Tris[(1‐methylimidazol‐2‐yl)methyl]amine–boric acid (1/1)

Cocrystallization of a poly­imidazole compound with boric acid results in the formation of the title compound, C₁₅H₂₁N₇·B(OH)₃, which has an extensive hydrogen‐bonding network. The O?N(im) separations (im is imidazole) range from 2.6991 (15) to 2.7914 (14) Å, with O—H?N angles ranging from 170.6 (18) to 175 (2)°. In addition, symmetry‐related boric acid mol­ecules form intermolecular hydrogen bonds, with an O?O distance of 2.7582 (14) Å, and symmetry‐related imidazole groups form π–π stacks, with a centroid‐to‐centroid separation of 3.533 Å.     

Réactivité de spirochromènes et de mérocyanines azahétérocycliques vis-à-vis de quelques agents nucléophiles

Under the action of nucleophile reagents such as water, hydrogen sulfide and sodium borohydride, the saturated azaheterocyclic spirochromenes are opened either on the azaheterocyclic side or on the benzopyran side or on both parts simultaneously. The merocyanines are either partially reduced or decomposed by the sodium borohydride.     

Conductance behavior of 1:1 electrolytes in 3-methyl-2-oxazolidone at 25°C

The conductance behavior of twenty-five 1∶1 electrolytes has been investigated in 3-methyl-2-oxazolidone (3Me20x) at 25°C. Conductance data were analyzed by the Lee-Wheaton equation, and all salts studied were found to be only slightly associated. Ionic limiting equivalent conductances were obtained using tris(iso-pentyl)butylammonium tetraphenylborate as a reference electrolyte. The relative values of the ionic limiting molar conductance are generally similar to those for other dipolar aprotic solvents. However, the order $$\lambda _0 (i - Pent_3 BuN^ + ) > \lambda _0 (Pent_4 N^ + )and\lambda _0 (Br^ - ) > \lambda _0 (ClO_4^ - )$$ is opposite to that found previously in the similar solvent 3-tert-bytyl-2-oxazolidone.     

Asymmetric Synthesis of α-Aminophosphonic Acids by Cycloaddition of N-Glycosyl-C-dialkoxyphosphonoylnitrones

Addition of dialkyl phosphites to the nitrone 6 , formed in situ from the oxime 5 and formaldehyde gave the hydroxylamines 7 (86%) and 8 (88%), which reacted with p-benzoquinone in the presence of ethylene via the C-dialkoxyphosphonoylnitrones 9 and 10 to yield the cycloaddition products 11 – 14 (80–85%) with a diastereoselectivity of about 50%. The cycloaddition products were transformed into the monoisopropylidene derivatives 15 – 18 and the diacetates 19 – 22 . Comparison of the NMR spectra and the specific rotations of the compounds 19 – 22 with those of the corresponding α-ammo-acid derivatives 23 – 26 of known configuration indicated preferential formation of the L -isomers. The cycloaddition products were transformed in good yield into the L -α-aminophosphonic acids 29 , 30 , 36 , and 39 .     

In-lens cryo-high resolution scanning electron microscopy: methodologies for molecular imaging of self-assembled organic hydrogels.

The micro- and nanoarchitectures of water-swollen hydrogels were routinely analyzed in three dimensions at very high resolution by two cryopreparation methods that provide stable low-temperature specimens for in-lens high magnification recordings. Gemini surfactants (gS), poly-N-isopropylacrylamides (p-NIP Am), and elastin-mimetic di- (db-E) and triblock (tb-E) copolymer proteins that form hydrogels have been routinely analyzed to the sub-10-nm level in a single day. After they were quench or high pressure frozen, samples in bulk planchets were subsequently chromium coated and observed at low temperature in an in-lens field emission SEM. Pre-equilibrated planchets (4-40 degrees C) that hold 5-10 microl of hydrogel facilitate dynamic morphological studies above and below their transition temperatures. Rapidly frozen samples were fractured under liquid nitrogen, low-temperature metal coated, and observed in-lens to assess the dispersion characteristics of micelles and fragile colloidal assemblies within bulk frozen water. Utilizing the same planchet freezing system, the cryoetch-HRSEM technique removed bulk frozen water from the hydrogel matrix by low-temperature, high-vacuum sublimation. The remaining frozen solid-state sample faithfully represented the hydrogel matrix. Cryo- and cryoetch-HRSEM provided vast vistas of hydrogels at low and intermediate magnifications whereas high magnification recordings and anaglyphs (stereo images) provided a three-dimensional prospective and measurements on a molecular level.     

Enantiospecific total synthesis of (-)-polyoxamic acid using 2,3-aziridino-gamma-lactone methodology

The non-natural enantiomer of polyoxamic acid was synthesized in six steps from 2,3-aziridino-gamma-lactone 7 with an overall yield of 10%. The key step of the strategy is a deprotection-protection sequence on the nitrogen atom of the aziridine ring required for aziridine activation toward nucleophilic ring opening.     

Capillary electrophoretic chiral separations using cyclodextrin additives: III. Peak resolution surfaces for ibuprofen and homatropine as a function of the pH and the concentration of β-cyclodextrin

Our recent extended peak resolution equation of capillary electrophoresis has been combined with the multiple equilibria-based electrophoretic mobility model of chiral separations to describe peak resolution as a function of the composition of the background electrolyte (pH and the β-cyclodextrin concentration) and a function of the operating variables (effective portion of the applied potential, dimensionless electroosmotic flow coefficient). Using the previously determined model parameters, the resolution surfaces were calculated for a Type I chiral separation (ibuprofen), and a Type III chiral separation (homatropine). In Type I separations resolution can be obtained only over a narrow pH range in the vicinity of the pK_a value, and above a minimum value, the concentration of β-cyclodextrin plays a lesser role. In Type III separations, the pH- and β-cyclodextrin concentration-dependent resolution surface has two lobes, on which the migration order of the enantiomers is opposite. This can be an advantage in trace component analysis. In both Type I and Type III separations, peak resolution varies strongly with the dimensionless electroosmotic flow coefficient when its value is changed in the − 1 to 1 range. The loci of the pH-dependent and the β-cyclodextrin concentration-dependent resolution maxima do not shift significantly when the dimensionless electroosmotic flow coefficient is changed. This fact provides the analyst with an additional resolution enhancement tool that does not alter the selectivity of the separation. The utility of the model and its theoretical predictions has been demonstrated by comparing measured and calculated R_s values for ibuprofen and homatropine.     

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     

1-bromo-1-ethoxycyclopropane: a new reagent for cyclobutanone synthesis

A variety of of cyclobutanones have been prepared in high yield from 1-bromo-1-ethoxycyclopropane via lithiation, addition to aldehydes or ketones, and mild acid-catalyzed rearrangement of the adducts.     

Raman study of SO2 at high pressure: aggregation,phase transformations,and photochemistry

We report Raman measurements made on SO₂ in a diamond cell up to 75 kbar showing two new phases, solid II and solid III, which differ from known, zero-pressure solid I. Spectra indicate that SO₂ molecules aggregate in solid III to possibly form a cyclical trimer; solid III is shown to be photochemically active to blue radiation.