Semiempirical molecular orbital studies on p-quinodimethane. Electronic structure assignments based on total energies and comparisons with photoelectron and absorption spectra

The electronic structure of p-quinodimethane has been investigated using both CNDO/S and INDO molecular orbital approximations. It is found that the energetically favorable configuration is a “quinoid” construction leading to a spin-paired singlet ground state. Comparisons of the calculated excitation energies and orbital orderings with optical and photoemission measurements are consistent with this assignment. The “quinoid” configuration is found to be energetically unfavorable toward the formation of a low-lying triplet or “biradical”-like state. Charge density distributions, however, suggest a high ground state chemical reactivity.     

Intramolecular hydrogen bonding and calixarene-like structures in p-cresol/formaldehyde resins

The nature of the strong hydrogen bonds found in p-cresol/formaldehyde (PCF) resins, compared to ordinary phenolic compounds, is studied. The evidence from FTIR spectroscopy indicates that this strong interaction is due to intramolecular hydrogen bonding from “calixarene-like” structures. The formation of this structure in PCF is enabled by its “linear” (all-ortho-linkage) structure, which is not present in branched resins. Additionally, a transition is observed at around 175 to 200°C where the intramolecular hydrogen bonded structure is lost. This structure cannot be recovered upon cooling or annealing due to restrictions on conformational rotations that are coupled to a new pattern of intermolecular hydrogen bonding. However, the structure is reformed by dissolving the resin in solution and casting new films.     

Dual fluorescence of 4-N,N-dimethylaminopyridine. Role of hydrogen-bonded complex in the ground state

A correlation is shown between the appearance of the dual fluorescence of 4-N,N-dimethylaminopyridine (DMAP) solutions and the formation of hydrogen-bonded of complexes in the ground state. A comparative absorption study between pyridine, N,N-dimethylaniline and DMAP shows that the hydrogen-bonded complex is situated on the amino nitrogen of DMAP. A “pretwisted” conformation of DMAP in the ground state isassumed due to this hydrogen-bonded complex. Simulations by intermolecular interaction calculations and spectroscopic calculations (CNDO/s) confrim the “twisting” influence of water molecules (and/or any other hydrogen bonding) on the amine in the ground state. This “pretwisting” in the ground state by hydrogen bonding is common in many other aromatic amines. Moreover, the deforming role of hydrogen bonding in the ground state seems to be a general phenomenon in flexible aromatic molecules.     

Stereoselective propene polymerization at a metallocene/alumoxane catalyst derived from the chirally-substituted “meso-like” (p-R,p-S)-bis[1-(neoisopinocamphyl)-4,5,6,7-tetrahydroindenyl]-zirconium dichloride

Isopinocamphyl-tosylate (2) was treated with indenyllithium to yield 3-(neoisopinocamphyl)-indene (3). Treatment of 3 with methyllithium gave 1-(neoisopinocamphyl)indenyllithium (4) which was then treated with 0.5 molar equivalents of ZrCl₄(thf)₂ to give a 52:48 mixture of one of the “racemic-like” isomers of bis[1-(neoisopinocamphyl)indenyl]ZrCl₂ (5A) and its “meso-like” diastereomer 5C. Hydrogenation of the 5A/5C mixture (50 bar H₂, Pt) furnished a mixture of the corresponding tetrahydroindenylzirconium complexes 6A and 6C, from which the “meso-like” bis[1-(neoisopinocamphyl)-4,5,6,7-tetrahydroindenyl]zirconium dichloride diastereoisomer (6C) was isolated. Treatment of 6C with an excess of methylalumoxane in toluene/propene generated an active -olefin polymerization catalyst. At −30°C partly isotactic polypropylene ( _η = 39000) was obtained. The catalyst derived from the chirally-substituted “meso-like” metallocene complex 6C produced polypropylene predominantly under enantiomorphic site control.     

Are pore size distributions in microfiltration membranes measurable by two-phase flow porosimetry?

The issue of evaluating equivalent pore diameter distributions in membrane microfilters from gas-liquid (g-l) porosimetry data has been critically examined. Experiments performed with one isotropic and one composite anisotropic membrane in both possible orientations revealed conspicous dependence of the obtained (g-l) porosimetry peaks on imposed pressure ramp rates, p. Interference of this kinetic effect can be eliminated from the measured data by extrapolation to p = 0. The ramp rate effect is most likely caused by tortuous pore length distribution, and relatively long times required for liquid expulsion. For two experiments, the observed effects of p could be reconciled with predictions of the Schlesinger-Bechhold theory [Bechold et al., Kolloid Z., 55 (1931) 172–198]. The data obtained with the thin top layer of the composite membrane facing intruding air directly did deviate somewhat from the theory. Pores characterized by (g-l) porosimetry are likely of the “throat type”, and their size distribution is considerably more narrow than that obtained for the “node-type” pores by SEM-image analysis [Zeman and Denault, J. Membrane Sci., 71 (1992) 221–231]. A single bivariate distribution function was constructed for these two distinct pore populations. Flow-weighted or number fraction distributions can be calculated from the extrapolated porosimetry data. For narrow ranges of “throat” diameters, these distributions are fairly similar.     

A combined experimental and theoretical study on photoinduced intramolecular charge transfer in trans-ethyl p-(dimethylamino)cinamate

Intramolecular charge transfer (ICT) behavior of trans-ethyl p-(dimethylamino)cinamate (EDAC) in various solvents has been studied by steady-state absorption and emission, picosecond time-resolved fluorescence spectroscopy and femtosecond transient absorption experiments as well as time-dependent density functional theory (TDDFT). Large fluorescence spectral shift in more polar solvents indicates an efficient charge transfer from the donor site to the acceptor moiety in the excited state compared to the ground state. The energy for 0,0 transition (ν_0,0) for EDAC shows very good linear correlation with static solvent dielectric property. The relaxation dynamics of EDAC in the excited state can be effectively described by a “three state” model where, the locally excited (LE) state converts into the ICT state within 350 ± 100 fs. A combination of solvent reorganization and intramolecular vibrational relaxation within 0.5–6 ps populates the relaxed ICT state which undergoes fluorescence decay within few tens to hundreds of picoseconds.     

Unusual sulfated marine steroids from the ophiuroid ophioderma longicaudum

Polar extracts of the ophiuroid Ophioderma longicaudun contain unusual sterol sulfates together with a mixture of common 3β-hydroxysterol sulfates. The more polar compound has been shown to be 5β-cholestane-3, 4,11β,12β,21-pentol 3,21-disulfate 1. A second group of unusual compounds are disulfated 3,21-dihydroxysteroids. After solvolysis to remove the sulfate groups they have been identified as : (20R)-5-cholestane-3, 21-diol 4a, (20R)-cholest-5-ene-3,21-diol 5a, (20R22E)-cholest-5,22-diene-3,21-diol 6a and (20R)-24-methylcholest-5,24(28)-diene-3,21-diol 7a. Analysis of the “non-sulfated” sterol fractions has shown the presence of common 3β-hydroxy sterols.     

Study of the secretion from a scale insect (Ceroplastes ceriferus) diterpenoids and sesterterpenoids

Structures of four new minor sesterterpenoids 1–3,18 and three new diterpenoids 12–14 isolated from the secretion of Ceroplastes ceriferus are described. All four sesterterpenoids are 14-membered monocyclic compounds; one of them has been chemically correlated with cericeroic acid¹ of proven absolute stereochemistry. The “cyclic wax” was a complex mixture of esters consisting of fatty acids (C₁₀-C₃₂) and unusual cyclic alcohols (diterpenoids and sesterterpenoids)     

Spectres de rotation de la molécule de chlorobenzéne : I. Variétés isotopiques monosubstituées et structure rs

The microwave spectra of chlorobenzene “(1)-³⁵Cl”, all eight mono-[“(1)-³⁷Cl”, “(1)-³⁵Cl, (2)D”, “(1)-³⁵Cl, (3)D”, “(1)-³⁵Cl, (4)D”, “(1)-³⁵Cl, (1)-¹³C”, “(1)-³⁵Cl, (2)-¹³C”, “(1)-³⁵Cl, (3)-¹³C”, “(1)-³⁵Cl, (4)-¹³C”], one di[“(1)-³⁵Cl, (2,6)D₂,”] and one trisubstituted species [“(1)-³⁷Cl, (2,6)D₂”] have been investigated. From the moments of inertia of the vibrational ground state the r_s structure was derived. The reliability of the two small a coordinates could be enhanced through use of the multiply substituted species. The errors of the moments of inertia were propagated to the structural parameters. It could be shown that the benzene ring is deformed. However the quantitative deformation could not be established due to the rather large errors of some structural parameters.     

Carbohydrate pyrolysis mechanisms from isotopic labeling: Part 1: The pyrolysis of glycerin: Discovery of competing fragmentation mechanisms affording acetaldehyde and formaldehyde and the implications for carbohydrate pyrolysis

The flash pyrolysis of glycerin was investigated by the use of isotopic labeling with ¹³C, in conjunction with GC/MS analysis of the products. The formation of acetaldehyde and acrolein was shown to occur by unimolecular reaction. Acetaldehyde was found to be produced by at least two competing mechanisms. Mechanism “A” delivered C-2 of glycerin to the carbonyl group of acetaldehyde, whereas mechanism “B” delivered C-2 of glycerin to the methyl group of acetaldehyde. Formaldehyde was exclusively derived from C-1 or C-3 by either mechanism. The partition between the two mechanistic paths was found to be influenced by the presence of potassium salts and acids, but not by the presence of benzoyl peroxide or galvanoxyl. Mechanism “A” is postulated to be a concerted cyclic version of the Grob Fragmentation, proceeding through an intermediate which is hydrogen-bonded between the 1- and 3-hydroxyl groups to simultaneously generate enol-acetaldehyde, formaldehyde and H₂O. Mechanism “B” as favored by the presence of alkali is postulated to involve hydrogen-bonding between adjacent hydroxyl groups and to be an alkaline version of the pinacol rearrangement followed by retro-aldol fragmentation of the intermediary 3-oxopropoxide anion. These mechanistic classes are both fundamentally important, not only for their effect on glycerin, but for being able to provide numerous means of initial pyrolytic carbon–carbon bond breakage along carbohydrate carbon-chains, given the numerous 1,2,3-triol interactions that are possible. Further nomenclature is introduced to refine the distinctions among isotopomers and isotopologs (ipsomer, ipsolog, naturalomer, “nominal” isotopolog, ubiquilog), as extensions of a concept previously adopted by IUPAC.     

Complexes of ytterbium(III) nitrate and triflate with homoazacalix[n]arenes (n=3, 4)

The synthesis and crystal structures of three ytterbium(III) complexes of homoazacalixarenes are reported. In all cases, the complexes are obtained without addition of a base, the protons of the complexing phenolic moieties being transferred to the amine groups upon complexation. The 1:1 complex between p-chloro-N-benzylhexahomotriazacalix[3]arene (1) and Yb(NO₃)₃ is analogous to the neodymium(III) complex reported previously, with the metal ion bound to three phenoxide groups and three nitrate ions. A second 1:1 complex is obtained with Yb(NO₃)₃ and p-methyl-N-benzyltetrahomodiazacalix[4]arene (2), in which the metal ion is bound to two phenoxide groups only. The replacement of nitrate by triflate counter-ions results in the formation of a 1:2 ‘sandwich’ complex with 1, in which two divergent calixarene molecules are bridged by the metal ion, the counter-ions being non-bonding.     

Direct and indirect calorimetric studies of stress responses of chlorella cells to infection with the mycoplasma, Acholeplasma laidlawii

This paper reports the defence responses of plant cells to the stress of infection by mycoplasma using an algae model of Chlorella vulgaris under attack by the Mollicute, Acholeplasma laidlawii, which is normally a pathogen of animal systems and higher plants. When the two unicellular organisms were mixed, there was a significant rise in the heat flow rate from 30 min after the chlorella cells were mixed with the mycoplasma cells and were thermally equilibrated in the instrument. There were two peaks in the formation of the superoxide anion radical (O₂√⁻). The first one appeared immediately after mixing the two organisms and essentially was completed in 10 min. This coincided with the raised oxygen uptake rate (OUR) for the supply of the superoxide that can destroy the foreign organisms. The second transient “burst” of superoxide anion radical production was observed after 40–50 min, and correlated with both the continued high level of OUR and the observed “heat burst”. The increased OUR by the chlorella cells at the early stages of infection may be the trigger for further, more prolonged defence reactions by the cells treated with mycoplasma.

The continued infection with mycoplasma, depressed the rate of growth and division of the chlorella cells. This might be caused by competition of the former for substrates required by the host cells. The heat flow rate was also reduced, reflecting the depressed metabolic activity of the plant cells after prolonged exposure to the pathogen.     

Hashish : Synthesis of (+)-Δ-tetrahydrocannabinol

Alkylation of olivetol with p-menth-4-en-3,8-diol (3) afforded Δ⁴-tetrahydrocannabinol, an unknown isomer of the hashish active constituent Δ¹-THC, together with the “abnormal” isomer. The attack of olivetol onto the positions 3 and 5 and resp. 3 and 8 of (3) yielded derivatives of 2,6-methano-2H,1-benzoxocin-7-ol and new 2,3,4,4a-tetrahydroxanthenes.     

The crystal structures of 2-(2′-pyridyl)phenyltellurium(II) bromide and of the inclusion compound bis[2-(2′-pyridyl)phenyltellurium(II) chloride]·p-ethoxyphenyl-mercury(II) chloride

In 2-(2′-pyridyl)phenyltellurium(II) bromide (1) the coordination about tellurium may be described as pseudo-trigonal bipyramidal wth bromine (Te---Br = 2.707(11) Å) and nitrogen (Te---N) = 2.236(11) Å) atoms occupying axial positions. The equatorial plane comprises a carbon atome (Te---C = 2.111(6) Å) and two lone pairs of electrons. There are no significant intermolecular interactions between the six independent molecules in the unit cell. Bis[2-2′-pyridyl)phenyltellurium(II) chloride]·p-ethoxy-phenylmercury(II) chloride (2) may be regarded as an “inclusion compound” obtained by replacement of two RTeX (X = Cl or Br) molecules by two p-ethoxyphenylmercury(II) chloride entities. There is approximately linear coordination about mercury (C---Hg---Cl = 179.2°(4), Hg-C = 2.044(14) and Hg---Cl = 2.328(4) Å) and 2-(2′-pyridyl)phenyltellurium(II) chloride, with a structure similar to that of (1) above (Te---N = 2.2366(6), Te---Cl = 2.558(1), Te---C = 2.080(25) Å). There are no significant intermolecular contacts.     

Chemistry of the podocarpaceae—V The identification of miropinic acid and isomiropinic acid from Podocarpus ferrugineus D. Don

By reinvestigation of “isomiropinic” and “miropinic” acids from the bled resin of Podocarpus ferrugineus, the compounds have been identified as sugiol and isopimaric acid respectively. The identity of the resin acid from Dacrydium biforme has been confirmed as isopimaric acid.     

Principal components of phenolics to characterize red Vinho Verde grapes: Anthocyanins or non-coloured compounds?

Phenolic profile of 10 different varieties of red “Vinho Verde” grapes (Azal Tinto, Borraçal, Brancelho, Doçal, Espadeiro, Padeiro de Basto, Pedral, Rabo de ovelha, Verdelho and Vinhão), from Minho (Portugal) were studied. Nine Flavonols, four phenolic acids, three flavan-3-ols, one stilben and eight anthocyanins were determined. Malvidin-3-O-glucoside was the most abundant anthocyanin while the main non-coloured compound was much more heterogeneous: catechin, epicatechin, myricetin-3-O-glucoside, quercetin-3-O-glucoside or syringetin-3-O-glucoside. Anthocyanin contents ranged from 42 to 97%. Principal component analysis (PCA) was applied to analyse the date and study the relations between the samples and their phenolic profiles. Anthocyanin profile proved to be a good marker to characterize the varieties even considering different origin and harvest. “Vinhão” grapes showed anthocyanins levels until twenty four times higher than the rest of the samples, with 97% of these compounds.     

Sythesis and crystal structure of a new nickel(II)-terpyridine complex: [Ni(terpy)(NO2)(ONO)(H2O)]

The compounds Ni(terpy)Cl₂ · 3H₂O and NaNO₂ react readily in aqueous solution to give the complex [Ni(terpy)(NO₂)(ONO)(H₂O)], which has been structurally characterized by an X-ray diffraction study. The hexacoordinate nickel atom bonds to three nitrogen atoms of the terpyridine ligand, to a “nitro” group, to a “nitrite” group and to a water molecule. Distortion from an octahedron to a trigonal prism, for the complex, has been evaluated. An infrared study of the complex has been carried out and shows bands of nitro and nitrite coordination. Magnetic study reveals a Curie-Weiss behavior, giving an effective magnetic moment at room temperature 3.12 B.M.     

Co-aggregation of fullerene C60 and thiophene in the non-aromatic solvent cyclohexene

Co-aggregation of fullerene C₆₀ and thiophene has been studied calorimetrically in cyclohexene at 25 °C. The total aggregation heat is found to depend on initial concentration of thiophene and fall between −1.9 and −5.8 kJ mol⁻¹. The corresponding thiophene/fullerene molar ratio (“co-aggregation number”) ranges from 7 to 12. The data are rationalized by formation of heteromolecular nanoaggregates with intermolecular contacts of both fullerene–thiophene and fullerene–fullerene types. A physical model describing interaction between fullerenes and π-donors in solution is substantiated and used to explain heterogeneity of composites containing fullerenes.     

Oxidative degradation of pentachlorophenol by an iron(III)-porphyrin catalyst bound to humic acid via formaldehyde polycondensation

Introduction of functional groups into humic substances is a novel trend in humic technology. In the present study, a biomimetic catalyst, 5-(p-hydroxyphenyl)-5,10,15,20-tetrasulfonatophenyl porphine iron(III) (FeTPPSOH), was introduced into humic acid via formaldehyde polycondensation. In the presence of KHSO₅, the self-degradation of the prepared catalysts, which were called “resol,” was retarded. In addition, the catalytic activity, which was evaluated by the percent pentachlorophenol (PCP) disappearance and levels of dechlorination, was significantly greater for the resol catalysts. The results of the present study show that resol catalysts effectively enhance oxidative degradation of PCP. The byproducts of oxidation were investigated by GC/MS analysis of n-hexane extracts of the reaction mixtures. This analysis demonstrated that another advantage of resol catalysts is that they prevent the formation of more harmful dimers, such as octachlorodibenzo-p-dioxin.     

Transition metal complexes with long-chain amines thermal behaviour and crystal structure of (n-CaH2a+1NH2)2ZnCl2

The thermal behaviour of (n-C_aH_2n+1NH₂)₂ZnCl₂ complexes with n = 6, 8, … 16 has been investigated by DSC and by temperature variable IR and X-ray powder diffraction techniques. Complexes with n = 12,14,16 show solid—solid phase transition which are “melting” transitions of the hydrocarbon regions of the structure. The crystal structure of both the low and the high temperature polymorphs is characterized by the piling of sandwiches, each formed by an “inorganic” layer sandwiched between two alkylammonium layers.