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Silver(I)-2,6-bis(ethanolimidazolemethyl)pyridine hydroxide (4a) and silver(I)-2,6-bis(propanolimidazolemethyl)pyridine hydroxide (4b) are water-soluble silver(I)-carbene complexes that were synthesized in high yield by reacting silver(I) oxide with N-substituted pincer ligands 3 (a = 2,6-bis(ethanolimidazoliummethyl)pyridine diiodide, b = 2,6-bis(propanolimidazoliummethylpyridine)pyridine dibromide). The X-ray crystal structure of 4a is a one-dimensional linear polymer, whereas the mass spectroscopy confirms a monomer in the gas phase. A change in the anion of 4a from a hydroxide to a hexafluorophosphate formed a silver(I)-carbene complex 4c that is dimeric in structure and insoluble in water. The bactericidal activities of the water-soluble silver(I)-carbene complexes were found to be improved over that of silver nitrate.

Observations of 85 stars were obtained at wavelengths between 451 and 800 nm with the Mark III Stellar Interferometer on Mount Wilson, near Pasadena, California. Angular diameters were determined by fitting a uniform-disk model to the visibility amplitude versus projected baseline length. Half the angular diameters determined at 800 nm have formal errors smaller than 1%. Limb-darkened angular diameters, effective temperatures, and surface brightnesses were determined for these stars, and relationships between these parameters are presented. Scatter in these relationships is larger than would be expected from the measurement uncertainties. We argue that this scatter is not due to an underestimate of the angular diameter errors; whether it is due to photometric errors or is intrinsic to the relationship is unresolved. The agreement with other observations of the same stars at the same wavelengths is good; the width of the difference distribution is comparable to that estimated from the error bars, but the wings of the distribution are larger than Gaussian. Comparison with infrared measurements is more problematic; in disagreement with models, cooler stars appear systematically smaller in the near-infrared than expected, warmer stars larger.

The reaction of [Ir(COE)Cl] 2 and Et 2 PhP in a 1:6 ratio affords the iridium(I) complex (Et 2 PhP) 3 (Cl)Ir ( 4 ) as orange crystals in good yield (80%). The 1 H- and 31 P-NMR data are consistent with the structure of 4 . The structure of 4 was determined by X-ray crystallography. The sterically hindered iridium(III)-silyl complex 5 is prepared in 63% yield by the reaction of 4 and H 3 Si(C 6 H 3 Mes 2 -2,6) ( Eq. 5 ). Complex 5 is isolated as colorless crystals that are thermally stable up to the temperature at which they melt (162–163 °C). The 31 P- and 29 Si-NMR and IR spectroscopic data and the X-ray crystallographic analysis are consistent with the structure of 5 .

The reduction of terphenylgermanium(II) or terphenyltin(II) chlorides with alkali metals was investigated. Treatment of Ar'GeCl or ArGeCl (Ar' = C(6)H(3)-2,6-Dipp(2), Dipp = C(6)H(3)-2,6-Pr(i)(2); Ar = C(6)H(3)-2,6-Trip(2), Trip = C(6)H(2)-2,4,6-Pr(i)(3)) with lithium, sodium, or potassium afforded the neutral alkyne analogues Ar'GeGeAr', 1, ArGeGeAr, 2, the singly reduced radical species NaArGeGeAr, 3, or KAr'GeGeAr', 4, or the doubly reduced compounds Li(2)Ar'GeGeAr', 5, Na(2)ArGeGeAr, 6, or K(2)ArGeGeAr, 7. Similarly, reduction of Ar'SnCl or ArSnCl afforded the neutral Ar'SnSnAr', 8, or ArSnSnAr, 9, the radical anions [(THF)(3)Na[rSnSnAr]], 10, [K(THF)(6)][Ar'SnSnAr'], 11, [K(THF)(6)][ArSnSnAr], 12, [K(18-crown-6)(THF)(2)] [ArSnSnAr], 13, or the doubly reduced Na(2)ArSnSnAr, 14, K(2)Ar'SnSnAr', 15, or K(2)ArSnSnAr, 16. The compounds were characterized by UV-vis, (1)H and (13)C NMR or EPR spectroscopy. The X-ray crystal structures of all compounds were determined except those of 2 and 9. The neutral 1 and 8 displayed planar, trans-bent CMMC (M = Ge and Sn) cores with M-M-C angles of 128.67(8) and 125.24(7) degrees, respectively. The M-M bond lengths, 2.2850(6) and 2.6675(4)A, indicated considerable multiple character and a bond order approaching two. Single and double reduction of the neutral species resulted in the narrowing of the M-M-C angles by ca. 12-32 degrees and changes in the Ge-Ge and Sn-Sn bond lengths. One-electron reduction afforded a slight (ca. 0.03-0.05A) lengthening of the Ge-Ge bonds in the case of germanium species 3 and 4 and a greater lengthening (ca. 0.13-0.15A) for the Sn-Sn bonds in the tin compounds 10-13. The addition of another electron yielded salts of the formal dianions [Ar'MMAr'](2)(-) and [ArMMAr](2)(-) which are isoelectronic to the corresponding doubly bonded, neutral arsenic and antimony derivatives. All the dianion salts were obtained as contact ion triples with two alkali metal cations complexed between aryl rings. The Ge-Ge bonds in the dianions of 5-7 were longer, whereas the Sn-Sn distances in the dianions in 14, 15, and 16 were shorter than those in the monoanions. Unusually, the Li(2)Ar'GeGeAr' salt, 5, displayed a longer Ge-Ge bond (by ca. 0.06A) than those of its Na(+) or K(+) analogue salts which was attributed to the greater polarizing power of Li(+). It was concluded that the M-M bond lengths in 3-7 and 10-16 are dependent on several factors that include M-M-C angle, Coulombic repulsion, alkali metal cation size, and the character of the molecular energy levels. The M-M bonding in the neutral compounds was accounted for in terms of a second-order Jahn-Teller mixing of sigma- and a pi-orbital which afforded bond orders near two for the neutral compounds, 1, 2, 8, and 9. Calculations on MeMMMe (M = Ge or Sn) model species showed that the LUMO corresponded to an orbital that had n(+) lone pair character. The slight Ge-Ge bond length increase upon one-electron reduction is consistent with these results, and the further bond lengthening upon double reduction is consistent with increased Coulombic repulsion. The greater Sn-Sn bond length increase seen for one-electron reduction of the tin species is probably due to the increased p-character of orbitals comprising the Sn-Sn sigma-bond when the Sn-Sn-C angle is decreased by ca. 30 degrees. Upon further reduction, the slight decrease in the Sn-Sn bond is probably a result of the reduced importance of Coulombic repulsion due to the larger size of tin and a widening of the Sn-Sn-C angles which may shorten the Sn-Sn sigma-bond.

The synthesis of the tetranuclear Ag44+ cluster stabilized by a N-heterocyclic carbene macrocycle (3)[PF6]4. Decomposition of the tetranuclear cluster (3)[PF6]4 in light forms the dimeric species (2)[PF6]2. In situ synthesis of the dimeric silver N-hetrocyclic carbene complex (2)[Br]2 in water.

The pincer ligands 2,6-bis(n-butylimidazoliummethyl)pyridine dihalide (halide = Cl (3a), Br (3b) are easily obtained by the reaction of n-butylimidazole with 2,6-bis(chloromethyl)pyridine and 2,6-bis(bromomethyl)pyridine in a 2:1 molar ratio, respectively. Ligand 3a readily reacts with Ag2O in CH2Cl2 to yield the bis(silver carbene) complex [2,6-bis(n-butylcarbenemethyl)pyridine][AgCl]2 (4) in 70% yield. The reaction of 4 with (PhCn)2PDCl2 in CH2Cl2 yields the palladium carbene complex Pd[2,6-bis(N-n-butylcarbenemethyl)pyridine]Cl2 (5). The reaction of equimolar amounts of 4 and [Rh(COD)Cl]2 (COD = cyclooctadiene) in CH2Cl2 gave the dirhodium(I) complex [2,6-bis(N-n-butylcarbenemethyl)pyridine][Rh(COD)Cl]2 (6). All compounds were characterized by 1H and 13C NMR spectroscopy. The solid-state structures of 4 and 6 were determined by X-ray crystallography.

The oxidative addition reaction of (Et 3 P) 3 IrCl with H 3 Si(C 6 H 3 -Mes 2 -2,6) (Mes = 2,4,6-trimethylphenyl) affords the sterically hindered iridium(III)-silyl complex (Et3P) 3 (H) 2 Ir[Si(H)Cl(C 6 H 3 -Mes 2 -2,6)] (1) via a H/Cl exchange reaction at silicon. Complex 1 is characterized by 1 H-, 1 3 C-, 3 1 P-, and 2 9 Si-NMR and IR spectroscopy. The solid state structure of 1 is determined by X-ray crystallography. Complex 1 undergoes a halide abstraction reaction with LiB(C 6 F 5 ) 4 (OEt 2 ) to afford the cationic iridium(III)-silylene complex [(Et 3 P) 3 (H) 2 Ir=Si(H)(C 6 H 3 -Mes 2 -2,6)][B(C 6 F 5 ) 4 ] (4). Complex 4 is characterized by 1 H-, 1 3 C-, 3 1 P-, and 2 9 Si-NMR spectroscopy.

The rings [Fe2(μ-SiRR′)2(CO)8] (R=Me, R′=Me or Cl) react with HMPA to give the base-stabilized silylenes [Fe(SiRR′(HMPA))(CO)4]. The reactions of [Fe2(μ-SiMe2)2(CO)8] with DABCO, THF and PMe3 have also been examined. The crystal structures of both [Fe2(μ-SiMe2)2(CO)8] and [Fe2(μ-SiMeCl)2(CO)8] show planar rings, CH⋯OC short contacts and relatively short but non-bonding SiSi distances. The possibility of through-ring MM or EE bonding in complexes of the type [M2(μ-ER2)2(CO)8] (M is a transition metal and E is a Group 14 element) is analyzed with the help of theoretical calculations based on density functional theory. For compounds with 20 ring electrons (or a framework electron count, FEC, of eight), regular M2E2 rings are expected, with no short through-ring distances. The framework electron counting rules, geometry optimization of several model complexes and a structural database analysis consistently indicate that through-ring bonding exists only when the FEC is less than eight. In that case, the isomer with a short metal–metal bond is found to be significantly more stable than that with a short EE distance.

The synthesis and characterization of several m-terphenyl stabilized germylyne transition metal complexes and related metallogermylenes are reported. The reaction of Na[M(η5-C5H5)(CO)3] (M = Cr, or...

The synthesis and characterization of platinum−silicon four-membered rings of two different structural types is reported. The reaction of Pt(PR3)3 (R = Et or Pr) and Si(Hex)H3 (Hex = n-hexyl) gives [Pt(PR3)2SiH(Hex)]2 (R = Et, 1a; R = Pr, 1b) and [(R3P)Pt(μ-Si(μ-H)(Hex)(Pt(PR3)2)H)]2 (R = Et, 2a; R = Pr, 2b). The ratio of 1a to 2a is highly dependent on the reaction conditions, whereas 1b is always the major product in the PPr3 substituted case. Evidence that suggests that 1a and 1b are precursors to 2a and 2b, respectively, is presented. X-ray crystallography of the rings 1b and 2a shows they contain short Si−Si and Pt−Pt contacts, respectively. The 29Si NMR chemical shifts for the two types of rings differ by over 250 ppm. A 2D-NMR study of these rings has provided detailed coupling information, including information on the agostic Pt−H−Si hydride in 2a.

The synthesis and characterization of the primary terphenyl silanes and chlorosilanes 2,6-Mes2C6H3SiCl3 (1a), 2,6-Trip2C6H3SiCl3 (1b), 2,6-Mes2C6H3SiHCl2 (2a), 2,6-Trip2C6H3SiHCl2(2b) 2,6-Mes2C6H3SiH3(3a), 2,6-Trip2C6H3SiH3 (3b) and 2,6-Mes2C6H3SiCl2SiCl3(4), (where Mes = 2,4,6-Me3C6H2 and Trip = 2,4,6-i-Pr3C6H2) are described. Compounds (1a, 1b), (2a, 2b) and (4) were prepared by the reaction between SiCl4, SiHCl3 or Si2Cl6, respectively, and the appropriate aryl lithium reagent (2,6-Mes2C6H3Li or 2,6-Trip2C6H3Li·OEt2). Compounds (3a) and (3b) were prepared by reduction of (1a) and (1b) with LiAlH4, respectively. The derivatives (1a) - (3a) were characterized by 1H, 13C, 29Si NMR and IR spectroscopy. The structures of (1a), (1b) and (4) have been determined. Crystal data with CuKα (λ = 1.54178 A) at 130 K: (1a), a = 9.074(2) A, b = 16.681(3) A, c = 15.842(3) A, β = 105.20(3)°, V = 2313.9(8) A3, Z = 4, space group P21/c, 2772 (I > 2σ(I)) data, R = 0.042; (1b), a = 12.197(2) A, b = 25.602(5) A, c = 11.098(...

We describe the Navy Prototype Optical Interferometer (NPOI), a joint project of the Naval Research Laboratory (NRL) and the US Naval Observatory (USNO) in cooperation with Lowell Observatory. The NPOI has recently begun operations at the Lowell Observatory site near Flagstaff, Arizona, obtaining its first images, of a binary star, in 1996 May and June and its first limb-darkening observations during 1996 November to 1997 February. This paper gives an overview of the NPOI, including the characteristics of optical interferometry that affect its design. The NPOI includes subarrays for imaging and for astrometry. The imaging subarray consists of six moveable 50 cm siderostats feeding 12 cm apertures, with baseline lengths from 2.0 to 437 m. The astrometric subarray consists of four fixed 50 cm siderostats feeding 12 cm apertures (35 cm apertures to be installed in 1998), with baseline lengths from 19 m to 38 m. The shared back end covers 450-850 nm in 32 channels. The NPOI features vacuum feed and delay systems, active group-delay fringe tracking, and a high degree of automation. The astrometric subarray also includes an extensive site laser metrology system to measure the motions of the siderostats with respect to one another and to the bedrock. For imaging stellar surfaces, arrays with equal spacing between elements are superior to arrays that have been laid out to optimize (u, v) coverage and that therefore have unequal spacing. The imaging subarray of the NPOI provides a number of equally spaced configurations with linear scales at ratios of ≈ 1.64. Unequally spaced configurations are available for a variety of other imaging programs. Coherence across either type of imaging configuration is maintained by "phase bootstrapping": the phases on the longest baselines, on which fringes may be too weak to track, are stabilized by tracking fringes on the shortest baselines. In principle, the four elements of the astrometric subarray provide enough independent baselines to solve for stellar positions and the array geometry simultaneously while observing each of 11 stars only once. The anticipated magnitude limit is 7 mag or better with 12 cm apertures and average seeing; with 35 cm apertures, we expect the limit to be one or more magnitudes fainter. The anticipated wide-angle astrometric precision of the NPOI is ≈ 2 mas. The best angular resolution of the imaging subarray will be ≈ 200 μas. Our experience with the Mark III interferometer suggests that we will be able to measure stellar diameters as small as 200 μas with 1% precision and binary star separations as small as ρ ≈ 65 μas (for Δm ≈ 0 mag) or ρ ≈ 200 μas (for Δm ≈ 3-4 mag). With its large bandwidth and phase bootstrapping, the imaging subarray should be able to make images 10 resolution elements across the disks of nearby late-type stars.

The effect of phosphotungstic acid (PTA) and iodine solution (IKI) staining was investigated as a method of enhancing contrast in the X-ray computed tomography of porcine anterior cruciate ligaments (ACL) and patellar tendons (PT). We show that PTS enhanced surface contrast, but was ineffective at penetrating samples, whereas IKI penetrated more effectively and enhanced contrast after 70 hours of staining. Contrast enhancement was compared when using laboratory and synchrotron based X-ray sources. Using the laboratory source, PT fascicles were tracked and their alignment was measured. Individual ACL fascicles could not be identified, but identifiable features were evident that were tracked. Higher resolution scans of fascicle bundles from the PT and ACL were obtained using synchrotron imaging techniques. These scans exhibited greater contrast between the fascicles and matrix in the PT sample, facilitating the identification of the fascicle edges; however, it was still not possible to detect individual fascicles in the ACL.

The reaction of 2 equiv of LiC6H3-2,6-Mes2 (Mes = 2,4,6-Me3C6H2−) with GeCl2·dioxane, SnCl2, or PbCl2 in ether solution has resulted in the isolation of rare examples of monomeric, σ-bonded, diaryl derivatives M{C6H3-2,6-Mes2}2 (M = Ge (1), Sn (2), or Pb (3)). The compounds 1−3 are thermally stable, purple, crystalline solids with V-shaped geometries and remarkably wide (ca. 114.5°) interligand bond angles. The monoaryl metal chloride derivatives [M(Cl){C6H3-2,6-Mes2}]2 (M = Ge (4) or Sn (5)) were isolated by treatment of the appropriate dichlorides with either 1 equiv of LiC6H3-2,6-Mes2 or 1 equiv of the diaryls 1 or 2. The orange germanium compound 4 has a dimeric structure in which the monomers are linked by a relatively weak, 2.443(2) A, Ge−Ge interaction. In sharp contrast, its yellow tin analogue 5 has a dimeric structure in which three-coordinate tin centers are associated by asymmetrically bridging chlorides. The compounds 1−3 constitute a unique, structurally characterized diaryl series for Ge, S...

Reaction of equimolar amounts of Fe2(CO)9 and SiPhH3 yields the triply bridged diiron complex [(OC)3Fe]2(μ-SiPhH)2(μ-CO) (1a). The analogous reaction of Fe2(CO)9 and SiPh2H2 provides the singly bridged diiron complex [(OC)4Fe−Fe(CO)3(SiHPh2)](μ-η2-H-SiPh2) (4). When 4 is photolyzed for 12 h, the complex [(OC)3Fe]2(μ-η2-H-SiPh2)2 (5) is obtained. Both 4 and 5 exhibit agostic Fe−H−Si interactions. Reaction of Fe2(CO)9 or Fe(CO)5 with the disiloxane Ph2HSi−O−SiHPh2 gives the singly bridged diiron complex [(OC)4Fe]2(μ-η2-Ph2Si−O−SiPh2) (6). All compounds have been characterized by 1H, 13C, and 29Si NMR and IR spectroscopy. Compounds 4−6 have been characterized by X-ray crystallography.

The reaction of Cp(PMe3)2RuCH2SiMe3 (1) with (η1-Cp*)SiHCl2 in refluxing toluene produces silyl Cp(PMe3)2RuSiCl2(η1-Cp*) (2) in good yields; Cp(PMe3)2RuSiCl3 (3) is a minor product of this reaction. The Cp* group in 2 is σ-bound to the silicon and is “static” on the NMR time scale at room temperature. The η1-Cp* configuration in 2 was confirmed by a single-crystal X-ray diffraction study. In the structure of 2, a “three-legged piano stool” geometry is observed around ruthenium whereas the silicon adopts a distorted tetrahedral geometry. The η1-Cp* group on silicon is in an anti relationship with one of the phosphines on ruthenium (dihedral angle = 179.5(3)°). Complex 2 exhibits reactions involving chloride substitution, Si−Cp* bond cleavage, and Ru−Si bond cleavage. The reaction of 2 with LiAlH4 produces Cp(PMe3)2RuSiH2(η1-Cp*) (6) in which the η1-Cp* group is more mobile compared to 2. The Si−Cp* bond of 2 is readily cleaved by acid (i.e HCl(g), pyridinium chloride) to give 3 and pentamethylcyclopentadie...

The one-step conversion of (Me3SiCH2)SiCl3 to polycarbosilanes by reaction with three molar equivalents of the alkali metals lithium, sodium, or sodium potassium alloy is reported. The expected network polysilanes were not isolated. Under similar conditions (Me3SiCH2)SiHCl2 gives polysilanes. The low molecular weight polymers have been characterized by elemental analyses, gpc, and FT-IR, NMR, UV-vis, and mass spectroscopies.

The synthesis and spectroscopic characterization of the imidazolium-linked cyclophanes [ 1 ][PF6]2 and [ 2 ][PF6]2 are described. Cyclophane [ 1 ][PF6]2 contains two imidazolium rings bridged by a 2,6-bis(methyl)-pyridine unit and a 2,5-bis(methyl)-pyrrole unit. Cyclophane [ 2 ][PF6]2 contains two imidazolium rings bridged by two 2,5-bis(methyl)-pyrrole units. Cyclophanes [ 1 ][PF6]2 and [ 2 ][PF6]2 have been structurally characterized by X-ray crystallography.

The synthesis and structural characterization of a dicationic imidazolium-linked cyclophane and a dimeric N-heterocyclic carbene complex are described.

The synthesis and structural characterization of 2,6-Mes2C6H3(H)Si(OH)2 (1), a silanediol with a Si−H functionality, are described.

The synthesis and structural characterization of cis-(2,6-Mes2C6H3(H)2Si)Pt(H)(PPr3)2 (1), a mono(silyl)platinum(II) complex, are described.

Interferometry at optical wavelengths is very similar to radio interferometry, once the fundamental differences in detectors are accounted for. The Mount Wilson Mark III optical interferometer has been used for optical interferometry of stars and stellar systems. Success with the Mark III has lead to the current program at the Naval Research Laboratory to build the Big Optical Array (BOA), which will be an imaging interferometer. Imaging simulations show that BOA will be able to produce images of complex stellar systems, with a resolution as fine as 0.2 milliarcseconds.

The proposed Fast All Sky Telescope (FAST) is an interferometer which is intended to monitor the northern four-fifths of the celestial sphere every two days at 8.1 GHz and daily at 2.7 GHz. The design goal is to have a rms sensitivity of 10 mJy/beam at both frequencies. The array is planned to comprise 20 3-meter diameter antennas with a maximum baseline of 0.7 km. FAST will provide a valuable database that may be used to study time variability in a sensitivity limited sample of radio sources. This will significantly impact on the understanding of active Galactic and extragalactic radio sources, as well as on the understanding of radio wave scattering in the interstellar medium.

The binding of H2O to MeAl(OAr)2 (1: Ar = 2,6-di-tert-butyl-4-methylphenyl) in THF-d8 at −40 °C provides aquo complex 2, the structure of which was determined by X-ray crystallography. Complex 2 i...

Newcomb, a scientific interferometry mission at low costRobert D. Reasenberg, Robert W. Babcock, and James D. PhillipsSmithsonian Astrophysical ObservatoryHarvard-Smithsonian Center for Astrophysics60 Garden Street, Cambridge, MA 02138Kenneth J. JohnstonUnited States Naval Observatory34th Street & Massachusetts Avenue, N.W., Washington, DC 20392Richard S. SimonNational Radio Astronomy Observatory520 Edgemont Road, Charlottesville, VA 22903-2475ABSTRACTNewcomb is a design concept for an astrometric optical interferometer with nominal single-measurementaccuracy of 100 microseconds of arc (pas). In a 30 month mission life, it will make scientifically interestingmeasurements of 0-star, RR Lyrae, and Cepheid distances, probe the dark matter in our Galaxy via parallaxmeasurements of K giants in the disk, establish a reference grid with internal consistency better than 50 pas, and laygroundwork for the larger optical interferometers that are expected to produce a profusion of scientific results during thenext century. With an extended mission life, Newcomb could do a useful search for other planetary systems.The instrument is a highly simplified variant of POINTS. It has three (or four) interferometers stacked oneabove the other. All three (four) optical axes lie on a great circle, which is also the nominal direction of asirometricsensitivity. The second and third axes are separated from the first by fixed "observation angles" of 40.91 and 60.51 deg.The fourth axis would be at either 70.77 or 78.60 deg from the first. Each interferometer detects a dispersed fringe(channelled spectrum), which falls on a short CCD detector array. The optical passband is from 0.9 to 0.3 microns. Wewill move the beamsplitter assembly along the baseline direction up to mm to compensate the change in the opticalpath difference associated with a target away from the nominal interferometer axis. This is a recent design change thatadds complexity (moving parts in the interferometer) to achieve adequate field of view; it also enhances astrometricaccuracy. With a nominal baseline length of 30 cm, the deviation limit is reached by a star arc mm from the opticalaxis. The instrument will be constructed of stable materials such as ULE glass, and have minimal internal moving partsand simple laser metrology.A reference frame can be constructed using stars located in a regular pattern on the sky. We start with the 60points that are vertices of the regular truncated icosaheciron. The instrument axis separations are chosen as separationangles of the figure such that for each vertex there are four other vertices at the chosen angle. (There is no angleoffering higher multiplicity.) To the original set of 60 points, we add two additional such sets by rotating the figuredeg. With such a grid of 180 stars, we have shown POINTS-like grid lock-up even with large Sun-exclusionangles (of up to 75 deg). In the covariance studies, we assumed nine quarterly observation series and five biasparameters per series per observation angle; we estimated position, proper motion, and parallax for each star. In anextension of the study, we eliminated stars at random from the set and found that there was stable behavior with as fewas half the stars in the grid. Further, with 120 stars in the grid, the statistical uncertainty of star position increased byonly 3 1% due to degeneracy (i.e., with constant number of observations.)As with POINTS, additional stars can be observed with respect to the grid. With three interferometers and a fullgrid and without exceeding the deviation limit, the region accessible for observing with respect to at least two grid stars isover 85% of the sky outside of the Sun exclusion zone. This observable region can be enlarged by either including moresets of 60 points in the reference star grid, or adding the fourth interferometer, or increasing the allowed deviation.

Newcomb is a design concept for an astrometric optical interferometer with nominal single- measurement accuracy of 100 microseconds of arc ((mu) as). In a three-year mission life, it will make scientifically interesting measurements of O-star, RR Lyrae, and Cepheid distances, establish a reference grid with internal consistency better than 100 (mu) as, and lay groundwork for the larger optical interferometers that are expected to produce a profusion of scientific results during the next century. With an extended mission life, Newcomb could do a useful search for other planetary systems. The instrument is a highly simplified variant of POINTS. It has three (or four) interferometers stacked one above the other. All three (four) optical axes lie on a great circle, which is also the nominal direction of astrometric sensitivity. The second and third axes are separated from the first by fixed 'observation angles' of 40.91 and 60.51 deg. The fourth axis would be at either 70.77 or 78.60 deg from the first. Each interferometer detects a dispersed fringe (channeled spectrum), which falls on a CCD detector array nominally 8 k elements long and a small number of elements wide. With a nominal baseline length of 30 cm and optical passband from 0.9 to 0.3 microns, the Nyquist limit is reached by a star +/- 21 arcmin from the optical axis. The instrument will be constructed of stable materials such as ULE glass, and have neither internal moving parts nor laser metrology.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The synthesis of a dicationic imidazolium-linked cyclophane and a dimeric silver-N-heterocyclic carbene complex, that is the first silver complex with a N-heterocyclic carbene ligand involved in a pi-bonding interaction, is reported.

For the first time, four spectroscopic binaries have been directly resolved with the Mark III Stellar Interferometer. Observations in 1988 and 1989 were analyzed, and visual orbits for four binaries have been determined. The semimajor axes for Beta Tri, Alpha Equ, Alpha And and Beta Ari are approximately 0.008 arcsec, 0.012 arcsec, 0.024 arcsec and 0.037 arcsec, respectively. The magnitude differences between two components are 0.5, 0.7, 1.8 and 2.6 mag, respectively. All of the orbital elements for Alpha And and Beta Ari were determined from interferometric data only, and agree well with spectroscopic observations. Predictions of relative position between the two components for these binaries are consistent with the measurements to less than 0.001 arcsec. Combined with data from spectroscopy, masses and distance for the double-lined spectroscopic binary Beta Ari are derived, and the results indicate that both components of Beta Ari agree well with the empirical mass-luminosity relation.

Reliable stellar angular diameters can now be determined using the Mark III Optical Interferometer located on Mt. Wilson, California. The Mark III is a Michelson Interferometer capable of measuring the interferometric fringe visibility for stars using interferometer baselines varying from 3 to 31.5 meters in length. Angular diameters measured with the Mark III Optical Interferometer are presented for 12 stars at wavelengths of 450 and 800 nm.

This paper describes the design and the capabilities of the Naval Research Laboratory Big Optical Array (BOA), an interferometric optical array for high-resolution imaging of stars, stellar systems, and other celestial objects. There are four important differences between the BOA design and the design of Mark III Optical Interferometer on Mount Wilson (California). These include a long passive delay line which will be used in BOA to do most of the delay compensation, so that the fast delay line will have a very short travel; the beam combination in BOA will be done in triplets, to allow measurement of closure phase; the same light will be used for both star and fringe tracking; and the fringe tracker will use several wavelength channels.