Electronic Structures, (d-p)π Conjugation Effects, and Spectroscopic Properties of Polyoxometalates: M6O19 2− (M=Cr, Mo, W)

The electronic structures and bonding of isopoly oxometalates M₆O₁₉ ^2– (M=Cr, Mo, W) have been investigated by using ab initio and relativistic density functional methods. We have discussed the role of the central oxygen atom and the (d-p) conjugation interactions between the metal and bridging oxygen atoms. It is found that there exist 12 three-centered two-electron (d-p-d) bonds for the three M₄(-O)₄ planar rings in M₆O₁₉ ^2– ions and these hexametalates are considered to have quasi-aromaticity. The (d-p) conjugation effects play essential role in stabilizing these cluster compounds, and the reduced (d-p) conjugation effects account for the instability of the isopoly oxochromate ion, Cr₆O₁₉ ^2–. The vibrational spectra and electronic spectra of M₆O₁₉ ^2– ions are evaluated and assigned theoretically and the calculated spectra are in fairly good agreement with the measured experimental results.     

N-Silylation of 2,4,6-Trialkyl-2,4,6,8-tetraazabicyclo[3.3.0]octane-3,7-diones

N-Trimethylsilyl derivatives of 2,4,6,8-tetraazabicyclo[3.3.0]octane-3,7-diones have been prepared for the first time and the electrophilic substitution reaction of the trimethylsilyl group has been studied.     

Development of a stabilized low temperature infrared absorption cell for use in low temperature and collisional cooling experiments

We have constructed a stabilized low temperature infrared absorption cell cooled by an open cycle refrigerator, which can run with liquid nitrogen from 250 to 80K or with liquid helium from 80K to a few kelvin. Several CO infrared spectra were recorded at low temperature using a tunable diode laser spectrometer. These spectra were analyzed taking into account the detailed effects of collisions on the line profile when the pressure increases. We also recorded spectra at very low pressure to accurately model the diode laser emission. Spectra of the R(2) line in the fundamental band of 13CO cooled by collisions with helium buffer gas at 10.5K and at pressures near 1 Torr have been recorded. The He-pressure broadening parameter (gamma(0) = 0.3 cm(-1) atm(-1)) has been derived from the simultaneous analysis of four spectra at different pressures.     

Effect of capping and particle size on Raman laser-induced degradation of γ-Fe2O3 nanoparticles

Diol capped γ-Fe₂O₃ nanoparticles are prepared from ferric nitrate by refluxing in 1,4-butanediol (9.5 nm) and 1,5-pentanediol (15 nm) and uncapped particles are prepared by refluxing in 1,2-propanediol followed by sintering the alkoxide formed. X-ray diffraction (XRD) shows that all the samples have the spinel phase. Raman spectroscopy shows that the samples prepared in 1,4-butanediol and 1,5-pentanediol and 1,2-propanediol (sintered at 573 and 673 K) are γ-Fe₂O₃ and the 773 K-sintered sample is Fe₃O₄. Raman laser studies carried out at various laser powers show that all the samples undergo laser-induced degradation to α-Fe₂O₃ at higher laser power. The capped samples are however, found more stable to degradation than the uncapped samples. The stability of γ-Fe₂O₃ sample with large particle size (15.4 nm) is more than the sample with small particle size (10.2 nm). Fe₃O₄ having a particle size of 48 nm is however less stable than the smaller γ-Fe₂O₃ nanoparticles.     

苯并菲盘状液晶的合成、亲氟效应及分子对称性对介晶性的影响

New symmetrical and asymmetrical triphenylene-containing discotic liquid crystals with two different peripheral alkyl chains, known as sym-TP(OC6H13)3(OR)3 and asym-TP(OC6H13)3(OR)3, were synthesized. Their thermotropic liquid crystal properties were investigated through polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analyses. The asyrranetdcal discogens are 2,6,11-rialkoxy-3,7,10-trihexyloxytriphenylenes, with the alkyl chain carbon numbers varying from 3-10, 12, and 14, while the symmetrical compounds are 2,6,10-trialkyloxy-3,7,11-trihexyloxytriphenylene. Two fluoroalkoxy substituted triphenylene discogens, 2,6,10-td(4,4,4-trifluorobutoxy)-3,7,11-trihexyloxytriphenylene and its asymmetrical isomer 2,6,11-tri(4,4,4-trifluorobutoxy)-3,7,10-trihexyloxytdphenylene were prepared. These two compounds show higher melting and clearing points than their alkoxy analogs, which implies that fluorophilic effect exists in the formation and stabilization of discotic columnar mesophase. The triphenylene derivatives TP(OC6H13)3(OR)3 with two different peripheral chains, symmetrically or asymmetrically attached on triphenylene cores, have lower melting points and clearing points than those of the higher symmetrical compounds TP(OR)6 with the same total chain carbon numbers. The mixed-chain-triphenylenes with longer alkoxy chains (n=9,10,12,14) show columnarmesophase at room temperature.     

Alcohols,ethers, carbohydrates,and related compounds. II. The anomeric effect

The anomeric effect has been studied for a variety of compounds using the MM4 force field, and also using MP2/6-311++G(2d,2p) ab initio calculations and experimental data for reference purposes. Geometries and energies, including conformational, rotational barriers, and heats of formation were examined. Overall, the agreement of MM4 with the experimental and ab initio data is good, and significantly better than the agreement obtained with the MM3 force field. The anomeric effect is represented in MM4 by various explicit terms in the force constant matrix. The bond length changes are accounted for with torsion-stretch elements. The angle changes are accounted for with torsion-bend elements. The energies are taken into account with a number of torsional terms in the usual way. A torsion-torsion interaction is also of some importance. With all of these elements included in the calculation, the MM4 results now appear to be adequately accurate. The heats of formation were examined for a total of 12 anomeric compounds, and the experimental values were fit by MM4 with an RMS error of 0.42 kcal/mol.     

Far-UV laser flash photolysis in solution. A study of the chemistry of 1,1-dimethylsilene in hydrocarbon solvents

The far-UV (193 nm) laser flash photolysis of nitrogen-saturated isooctane solutions of 1,1-dimethylsiletane allows the direct detection of 1,1-dimethylsilene as a transient species, which (at low laser intensities) decays with pseudo-first-order kinetics (τ 10 μs) and exhibits a UV absorption spectrum with λ_max 255 nm. Characteristic rapid quenching is observed for the silene with methanol (k_McOH = (4.9 ± 0.2) × 10⁹ M⁻¹ s⁻¹), tert-butanol (k_BuOH = (1.8 ± 0.1) × 10⁹ M⁻¹ s⁻¹) and oxygen (k_O2 = (2.0 ± 0.5) × 10⁸ M⁻¹ s⁻¹). The Arrhenius activation parameters for the reaction with methanol have been determined to be E_a = −2.6 ± 0.6 kcal mol⁻¹ and log A = 7.7 ± 0.3.     

Preparation and characterization of spherical polymer packings from polybutadiene for size-exclusion chromatography

Porous polymer spherical particles for column packings in nonaqueous size-exclusion chromatography (SEC) were prepared from 1,2-syndiotactic polybutadiene by suspension and evaporation method. The polymer microbeads obtained were crosslinked by radical reaction between 2-vinyl groups in polybutadiene with ultraviolet radiation, to render them insoluble. These microbeads have wider chromatographic separation width than polystyrene column packings. In addition, the polybutadiene microbeads did not show the excessive retention observed with commercial polystyrene columns for polycyclic aromatic compounds. Therefore, a close correlation between the elution volume and M, for polycyclic aromatic compounds was observed with polybutadiene microbeads columns.     

NMR studies of solid state—solvent and H/D isotope effects on hydrogen bond geometries of 1:1 complexes of collidine with carboxylic acids

¹H and ¹⁵N NMR spectra of 10 complexes exhibiting strong OHN hydrogen bonds formed by ¹⁵N-labeled collidine and different proton donors, partially deuterated in mobile proton sites, have been observed by low-temperature NMR spectroscopy using a low-freezing CDF₃/CDF₂Cl mixture as polar aprotic solvent. The following proton donors have been used: HCl, formic acid, acetic acid, various substituted benzoic acids and HBF₄. The slow hydrogen bond exchange regime could be reached below 140 K, which allowed us to resolve ¹⁵N signal splittings due to H/D isotopic substitution. The valence bond order model is used to link the observed NMR parameters to hydrogen bond geometries. The results are compared to those obtained previously [Magn. Reson. Chem. 39 (2001) S18] for the same complexes in the organic solids. The increase of the dielectric constant from the organic solids to the solution (30 at 130 K) leads to a change of the hydrogen bond geometries along the geometric correlation line towards the zwitterionic structures, where the proton is partially transferred from oxygen to nitrogen. Whereas the changes of spectroscopic and, hence, geometric parameters are small for the systems which are already zwitterionic in the solid state, large changes are observed for molecular complexes which exhibit almost a full proton transfer from oxygen to nitrogen in the polar liquid solvent.     

Kunzeanones A, B, and C: novel alkylated phloroglucinol metabolites from Kunzea ambigua

Three new metabolites, kunzeanones A (1), B (2), and C (3), along with three known compounds, cryptostrobin (4), (+)-spathulenol (5), and (−)-globulol (6), were isolated from the non-polar fraction of the dried leaves of Kunzea ambigua (Myrtaceae), which shows ichthyotoxicity toward a small fish, medaka. The structures of these new compounds were elucidated as condensates of alkylated phloroglucinol with methylflavanone and germacrane-type sesquiterpene, respectively, on the basis of spectral analyses including 1-D and 2-D NMR spectra. The stereochemistries of kunzeanones A and B were determined by X-ray crystallographic analysis. A sesquiterpene, (+)-spathulenol (5), among the isolates was characterized as the ichthyotoxic principle of the extract.