Assignment of absolute configuration of a chiral phenyl-substituted dihydrofuroangelicin

A phenyl-substituted chiral dihydrofuroangelicin, 4-methyl-8-(2-E-phenylethenyl)-8,9-dihydro-2H-furo[2,3-h]- 1-benzopyran-2-one, synthesized in racemic form, has been resolved by HPLC chiral separation, and its absolute configuration determined by the non-empirical exciton chirality method. The solution conformation has been investigated through NMR and molecular modeling methods: two minima found by molecular mechanics and DFT methods are in keeping with observed 1H-1H 3J coupling constants and NOE effects. The experimental CD spectrum for the second eluted enantiomer shows a positive couplet between 230 and 350 nm (amplitude A = + 15.7); by application of the exciton chirality method, the absolute configuration of this enantiomer at C8 is determined as (S). The experimental spectrum is in very good agreement with the one evaluated by means of DeVoe coupled-oscillator calculations, using the DFT calculated geometries.     

On the origin of the optical activity in the d-d transition region of tris-bidentate Co(III) and Rh(III) complexes

Time-dependent density functional theory (TD-DFT) has been employed to calculate the rotatory strengths in the d-d transition region for various tris-bidentate Co(III) and Rh(III) complexes. Optimized structural parameters are also reported. Our results confirm a previously proposed relationship between the azimuthal distortion of a complex containing saturated tris(diamine) and its optical activity. Formally d-d transitions are forbidden and should not exhibit optical activity. However, it is shown here that the intensity of these bands originates from a coupling of even ligand combination (participating in the e(g) type LUMO) and an odd ligand combination (participating in the t(2)(g) HOMO). For complexes containing planar unsaturated ligands, the signs of the d-d bands observed from the single-crystal circular and linear dichroisms are in accordance with the TD-DFT predictions. It is shown that by using hypothetical Co(NH(3))(6)(3+) complexes it is possible to estimate the contribution from the azimuthal distortion to the total rotatory strengths of the saturated tris(diamine) complexes. A discussion is also provided of previous theoretical studies and the way in which these investigations rationalized the optical activity.     

On-line gas chromatographic monitoring of catalyst processes in a microfabricated chemical reactor

A microfabricated catalyst reactor, prepared from glass and polydimethylsiloxane, has been directly interfaced to a gas chromatograph permitting real time reaction monitoring allowing rapid catalyst characterisation.     

Determination of 1,4-dioxane impurity levels in Triton X-100 raw material by gas chromatography with mass spectrometric detection

Triton X-100 (octoxynol 9) is a commercially available surfactant used as a solvent detergent in numerous pharmaceutical applications including virus inactivation. A byproduct formed during its synthesis is 1,4-dioxane, the cyclic dimer of ethylene oxide and a possible carcinogen to humans. The United States Pharmacopoeia (USP) contains a labor-intensive 1,4-dioxane test for Triton X-100. The method couples vacuum distillation to extract the 1,4-dioxane from the Triton X-100 matrix followed by gas chromatography (GC) using a packed column with flame-ionization detection. In order to provide a more automated and specific test methodology, a headspace GC-mass spectrometry (MS) method has been developed for this application. Analyte quantitation is accomplished by the method of standard additions. The automated sample preparation, coupled with the specificity inherent in high-efficiency capillary column separations together with single-ion MS detection, results in an assay that is more efficient, accurate, and precise than the USP procedure. Performance characteristics of the headspace GC-MS method are contrasted with those characteristics of the USP methodology.     

Synthesis,chemistry, and dynamic NMR study of new atropisomeric 4-dialkylamino-5-chloro-1,2-dithiole-3-thiones

[reaction: see text] Reaction of N-(2-phthalimidoethyl)-N-alkylisopropylamines and S(2)Cl(2) gave atropisomeric (by dynamic NMR) 4-N-(2-phthalimidoethyl)-N-alkylamino-5-chloro-1,2-dithiole-3-thiones that quantitatively cycloadded to dimethyl or diethyl acetylenedicarboxylate to give a novel class of stable thioacid chlorides, which in turn reacted with 1 or 2 equiv of secondary amines to give thioamides.     

An extensible and systematic force field,ESFF, for molecular modeling of organic,inorganic, and organometallic systems

ESFF is a rule-based force field designed for modeling organic, inorganic, and organometallic systems. To cover this broad range of molecular systems, ESFF was developed in an extensible and systematic manner. Several unique features were introduced including pseudoangle and a dot product function representing torsion energy terms. The partial atomic charges that are topology-dependent are determined from ab initio (DFT) calculated electronegativity and hardness for valence orbitals. The van der Waals parameters are charge-dependent, and correlated with the ionization potential for atoms in various valence states. To obtain a set of well-defined and physically meaningful parameters, ESFF employs semiempirical rules to translate atomic-based parameters to parameters typically associated with a covalent valence force field. The atomic parameters depend not only on atom type, but also on internal type, thus resulting in a more accurate force field. This article presents the theory and the method used to develop the force field. The force field has been applied to molecular simulations of a wide variety of systems including nucleic acids, peptides, hydrocarbons, porphyrins, transition metal complexes, zeolites, and organometallic compounds. Agreement with the experimental results indicates that ESFF is a valuable tool in molecular simulations for understanding and predicting both crystal and gas phase molecular structures.     

Spatially resolved Raman and UV-visible-NIR spectroscopy on the preparation of supported catalyst bodies: controlling the formation of H2PMo11CoO40 5- inside Al2O3 pellets during impregnation

The physicochemical processes that occur during the preparation of CoMo-Al2O3 hydrodesulfurization catalyst bodies have been investigated. To this end, the distribution of Mo and Co complexes, after impregnation of gamma-Al2O3 pellets with different CoMoP solutions (i.e., solutions containing Co, Mo, and phosphate), was monitored by Raman and UV-visible-NIR microspectroscopy. From the speciation of the different complexes over the catalyst bodies, insight was obtained into the interaction of the different components in the impregnation solution with the Al2O3 surface. It is shown that, after impregnation with a solution containing H2PMo11CoO40(5-), the reaction of phosphate with the Al2O3 leads to the disintegration of this complex. The consecutive independent transport of Co2+ complexes (fast) and Mo6+ complexes (slow) through the pores of the Al2O3 is envisaged. By the addition of extra phosphate and citrate to the impregnation solution, the formation of the desired heteropolyanion can be achieved inside the pellets. Ultimately, the H2PMo11CoO40(5-) distribution could be controlled by varying the aging time applied after impregnation. The power of a combination of spatially resolved spectroscopic techniques to monitor the preparation of supported catalyst bodies is illustrated.     

Calculating excitation energies with the help of cumulants

Recently, it has been shown that the ground-state energy of a quantum many-body system can be written in terms of cumulants. In this article, we show that the energies of excited states can be expressed similarly. These representations are suitable for various approximation schemes, e.g., projection techniques. The explicit use of cumulants ensures size consistency in all approximations. The theory is then applied to the computation of the energy bands of a semiconductor with a diamond structure. As we focus especially on the effects of electron correlations, we consider a simplified model and rederive results obtained previously by the variational local ansatz method. © 1994 John Wiley & Sons, Inc.