferredoxin (adrenodoxin) was used to test the approach. The strategy involves six steps: (1) NMR signals from (1)H, (13)C, and (15)N nuclei unaffected or minimally affected by paramagnetic effects are assigned by standard multinuclear two- and three-dimensional (2D and 3D) spectroscopic methods with protein samples labeled uniformly with (13)C and (15)N. (2) The very broad, hyperfine-shifted signals from carbons in the residues that ligate the metal center are classified by amino acid and atom type by selective (13)C labeling and one-dimensional (1D) (13)C NMR spectroscopy. (3) Spin systems involving carbons near the paramagnetic center that are broadened but not hyperfine-shifted are elucidated by (13)C[(13)C] constant time correlation spectroscopy (CT-COSY). (4) Signals from amide nitrogens affected by the paramagnetic center are assigned to amino acid type by selective (15)N labeling and 1D (15)N NMR spectroscopy. (5) Sequence-specific assignments of these carbon and nitrogen signals are determined by 1D (13)C[(15)N] difference decoupling experiments. (6) Signals from (1)H nuclei in these spin systems are assigned by paramagnetic-optimized 2D and 3D (1)H[(13)C] experiments. For oxidized human ferredoxin, this strategy led to assignments (to amino acid and atom type) for 88% of the carbons in the [2Fe-2S] cluster-binding loops (residues 43-58 and 89-94). These included complete carbon spin-system assignments for eight of the 22 residues and partial assignments for each of the others. Sequence-specific assignments were determined for the backbone (15)N signals from nine of the 22 residues and ambiguous assignments for five of the others.     

Microfluidic routing of aqueous and organic flows at high pressures: fabrication and characterization of integrated polymer microvalve elements

This paper presents the first systematic engineering study of the impact of chemical formulation and surface functionalization on the performace of free-standing microfluidic polymer elements used for high-pressure fluid control in glass microsystems. System design, chemical wet-etch processes, and laser-induced polymerization techniques are described, and parametric studies illustrate the effects of polymer formulation, glass surface modification, and geometric constraints on system performance parameters. In particular, this study shows that highly crosslinked and fluorinated polymers can overcome deficiencies in previously-reported microvalve architectures, particularly limited solvent compatibility. Substrate surface modification is shown effective in reducing the friction of the polymer-glass interface and thereby facilitating valve actuation. A microchip one-way valve constructed using this architecture shows a 2 x 10(8) ratio of forward and backward flow rates at 7 MPa. This valve architecture is integrated on chip with minimal dead volumes (70 pl), and should be applicable to systems (including chromatography and chemical synthesis devices) requiring high pressures and solvents of varying polarity.     

A parallel implementation of the COLUMBUS multireference configuration interaction program

Summary In this work a parallel implementation of the COLUMBUS MRSDCI program system is presented. A coarse grain parallelization approach using message passing via the portable toolkit TCGMSG is used. The program is very well portable and runs on shared memory machines like the Cray Y-MP, Alliant FX/2800 or Convex C2 and on distributed memory machines like the iPSC/860. Further implementations on a network of workstations and on the Intel Touchstone Delta are in progress. Overall, results are quite satisfactory considering the complexity and the prodigious requirements, especially the I/O bandwidth, of MRCI programs in general. For our largest test case we obtain a speedup of a factor of 7.2 on an eight processor Cray Y-MP for that section of the program (hamiltonian matrix times trial vector product) which has been parallelized. The speedup for one complete diagonalization iteration amounts to 5.9. An absolute speed close to 1 GFLOPS is found. Results for the iPSC/860 show that ordinary disk I/O is certainly not sufficient in order to guarantee a satisfactory performance. As a solution for that problem, the implementation of a fully asynchronous distributed-memory model for certain data files is in preparation. On leave from: Bereich Informatik, Universität Leipzig, Augustusplatz 10/11, O-7010 Leipzig, Germany     

General synthetic route to benz[g]isoquinolines (2-azaanthracenes)

Benzylic zinc reagents add with high regioselectivity to 1-(phenoxycarbonyl) salts derived from pyridine-3-carboxaldehyde ( 1a ) or 3-acetylpyridine ( 1b ) to yield 1-(phenoxylcarbonyl)-4-benzyl-1,4-dihydropyridine-3-carboxaldehydes 5a, 5c or ketones 5b, 5d . Aromatizations of these dihydro analogues with sulfur led to the corresponding aldehydes 6a, 6c or ketones 6b, 6d . An alternate synthesis to the aldehydic precursors involved additions of benzylic zinc reagents to 1-(phenoxycarbonyl) salts formed from methyl nicotinates which led to the corresponding methyl 1-(phenoxycarbonyl)-4-benzyl-1,4-dihydronicotinates 7a, 7b . Aromatizations of 7a, 7b led to the corresponding pyridine esters 8a, 8b which on reduction with lithium aluminum hydride yielded the corresponding carbinols 9a, 9b . Oxidation of 9a, 9b by manganese dioxide afforded aldehydes 6e, 6f . Aldehydes 6a-f were readily converted into the benz[g]isoquinolines 10a-f on heating in polyphosphoric acid.     

Localized moments in the superconducting Li1+xTi2−xO4 spinel system

Electron spin resonance (ESR) and magnetic-susceptibility measurements on the Li_1+xTi_2?xO₄ spinel system ($\text{0 ≤ x ≤}\text{1}\text{3}$) indicate the presence of two types of localized moments in this material. In both cases, an unpaired electron is trapped as a Ti³⁺ ion in a crystal field that is predominantly octahedral, but with a strong tetragonal component. This type of crystal field cannot arise in the stoichiometric spinel. We propose two types of defect in the title spinel system: an oxygen vacancy and a hydroxyl ion. Unpaired electrons are trapped as Ti³⁺ ions adjacent to these defects, and it is argued that the strong tetragonal field is associated with the formation of a static (TiO)⁺ ion by a displacement of the titanium ion from the defect. Spin relaxation occurs via a thermal ionization of the trapped electron that appears to be associated with a static-dynamic transition in the titanium-ion displacement.