Neutron powder diffraction studies of sulfuric acid hydrates. I. The structure of sulfuric acid hemitriskaidekahydrate D2SO4.6(1/2)D2O

We report the first neutron diffraction data from D2SO4.6(1/2)D2O. The crystal is monoclinic, space group Cm, with four formula units per unit cell. At 4.2 K the unit cell dimensions are a = 6.253 26(4) A, b = 26.813 62(10) A, c = 5.908 45(2) A, and beta = 112.1939(3) degrees [V = 917.286(6) A3 and rho(deuterated) = 1664.14(2) kg m(-3)]. The deuteron positions refined from the neutron data are in agreement with those established by single crystal x-ray analysis [D. Mootz and A. Merschenz-Quack, Z. Naturforsch. B 42, 1231 (1987)], but not with those found from the ab initio simulation of Hirsch and Ojamae [Acta Crystallogr, Sect. B: Struct. Sci. 60, 179 (2004)]. The crystal consists of SO4(2-), D3O+ ions, and D2O molecules hydrogen bonded to form a layered structure in which sheets of "icelike" D3O+ and D2O are separated by layers of opposing SO4(2-) tetrahedra.     

The preparation of 3,3a,4,5‐tetrahydronaphth[1,2‐c]isoxazoles from dilithiated 1‐tetralone oxime and select aromatic aldehydes

Dilithiated 1‐tetralone oxime was condensed with several electron enriched aromatic aldehydes, such as 4‐methoxybenzaldehyde or lithiated 4‐hydroxybenzaldehyde, followed by acid cyclization to new tetrahydronaphthisoxazoles, 3,3a,4,5‐tetrahydronaphth[1,2‐c]isoxazoles, with a trans geometry of the C₃‐H and C_3a‐H protons that was confirmed by X‐ray single crystal analysis.     

Index sets of computable structures

The index set of a computable structure is the set of indices for computable copies of . We determine complexity of the index sets of various mathematically interesting structures including different finite structures, ℚ-vector spaces, Archimedean real-closed ordered fields, reduced Abelian p-groups of length less than ω², and models of the original Ehrenfeucht theory. The index sets for these structures all turn out to be m-complete Π _n ⁰ , d-Σ _n ⁰ , or Σ _n ⁰ , for various n. In each case the calculation involves finding an optimal sentence (i.e., one of simplest form) that describes the structure. The form of the sentence (computable Π_n, d-Σ_n, or Σ_n) yields a bound on the complexity of the index set. Whenever we show m-completeness of the index set, we know that the sentence is optimal. For some structures, the first sentence that comes to mind is not optimal, and another sentence of simpler form is shown to serve the purpose. For some of the groups, this involves Ramsey’s theory. Supported by the NSF grants DMS-0139626 and DMS-0353748. Supported by the NSF grant DMS-0502499 and by the Columbian Research Fellowship of the George Washington University. Supported by the NSF grant DMS-0353748. __________ Translated from Algebra i Logika, Vol. 45, No. 5, pp. 538–574, September–October, 2006.     

Zirconocene catalysis in the synthesis of 1,4-dimagnesium reagents: using competitive inhibition to suppress β-hydrogen abstraction

Substituted 1,4-dimagnesium reagents were synthesized by the zirconocene-catalyzed reaction of alkenes with ethylmagnesium reagents in the presence of a methylmagnesium containing additive. Improved selectivity for formation of dimagnesium reagents over monomagnesium reagents was obtained in the presence of the methylmagnesium containing additive. The ratio of mono- to dimagnesiated products was extrapolated from the ratio of alkene to diene in the products formed when the reaction was quenched with allyl bromide. The extent of the increase in the alkene/diene ratio was dependent on the type of organomagnesium halide, with greatest increases (59%) for the alkylmagnesium chlorides. A mechanism for improved selectivity by suppression of β-hydrogen abstraction in the catalytic cycle is presented. Quenching the 1,4-dimagnesium reagents with allyl bromide yielded decadienes.     

Chains and antichains in partial orderings

We study the complexity of infinite chains and antichains in computable partial orderings. We show that there is a computable partial ordering which has an infinite chain but none that is or , and also obtain the analogous result for antichains. On the other hand, we show that every computable partial ordering which has an infinite chain must have an infinite chain that is the difference of two sets. Our main result is that there is a computably axiomatizable theory K of partial orderings such that K has a computable model with arbitrarily long finite chains but no computable model with an infinite chain. We also prove the corresponding result for antichains. Finally, we prove that if a computable partial ordering has the feature that for every , there is an infinite chain or antichain that is relative to , then we have uniform dichotomy: either for all copies of , there is an infinite chain that is relative to , or for all copies of , there is an infinite antichain that is relative to .     

Synthesis and Crystal Structure Determination of Methyl 2-acetyl-5′-phenyl-2H-spiro[benzo[d]isothiazole-3,3′-pyrazole]-1,1-dioxide-2′(4′H)-carboxylate and Methyl 2-acetyl-5′-(2-thienyl)-2H-spiro[benzo[d]isothiazole-3,3′-pyrazole]-1,1-dioxide-2′(4′H)-carboxylate

Abstract  

Dilithiated C(α), N-carbomethoxyhydrazones were condensed with lithiated methyl 2-(aminosulfonyl)benzoate to afford intermediates that were isolated and not characterized but cyclized with acetic anhydride, which also resulted in N-acetylation. The X-ray crystal structure determinations of methyl 2-acetyl-5′-phenyl-2H-spiro[benzo[d]isothiazole-3,3′-pyrazole]-1,1-dioxide-2′(4′H)-carboxylate and methyl 2-acetyl-5′-(2-thienyl)-2H-spiro[benzo[d]isothiazole-3,3′-pyrazole]-1,1-dioxide-2′(4′H)-carboxylate products were a follow up for absorption spectra, and they confirmed their structures. Mechanistic intermediates to describe the reaction may include C-acylated intermediates that cyclize to spiro(N-benzoisothiazole dioxide-pyrazole) instead of N-carbomethoxypyrazole-ortho-benzenesulfonamides. Crystals of C₁₉H₁₇N₃O₅S 7 are monoclinic, P2₁/c, a = 11.899(2) ?, b = 17.562(4) ?, c = 9.484(2) ?, β = 111.03(3)°, Z = 4, V = 1849.9(6) ?³, R ₁ = 0.0857 and wR ₂ = 0.2216 for reflections with I > 2σ(I); crystals of C₁₇H₁₅N₃O₅S₂ 8 are orthorhombic, Pbca, a = 16.045(3) ?, b = 10.746(2) ?, c = 20.389(4) ?, Z = 8, V = 3516(1) ?³, R ₁ = 0.0841 and wR ₂ = 0.2179 for all reflections with I > 2σ(I).     

Absolute frequency measurement of iodine lines with a femtosecond optical synthesizer

We have used a single laser femtosecond optical frequency synthesizer together with a widely tunable Nd:YAG laser to measure the absolute frequency of several absorption lines in molecular iodine around 532 nm. The use of two different repetition frequencies allows us to determine the number of modes used for the frequency measurement unambiguously. The lines also provide data for the determination of improved ro-vibrational constants of the iodine molecule. Received: 3 July 2001 / Published online: 19 September 2001     

Activation of methane in microwave plasmas at high pressure

Methane is converted to C2 products in a microwave plasma under pressure up to 400 torr at maximum plasma power of 100 W. Steam is introduced with methane into the plasma zone in order to suppress coke formation. Major products are C2 hydrocarbons. Small amounts of benzene are also formed. Very small amounts of some unusual highly unsaturated hydrocarbons are also formed. Oxygenated products are CO and CO₂. The conversion and yields are related to experimental variables by an empirical second order linear model. The conversion of methane ranges from 10 to 60%. The yield of C2 products ranges from 5 to 68%. The major C2 product is acetylene.     

Implications for space radiation environment models from CREAM & CREDO measurements over half a solar cycle.

Flight data obtained between 1990 and 1997 from the Cosmic Radiation Environment Monitors CREAM & CREDO carried on UoSAT-3, Space Shuttle, STRV-1a (Space Technology Research Vehicle) and APEX (Advanced Photovoltaic and Electronics Experiment Spacecraft) provide coverage over half a solar cycle. The modulation of cosmic rays and evolution of the South Atlantic Anomaly are observed, the former comprising a factor of three increase at high latitudes and the latter a general increase accompanied by a north-westward drift. Comparison of particle fluxes and linear energy transfer (LET) spectra is made with improved environment & radiation transport calculations which account for shield distributions and secondary particles. While there is an encouraging convergence between predictions and observations, significant improvements are still required, particularly in the treatment of locally produced secondary particles. Solar-particle events during this time period have LET spectra significantly below the October 1989 event which has been proposed as a worst case model.     

Protoglobin-Catalyzed Formation of cis-Trifluoromethyl-Substituted Cyclopropanes by Carbene Transfer

Trifluoromethyl-substituted cyclopropanes (CF₃-CPAs) constitute an important class of compounds for drug discovery. While several methods have been developed for synthesis of trans-CF₃-CPAs, stereoselective production of corresponding cis-diastereomers remains a formidable challenge. We report a biocatalyst for diastereo- and enantio-selective synthesis of cis-CF₃-CPAs with activity on a variety of alkenes. We found that an engineered protoglobin from Aeropyrnum pernix (ApePgb) can catalyze this unusual reaction at preparative scale with low-to-excellent yield (6–55 %) and enantioselectivity (17–99 % ee), depending on the substrate. Computational studies revealed that the steric environment in the active site of the protoglobin forced iron-carbenoid and substrates to adopt a pro-cis near-attack conformation. This work demonstrates the capability of enzyme catalysts to tackle challenging chemistry problems and provides a powerful means to expand the structural diversity of CF₃-CPAs for drug discovery.