Time-Resolved Photochemistry of Stiffened Stilbenes.

Broadband transient absorption spectroscopy is used to study the photoisomerization of stiffened stilbenes in solution, specifically E/ Z mixtures of bis(benzocyclobutylidene) (t4, c4) and ( E)-1-(2,2-dimethyltetralinylidene)-2-2-dimethyltetraline (t6). Upon excitation to S1, all evolve to perpendicular molecular conformation P, followed by decay to S0, while the spectra and the kinetic behavior crucially depend on the size of the stiffening ring. In 4, contrary to all previously studied stilbenes, the trans and cis absorption and excited-state spectra are nearly indistinguishable, while the corresponding isomerization times are comparable: τi = 166 ps for t4 and τi = 64 ps for c4 in n-hexane, as opposed to 114 and 45 ps in acetonitrile, respectively. Faster isomerization in polar solvents agrees with the zwitterionic character of the P state. In t6, torsion to P is effectively barrier-less and completes within 0.3 ps, the S1 → P evolution being directly traceable through the transient spectra of stimulated emission and that of excited-state absorption. In n-hexane, the P state is remarkably long-lived, τP = 1840 ps, but the lifetime drops down to 35 ps in acetonitrile. The trans-to-cis photoisomerization yield for t6 is measured to be 20%, while for t4, it remains uncertain. We discuss the effects of stiffening and substitution on the formation and lifetime of the intermediate states through which the stilbene molecules evolve on the S1 energy surface.

Comment on "Glycine-functionalized copper(ii) hydroxide nanoparticles with high intrinsic superoxide dismutase activity" by K. Korschelt, R. Ragg, C. S. Metzger, M. Kluenker, M. Oster, B. Barton, M. Panthöfer, D. Strand, U. Kolb, M. Mondeshki, S. Strand, J. Brieger, M. N. Tahir and W. Tremel, Nanoscale, 2017, 9, 3952.

Detoxification of cigarette smoke employing copper compounds has a potential for the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans and should not be practiced before the degree of formation has been shown to be harmless.

Enzymatic synthesis of bromo- and chlorocarbazoles and elucidation of their structures by molecular modeling.

3-Chlorocarbazole, 3,6-dichlorocarbazole, dibromocarbazole, and 1,3,6,8-tetrabromocarbazole are emerging environmental contaminants which have been detected recently in water, sediment, and soil samples. However, their sources and occurrence have not been explained. Here, we report an enzymatic synthesis of bromo- and chlorocarbazoles by chloroperoxidase from Caldariomyces fumago in water. Density functional theory (DFT) method was used to predict the most stable products. Carbazole and chloroperoxidase were assayed in vitro in the presence of hydrogen peroxide, bromide, and chloride ions in different substrate ratio treatments against constant and varying enzyme concentrations. Halogenated carbazoles formed were identified by high-resolution gas chromatography coupled to mass spectrometry. In all treatments, bromination and chlorination took place, but the composition and concentration of compounds formed varied from one treatment to another. Mono-, di-, tri-, and tetra-substituted bromo- and chlorocarbazoles which include the reported environmental contaminants were synthesized. 3-Substituted and 3,6-substituted congeners were relatively higher in concentration. Enzyme concentration did not favor preferential formation of any of the compounds synthesized. However, their synthesis was influenced by halide concentration. Congeners with bromine and chlorine at position of C-3, C-3,6, C-1,3,6, and C-1,3,6,8 were calculated as the stable intermediate sigma complexes by DFT method. Regioselectivity in halogenation is discussed and hypothesis of the likely stable products in the environment explained. This study provides evidence that bromo- and chlorocarbazoles reported previously can be formed enzymatically in the environment, demonstrating the need to consider aromatic pollutants transformation and their potential toxicity enhancements in the management of water pollution and contaminated sites.

Verification of stereospecific dyotropic racemisation of enantiopure D and L-1,2-dibromo-1,2-diphenylethane in non-polar media.

The first example of a dyotropic rearrangement of an enantiomerically pure, conformationally unconstrained, vicinal dibromide confirms theoretical predictions: D and L-1,2-dibromo-1,2-diphenylethane racemise stereospecifically in refluxing benzene without crossover to the meso-isomer. An orbital analysis of this six-electron pericyclic process is presented.

Bis-dibenzo[a.i]fluorenylidene, does it exist as stable 1,2-diradical?

The geometries, energies, and electronic properties of the two possible configurations of bis-[dibenzo[a.i]fluorenylidene] were investigated theoretically by density functional theory DFT B3LYP at the UB3LYP/6-311 + G(2d,p) // UB3LYP/6-31 + G(d,p) level of theory. According to the performed calculations, it was found that the singlet is 3.4 kcal mol(-1) lower in energy compared to triplet state at room temperature. This gap is compared with those of other alkenes like ethylene, (61.9 kcal mol(-1)) tetra-tert-butyethylene, (6.4 kcal mol(-1)) and bis-fluorenylidene (19.5 kcal mol(-1)). These results confirm the experimental findings of the paramagnetic properties determined by Franzen and Joschek. The low singlet-triplet gap in the case of bis-[dibenzo[a.i]fluorenylidene] is the result of a steric destabilization of the singlet due to strain and stabilization of the triplet electronic state by delocalization of each free electron within each aromatic moiety. This correlates with the special electronic structure of the triplet state of this compound, where facial interaction of two hydrogen atoms lying close to the lobes of each p-orbital occupied with a single electron at the distorted double bond in the triplet electronic state.

Reduction of benzylic alcohols and α-hydroxycarbonyl compounds by hydriodic acid in a biphasic reaction medium.

The synthetic protocol for the reduction of alcohols to hydrocarbons by using hydriodic acid, first described by Kiliani more than 140 years ago, was improved to be more applicable to organic synthesis. Instead of a strongly acidic, aqueous solution, a biphasic toluene-water reaction medium was used, which allowed the conversion of primary, secondary and tertiary benzylic alcohols, in good yields and short reaction times, into the corresponding hydrocarbons. Red phosphorous was used as the stoichiometric reducing agent. Keto, ester, amide or ether groups are tolerated, and catalytic amounts of hydriodic acid (0.2 equiv) in the presence of 0.6 equiv phosphorous are sufficient to achieve conversion.

Laboratory studies on formation and minimisation of polychlorinated dibenzodioxins and -furans (PCDD/F) in secondary aluminium process.

The objectives of this work were to study the formation mechanisms of polychlorinated dibenzo-p-dioxins/polychlorinated dibenzofurans (PCDD/F) in thermal aluminium recycling processes by use of laboratory experiments. The pattern of isomers of PCDD/F indicates that de novo synthesis is important in aluminium smeltery. The mechanisms of PCDD/F formation in aluminium smelting are similar to that of various incineration processes of waste material. The results of bioanalysis (EROD-test) confirms the existence of de novo synthesis of PCDD/F, but points out to the existence to some additional, toxic compounds of unknown structure. To reduce the amount of PCDD/F the input of carbon at the metal should be reduced; in addition the metal smeltery plants should be cleaned from fly ash particles. It is suggested to use good primary methods in the technical plants like constant feeding of the metal into the oven will minimise PCDD/F concentration. The biological EROD-bioassay is a good tool to estimate PCDD/F-TEQ values also for this technical process simulated in the laboratory.

Degradative fate of 3-chlorocarbazole and 3,6-dichlorocarbazole in soil.

BACKGROUND, AIM, AND SCOPE: 3-Chlorocarbazole and 3,6-dichlorocarbazole were isolated from Bavarian soils. The stereospecific formation of the isomers of these chlorinated carbazols can be explained by quantum mechanical calculations using the DFT method. It was shown that chlorination of carbazole and 3-chlorocarbazole respectively is preferred via the sigma-complexes 3-chlorocarbazole and 3,6-dichlorocarbazole as the most stable products. The dioxin-like toxicological potential of 3,6-dichlorocarbazole, determined by the Micro-EROD Test, is in the range of some picogram TCDD equivalents per milligram carbazole. The degradative fate of 3-chlorocarbazole and 3,6-dichlorocarbazole was analysed within a long-term study (57 days) in soil. MATERIALS AND METHODS: The soil was extracted by ASE (accelerated solvent extraction) and a further clean-up procedure with column chromatography and chromatography with C18-SPE stationary phases. Quantification of 3-chlorocarbazole and 3,6-dichlorocarbazole was performed employing the isotope-dilution method. The samples were measured with high-resolution GC/MS. RESULTS: The degradation (ln(c/c(0)) vs. time with best-fit line) showed in almost every storage condition a very small degradation (slopes (h(-1)) in -10(-4) range). However, the decay for the controls were two to three times (-28°C) and six times (with sodium azide) higher, than the decrease of 3-chlorocarbazole and 3,6-dichlorocarbazole in the samples of environmental conditions. DISCUSSION: Especially because of the toxicological potential of 3-chlorocarbazole and 3,6-dichlorocarbazole the proven degradative fate is of large interest. The results show that the analysed carbazoles are not readily degradable in this time period. CONCLUSIONS: The expected results of exponential decay behaviour could not be proven. RECOMMENDATION AND PERSPECTIVES: Longer-lasting studies are expected to reveal more accurate half-lives, although it has been shown here, that the compounds are not readily degradable in their native soil environment.

What is the preferred structure of the Meisenheimer-Wheland complex between sym-triaminobenzene and 4,6-dinitrobenzofuroxan?

The geometries, energies, and electronic properties of possible configurations of Meisenheimer-Wheland (M-W) complexes of sym-triaminobenzenes and 4,6-dinitrobenzofuroxan (DNBF) were investigated theoretically by MP2 and a variety of DFT methods. The pi-pi complex is preferred thermodynamically by more than 15 kcal/mol over the sigma-complexes for the unsubstituted species. However, the N-substituents of the 1,3,5-triaminobenzenes influence the relative stabilities of the alternative configurations significantly. The sigma-syn configuration of the M-W complex of 1,3,5-tris(N-piperidyl)benzene and DNBF has the lowest energy, followed closely by the sigma-anti and pi-pi forms. The small energy differences between different configurations are consistent with the dynamic interconversion of three homomeric structures observed experimentally by NMR. The ca. 1.63 A C-C interring bond exchanges among three equivalent sites. Quantum theory of atoms in molecules (QTAIM) analysis provided insights into the nature of the intermonomer interactions. Charge transfer and sigma bonding account for the stability and remarkably large binding energies of the M-W complexes.

Single-electron self-exchange between cage hydrocarbons and their radical cations in the gas phase.

We show that the radical cations of adamantane (C(10)H(16)(*+), 1H(*+)) and perdeuteroadamantane (C(10)D(16)(*+), 1D(*+)) are stable species in the gas phase. The radical cation of adamantylideneadamantane (C(20)H(28)(*+), 2H(*+)) is also stable (as in solution). By using the natural (13)C abundances of the ions, we determine the rate constants for the reversible isergonic single-electron transfer (SET) processes involving the dyads 1H(*+)/1H, 1D(*+)/1D and 2H(*+)/2H. Rate constants for the reaction 1H(*+)+1D <==> 1H+1D(*+) are also determined and Marcus' cross-term equation is shown to hold in this case. The rate constants for the isergonic processes are extremely high, practically collision-controlled. Ab initio computations of the electronic coupling (H(DA)) and the reorganization energy (lambda) allow rationalization of the mechanism of the process and give insights into the possible role of intermediate complexes in the reaction mechanism.

2-Norbornyl ion-pair leakage in electrophilic addition of HCl to nortricyclene and norbornene.

Ion pair "leakage" pathways, located computationally by means of multidimensional potential energy surface scans, rationalize the unsymmetrical D-label scrambling observed experimentally in the DCl addition products of nortricyclene and norbornene. "Classical" addition transition structures can interconvert ("leak") to symmetrical nonclassical 2-norbornyl ion pair species, either TS's or a minimum, before products form.

Correlation of PCDD/F and PCB at combustion experiments using wood and hospital waste. Influence of (NH(4))(2)SO(4) as additive on PCDD/F and PCB emissions.

The objectives of this work were to study the emissions of polychlorinated dibenzo-p-dioxins/polychlorinated dibenzofurans (PCDD/F) and polychlorinated biphenyl (PCB) during the combustion of wood, wood and hospital waste as well as wood, hospital waste and (NH(4))(2)SO(4) in a pilot plant and to determine the relation of the toxic emissions to the fuel properties. The presence of PVC (experiments b and c) increases PCDD/F and PCB formation compared to burning only eco-farm wood (a). However, analysis and quantification of PCDD/F and PCB showed that the amount of 5% of (NH(4))(2)SO(4) in the fuel mixture significantly impacted the dioxins emissions and suppressed it by approximately 50%. In addition, longer duration of the combustion experiment by using (NH(4))SO(4) resulted in a stronger inhibition of the toxic emissions. Overall, the lowest values were observed during the combustion of the fuel blends containing wood only. Relationships of PCDD/F (WHO-TEQ) and individual congeners of WHO-PCDD/F, WHO-PCB and six PCB indicator congeners were also found. As a result, all single PCB congeners were identified as PCDD/F (WHO-TEQ) surrogates in the flue gas. Furthermore, the correlation between PCDD/F and PCB congeners was examined by statistical methods such as cluster analysis (CA). Closely neighboured pairs on the dendrogram were additionally confirmed by a high linear coefficient of determination (R(2)). (NH(4))(2)SO(4)-containing samples did not affect the correlations between PCDD/F and PCB at the concentrations given.

Electrophilic bromination of alkenes: environmental, health and safety aspects of new alternative methods.

More than twenty new alternative methods for bromination of alkenes have been evaluated taking into consideration their resource demands, waste production as well as environmental, health and safety aspects. The cost of bromine and the substances designated to circumvent the application of molecular bromine have also been taken into account. As bromine is only one of several problematic substances being used, its avoidance-by applying bromine supported on solid material or by performing the in situ generation of bromine-does not significantly reduce the technological requirements. On the contrary, the resource demands and amount of waste produced by most new methods are significantly higher compared to the standard methods, especially if the recycling of a carrying agent is not efficient. The method using hydrobromic acid and hydrogen peroxide can be regarded as a competitive alternative to the standard method. The application of certain carrying agents could be interesting, because solvents such as carbon tetrachloride or chloroform used during synthesis could be replaced with less problematic ones during work-up. However, problems associated with these alternatives are not resolved as yet.

Stable ion and electrophilic chemistry of the sterically crowded stilbene 1,1'-Bi(benzocyclobutenylidene) and its derivatives.

Generation and NMR studies of novel carbocations and carboxonium ions are reported from sterically hindered stilbene 1,1'-bi(benzocyclobutenylidene) 1, its dimethoxy derivative 5, and from their skeletally rearranged derivatives, namely, the spirocyclic ketone 6, diastereomeric alcohols 7 and isomeric diols 8. Quenching experiments on the carbocations under various conditions resulted in the formation/isolation of several novel covalent adducts. Acid-catalyzed isomerization of the diols 8 produced a remarkable dimeric molecule, whose structure was confirmed by X-ray analysis. Reactions of hindered stilbenes 1 and 5 with Br 2/CDCl 3 were examined via NMR experiments. The experimentally observed carbocations were also studied computationally by GIAO-DFT and by NICS.

From dioxin to drug lead--the development of 2,3,7,8-tetrachlorophenothiazine.

Polychlorinated dibenzo-p-dioxins are persistent environmental pollutants. The most potent congener, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), causes a wasting syndrome and is a potent carcinogen and immunosuppressant in the rat at high doses. However, low doses cause opposite effects to some of those observed at higher doses, resulting in chemoprevention, stimulation of the immune system, and longevity in experimental animals. The new TCDD analogue, 2,3,7,8-tetrachlorophenothiazine (TCPT), was developed to take advantage of the low-dose effects of dioxins that have potential application as therapeutics. Its development marked a deviation from the traditional scope of phenothiazine drug design by deriving biological effects from aryl substituents. TCPT was synthesized in three steps. The key ring-closing step was performed utilizing a Buchwald-Hartwig amination to provide TCPT in 37% yield. Its potency to induce CYP1A1 activity over 24 h was 370 times lower than that of TCDD in vitro. The elimination half-life of the parent compound in serum was 5.4 h in the rat and 2.7 h in the guinea pig, compared to 11 and 30 days, respectively, for TCDD. These initial findings clearly differentiate TCPT from TCDD and provide the basis for further studies of its potential as a drug lead.

The first intermediates in the bromination of bicyclo[3.3.1]nonylidenebicyclo[3.3.1]nonane, combination of experiments and theoretical results.

Bicyclo[3.3.1]nonylidenebicyclo[3.3.1]nonane (1) and adamantylideneadamantane (Ad=Ad) are two caged olefins with closely related structures at the double bond. Both compounds react instantaneously with Br2 in chlorinated hydrocarbon solvents to give mixtures of olefin-Br2 aggregates identified as the 1:1 pi-complex and bromonium tribromide, bromonium pentabromide ion pairs. The stoichiometry, formation constants and the electronic spectra of all the species present at equilibrium (pi-complex and bromonium ions), obtained by addition of bromine to alkene 1, have been determined in 1,2-dichloroethane at 25 degrees C and compared with the values that characterize the corresponding aggregates arising from Ad=Ad. The absence of the two bridging CH2 groups in 1 significantly affects all the formation constants. Moreover, at variance with Ad=Ad, olefin 1 reacts with bromine to give, depending on reagent concentration, a substitution product. DFT (B3LYP) and ONIOM computations of 1:1 Br2-olefin complexes for 1 and Ad=Ad confirm that the association energy is larger for the complex 1-Br2. The higher stability of this species seems to be correlated to the greater IP of 1 with respect to Ad=Ad which is able to compensate the reduced polarizability. The experimental value of the formation constant found for the complex 1-Br2, 643 vs 289 M(-1) further supports the primary role exerted by dispersion interactions in alkene-Br2 pi-complexes.

Primary measures for reduction of PCDD/F in Co-combustion of lignite coal and waste: effect of various inhibitors.

Co-combustion of coal and waste in power plants poses both environmental and economic challenges, especially because of the high polychlorinated dibenzo-p-dioxin and furan (PCDD/F) emissions from solid waste. In this study, we performed a series of experiments focusing on the prevention of PCDD/F formation by the use of various inhibitors added to the fuel before combustion. A mixture of lignite coal, solid waste, and poly(vinyl chloride) (PVC) was thermally treated in a laboratory-scale furnace at 400 degrees C. Twenty different additives were investigated at a level of 10 wt% of the total fuel during the experiments. We have divided them into four general groups according to their chemical nature: metal oxides, N-containing compounds, S-containing compounds, and N- and S-containing compounds. The resulting values showed a significant reduction of PCDD/F levels when N- and S-containing compounds were used as additives to the fuel. Principle component analysis (PCA) was used to illustrate the effect of the 20 different inhibitors on the congener patterns emitted. As a result, the most effective inhibitors for PCDD/F formation in flue gases were determined to be (NH4)2SO4 and (NH4)2S2O3; they are inexpensive and nontoxic materials. Both compounds can suppress the formation of toxic compounds such as PCDD/Fs by more than 98-99%, and the most toxic PCDD/F congeners were not detectable in most of the samples. Thus, these compounds were also studied as a lower percentage of the fuel. (NH4)2SO4 resulted in a greater than 90% reduction of PCDD/F even when composing only 3% of the fuel combusted. However, less than 5% (NH4)2S2O3 resulted in far weaker inhibition. The PCDD/F homologue distribution ratio for samples with varying percentages of (NH4)2SO4 and (NH4)2S2O3 was also investigated. Higher percentages of the inhibitors produced a lower percentage of lower chlorinated PCDDs. The opposite effect was found for PCDFs.

Selective adsorption of polychlorinated dibenzo-p-dioxins and dibenzofurans by the zeosils UTD-1, SSZ-24, and ITQ-4.

Zeosils are microporous solids with a pure silica framework. Due to their hydrophobic properties, zeosils are ideal host materials for the adsorption of hydrophobic guest molecules. We tested zeosils with different pore diameters (UTD-1, SSZ-24 and ITQ-4 as well as CIT-5) for the selective adsorption of the polychlorinated dibenzo-p-dioxins and dibenzofurans. This group of highly toxic substances contains 210 congeners that possess similar chemical properties, but differ in their size and shape. In the experiment, polychlorinated dibenzo-p-dioxins and dibenzofurans were extracted from fly ash of a waste incinerator, adsorbed on amorphous silica, then thermally desorbed and flushed over a sequential arrangement of the zeosils at elevated temperature by a stream of nitrogen. ITQ-4 with the smallest pore diameter was placed first, followed by SSZ-24 and, finally, by UTD-1 with the largest pore diameter. After the experiment, the zeosils were analysed for their contents of the different congeners. The results show that the sorption of the congeners occurs selectively and that it is governed by the size and the shape of the dioxin molecules, which in turn depend on the number of chlorine atoms and the pattern of chlorine substitution (regioisomers). Geometrical reasoning as well as molecular dynamics calculations on the zeosil structures and on the dioxin molecules were helpful in rationalising the results. This work represents an especially complex case of the molecular sieving effect and may lead to a selective on-line monitoring of the concentrations of dioxin molecules in waste gases of industrial combustion processes. The size- and shape-selective sorption of dioxin molecules may also bear some resemblance to the molecular recognition process that occurs in nature at the aryl hydrocarbon receptor.