On the origin of changes in topicity observed in Diels–Alder reactions catalyzed by Ti–TADDOLates

New C₂ symmetric TADDOLs containing different groups at the 2-position of the dioxolane ring have been prepared. The Ti catalysts derived from these have been studied in the Diels–Alder reaction of cyclopentadiene and (E)-2-butenoyl-1,3-oxazolidin-2-one. Substituents at the C-2 position of the dioxolane ring can play an important role in determining the selectivity as well as the nature of the major isomer. This effect is more important for TADDOLs containing bulky aromatic groups such as 3,5-dimethylphenyl- or 1-naphthyl at the -positions. Experimental evidence supports the hypothesis that π–π interactions between aromatic groups at the C-2 and the ones at the -positions are critical in this respect.     

Peculiarities of the excitation energy transfer in europium and terbium aromatic carboxylates and nitrate complexes with sulfoxides: Blocking effect

The influence of modification of the aromatic ligands on the excitation energy transfer to Ln³⁺ ions in europium and terbium carboxylates and nitrates was examined. The luminescence excitation spectra of three groups of the europium and terbium compounds: phenyl-, diphenyl-, triphenylacetates, phenoxyacetates and triphenylpropionates; 1- and 2-naphthylcarboxylates and 2-naphthoxyacetates; lanthanide nitrates with diarylsulfoxides (diphenyl- and dibenzylsulfoxides) and dialkylsulfoxides were investigated. The spectra of adducts of terbium phenylcarboxylates with 1,10-phenanthroline were also analyzed. The effect of the aliphatic bridges, which decouple the π–π- or p–π-conjugation in the ligand, on the energy transfer to Ln³⁺ ions (so-called blocking effect) was investigated. It was shown, that this decoupling leads to significant lowering of the energy of “ligand–metal ion” charge transfer states (LM CTS) in the europium carboxylate salts, just down to 27,800 cm⁻¹ in europium 2-naphthoxyacetate. As a consequence, the probability of the LM CTS participation in the excitation energy dissipation processes increases. A channel of the excitation energy dissipation in the region of 31,750 cm⁻¹ related to ligand electronic transitions was found in the europium and terbium nitrates with sulfoxides. It was demonstrated that a part of the energy absorbed by the aromatic ligand having aliphatic bridge can be emitted as the ligand fluorescence.     

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.     

Guest and host deuterium effects of the large ZFS parameters D of dimethylbenzaldehydes in durene single crystals

The ZFS parameters D of 2,4-, 2,5- and 3,4-dimethylbenzaldehyde-1h₁ and -1d₁ guests in perhydrogenated and perdeuterated durene single crystals are determined by comparing the experimental and calculated resonance curves. It is found that the deuterium substitution of the guest aldehydic group in a given host leads to the decrease of the D values and to the increase of the energy gaps ΔE_T between the zero-point levels of the ³nπ* and ³ππ* states of the guests. On the other hand, the perdeuteration of the host results in the decrease of ΔE_T with a corresponding increase of the D value of a given guest. The D value of 1 cm⁻¹ determined for 2,5-dimethylbenzaldehyde-1h₁ in perdeuterated durene is the lategest ever found for an aromatic carbonyl compound. Correlations between D and ΔE_T indicate that the ZFS parameters D of the guests are determined by contributions from both spin-spin and spin-orbit interactions between the ³nπ* and ³ππ* states. The large guest and host deuterium effects observed on the D values are attributed to the changes of the gaps ΔE_T of the guests.     

Liquid-crystalline behaviour of 4-(ω-aminoalkyloxy)-4'-cyanobiphenyls and their side chain epoxy polymers

Liquid-crystalline side chain polymers were obtained from the ring-opening reaction of epoxy resin with mesogenic amines, 4-(ω-aminoalkyloxy)-4'-cyanobiphenyl. The amines with the alkyl group ranging from five to nine methylene units were synthesized, and were found to exhibit an enantiotropic nematic phase. Polymerization of these amines with ethylene glycol diglycidyl ether at 100°C yielded the polymers of low glass transition and melting temperatures. The mesomorphic properties of the amines and the resulting epoxy polymers are reported.     

Aromaticity strongly affected by substituents in fulvene and heptafulvene as a new method of estimating the resonance effect

Very good dependences of the variation in π-electron delocalisation expressed by the aromatic character of the ring, defined by the geometry-based HOMA model and NICS index in mono 6-substituted fulvene and 8-substituted heptafulvene derivatives on the nature of substituent allows us for defining the substituent through resonance effect (by means of σ_p⁺ and σ_p⁻ scale of the substituent effect). The analysis is based on 35 optimised systems at DFT B3LYP/6-311+G^** level of theory.     

Ring-current aromaticity in open-shell systems

The ab initio ipsocentric approach to calculation and mapping of induced orbital current density is extended to open-shell π systems. ROHF/6-31G^** calculations document the (magnetic) aromaticity of planar 4nπ triplet states of 4π/8π annulenes and isobenzofulvene, and quintet excited-state azulene. An orbital model for currents rationalises the generalised form of Baird’s rules, by which 4n + 2 annulenes are aromatic (antiaromatic) in states of even (odd) total spin (vice versa for 4nπ annulenes). In contrast to geometric criteria, ring-currents predict antiaromaticity for the doublet naphthalene radical anion.     

Aromatic guest inclusion by a tripodal ligand: Fluorescence and structural studies

The newly synthesized simple tripodal ligand tris-[2-(naphthalen-2-yloxy)-ethyl]-amine (L₁) act as a fluorescence signaling system for aromatic guest. It forms inclusion complexes with several electron deficient aromatic compounds. This inclusion phenomenon has been studied by steady-state fluorescence spectroscopy and solid-state structural analysis. Electron-rich L₁ shows dramatic color change and a concomitant quenching of luminescence in solution as well as solid phase when titrated with several other electron deficient aromatic guest molecules. Rather high selectivity towards the picric acid was observed. L₁ simultaneously forms inclusion complex and organic salt co-crystal with the composition [(L₁H⁺) (Pic⁻)]  PicH (PicH = picric acid) when crystallized in the presence of picric acid. In the solid state, it forms a strong π–π, C–Hπ and C–HO type interactions.     

Nickel-catalyzed multicomponent connection of dimethylzinc, alkynes, 1,3-butadiene, aldehydes, and amines

A nickel(0) species catalyzes the five-component connection reaction of dimethylzinc, alkynes, 1,3-butadiene, aldehydes, and amines, furnishing either 1 or 2 selectively in good yields depending on the kinds of amines, aromatic or aliphatic amines, respectively.     

Liquid phase hydrogenation and hydrodenitrogenation of aromatic nitrogen-containing environmental pollutants

Carcinogenic aromatic nitro-compounds are hydrogenated at 80–140 °C in the presence of a silica sol–gel entrapped combined palladium-[Rh(cod)Cl]₂ catalyst to give hydroaromatic amines and nitrogen-free hydrocarbons. The process involves initial transformation of the nitro to an amino function. Further hydrogenation causes denitrogenation and saturation of the aromatic moieties. Using 1-aminonaphthalene as a model substrate reveals simultaneous formation of 1- and 5-aminotetralin. While the former amine is readily converted into tetralin and 1-aminodecalins, the 5-aminotetralin gives, in a slow process, only the aminodecalins. The latter compounds are slowly denitrogenated to decalins. The catalytic hydrogenation of the aromatic compounds is accompanied by NH₃ elimination by which secondary amines are formed in a reversible fashion. The entrapped catalyst is leach-proof and recyclable. However, its catalytic activity in the different steps changes during the recycling. The high activity of the combined catalyst results from synergism between the two different metal nuclei.     

Umbrella inversions of cyclononatetraenylidenes at ab initio and DFT

Barriers of umbrella inversions for non-planar triplet 2-, 3-, 4-, or 5-X-2,4,6,8-cyclononatetraenylidenes, inverting through planar transition states, appear roughly four times lower in energy than their corresponding singlet states, at ab initio and DFT levels (X = H, F, Cl, Br). Relative activation energies for these racemization (), follow electronegativity for both singlet and/or triplet states of 2-X-2,4,6,8-cyclononatetraenylidenes (F > Cl > Br > H). This trend does not hold for species with halogens further away from the carbenic center: 3-, 4-, or 5-X-2,4,6,8-cyclononatetraenylidenes. Frequency calculations show one negative force constant for all planar species (transition states), while they appear positive for non-planar minima.     

Gas chromatographic determination of aromatic amines in water samples after solid-phase extraction and derivatization with iodine II. Enrichment

A procedure for the enrichment of aromatic amines via solid-phase extraction was developed. A HR-P phase based on styrene–divinylbenzene was used for the investigations, generally followed by derivatization with iodine and determination via GC–ECD. The recoveries of 53 aromatic amines in a drinking water matrix at pH 9 were determined. Most anilines showed relative recoveries between 80–120% with relative standard deviations of ≤5% at concentration levels between 10 and 20 μg l⁻¹. The comparison with a wastewater matrix led to similar results. The enrichment procedure was applied to real samples, e.g., samples of ammunition wastewater.     

Dansylation of aromatic, aliphatic, and medicinal carboxylic acid compounds in 1 M Na2CO3 buffer

Dansyl chloride (DNS-Cl) is a sulfonyl chloride compound which is utilized as a fluorescent probe for quantitative analysis or structural studies of complex molecules. The fluorescent emission was sufficiently strong to permit detection of less than 10 μg of the carboxyl compounds studied here. The dansylation of aromatic carboxyl compounds (i.e. aspirin), aromatic primary amines, and aliphatic carboxyl compounds was accomplished in 1 M Na₂CO₃ buffer at pH 11. The fluorescent labeled analytes were then isolated by thin layer chromatography (TLC) or the aqueous mixture was pre-extracted with proline (or glycine) to eliminate back-ground emission originating from the hydrolized or sulfonic acid form of DNS-Cl. Fluorescent labeled analytes are clearly discerned under ultraviolet light. Limits of detection for dansylated carboxyl compounds was 1-5 μg, however for amines it was approximately 1 μg. Dansylation of aromatic primary amines proceeded much faster (15 min) than that of carboxyl compounds (≥1.5 h) at 25 °C. Despite the aqueous solubility of analytes, which ranged from less than 0.004-5.30 mg/ml, the dansylation of carboxyl compounds was effective. Various organic solvents for extracting derivatives from aqueous mixture were evaluated.     

Neue C-spektroskopische untersuchungen zur struktur von allylmetall-verbindungen

High-resolution ¹³C NMR spectra of terminally monodeuterated allyl(earth)-alkalimetal compounds confirm earlier structure assignments: allylmagnesium bromide exists in a σ-covalent structure, which rapidly and reversibly goes over into its “metallomeric” mirror-image; allyllithium forms a distorted π-complex; allylpotassium and allylcaesium have the shape of perfectly symmetrical or slightly asymmetrical π-complexes. No NMR spectroscopic evidence is found in tetrahydrofuran solution for the presence of aggregates and mixed aggregates resulting from dimerization or oligomerization of monomeric allylmetal species.     

Photophysics of inter- and intra-molecularly hydrogen-bonded systems: Computational studies on the pyrrole–pyridine complex and 2(2′-pyridyl)pyrrole

The role of electron and proton transfer processes in the photophysics of hydrogen-bonded molecular systems has been investigated with ab initio electronic-structure calculations. We discuss generic mechanisms of the photophysics of a hydrogen-bonded aromatic pair (pyrrole–pyridine), as well as an intra-molecularly hydrogen-bonded π system composed of the same molecular sub-units (2(2′-pyridyl)pyrrole). The reaction mechanisms are discussed in terms of excited-state minimum-energy paths, conical intersections and the properties of frontier orbitals. A common feature of the photochemistry of these systems is the electron-driven proton transfer (EDPT) mechanism. In the hydrogen-bonded complex, a highly polar charge transfer state of ¹ππ^* character drives the proton transfer, which leads to a conical intersection of the S₁ and S₀ surfaces and thus ultrafast internal conversion. In 2(2′-pyridyl)pyrrole, out-of-plane torsion is additionally needed for barrierless access to the S₁–S₀ conical intersection. It is pointed out that the EDPT process plays an essential role in the fluorescence quenching in hydrogen-bonded aromatic complexes, the function of organic photostabilizers, and the photostability of biological molecules.     

Surface chemistry of nanoporous carbon and the effect of pH on adsorption from aqueous phenol and 2,3,4-trichlorophenol solutions

Granular nanoporous activated carbon prepared from polyacrylonitrile (PAN) was investigated as a means of removing weak aromatic acids from aqueous solutions. This carbon is highly nanoporous, the BET surface area being 544 m²/g with V_tot=0.278 cm³/g, and V_micro=0.266 cm³/g. Aqueous treatment reduced the surface area to 364 m²/g.

Granular nanoporous carbon prepared from PAN contains O and N related surface functional groups. The surface concentration of both oxygen and nitrogen atoms was found by XPS to be 5.3%. Surface groups containing these hetero atoms are responsible for the acid–base character of this carbon in aqueous solutions. The pH_PZC of the carbon is 8.4. The microporous pore network produces a wide hysteresis loop, observed when the granular carbon was studied by continuous titration. This loop, which was found to diminish, but not to disappear completely when the particle size was reduced, is attributed to irreversible hydrolysis of surface esters and/or lactones. The surface concentration of the functional groups titrated by the equilibrium Boehm method is 449.2 μeq./g, of which 112.9 μeq./g have acidic and 336.3 μeq./g have basic character. Within the acidic species three subgroups, namely the regions pK_a<6.37, 6.37K_a<10.25 and 10.25K_a<15.74.

The acid–base behavior influences the adsorption performance of the granular carbon in aqueous solutions of weak aromatic acids, in this case phenol and 2,3,4-trichlorophenol. Both the adsorption capacity and the overall interaction parameter, K (both derived from a fit to the Langmuir equation) depend on the adsorbed species and on the pH. The former is a consequence of the different water solubilities of the solute molecules, while the latter stems from the pH sensitivity of both the surface functional groups and these weak acids. The K values show a sequence pH=3K values indicate different adsorption mechanisms. The effect of pH is more marked for 2,3,4-trichlorophenol.     

Spectroscopic study of harmane in micelles at 77 K using fluorescent probes

Steady-state and time-resolved emission for spectroscopic techniques at 77 K, and molecular orbital calculations using PM3-MOPAC/93 and HAM/3-CI have been used to study the two forms of harmane, the neutral (HN) and the monoprotonated (HH), in different environments. In hydrophobic media, for (HN), four species were determined and in hydrophilic medium, for (HH), we found just one species. The photophysical properties of all these species were determined, and we verified that each one of them displays distinct photophysical properties from one to another. For example, for monomer of (HN), the lowest electronic singlet state S₁ is (π,π^*) and the lowest electronic triplet state T₁ is (π,π^*), due to the phosphorescence lifetime it is t=0.8 s. For the (HH) monomer, the S₁ is (π,π^*) and T₁ is (π,π^*) and the spin–orbital coupling is inefficient. These determinations were used to characterise and to identify the harmane species that is solubilised into the interior of neutral (triton X-100), anionic (dodecyl lithium sulphate) and cationic (hexadecyltrimethyl ammonium bromide) micelles, all of them were prepared under physiological conditions. The results indicated that active species in the interior of the micelles is a hydrogen bond complex between (HN) and micellar environments that is anchored in the aqueous region of micelles.     

Liquid-phase microextraction and GC for the determination of primary, secondary and tertiary aromatic amines as their iodo-derivatives

Presence of iodine in aromatic amines, introduced by their reaction with iodine, and other electron withdrawing substituents such as chlorine and nitro, has been found to afford excellent liquid-phase microextraction (LPME) in toluene and separation by gas chromatography in the determination of primary, secondary and tertiary aromatic amines. The effect is due to decreased basic nature of amines when electronegative substituents are present. Single drop microextraction (SDME) of the amines in 2 μl of toluene and injection of the whole extract into GC, or LPME into 50 μl of toluene and injection of 2 μl of extract, were used. LPME has been found more robust and to give better extraction in shorter period than SDME. In SDME-GC-FID, the average correlation coefficient was 0.9939 and average limit of detection 25 μg l⁻¹ (range 12-61 μg l⁻¹) whereas the corresponding values in LPME-GC-MS were, respectively, 0.9953 and 33 ng l⁻¹ (range 18-60 ng l⁻¹). The method has been applied to determine aromatic amines in river water, dye factory effluents and food dye stuffs. The LPME was found as robust, rugged and simple extraction method.     

Potentiometric studies of binary and ternary complexes of amoxycillin

The formation equilibria for the binary complexes of copper(II), nickel(II), cobalt(II) and zinc(II) with amoxycillin (Amx) and for the ternary complexes M(Arm)(Amx), where Arm refers to aromatic amines, namely 2,2′-dipyridyl or 1,10-phenanthroline, were investigated by a potentiometric technique. The protonation constants of amoxycillin and conditional stability constants of the formed complexes were determined at 25° (=0.1M NaNO₃). Probable mode of chelation with amoxycillin is discussed.     

A study on copper(II)-Schiff base-azide coordination complexes: Synthesis, X-ray structure and luminescence properties of [Cu(L)(N3)]X (L = Schiff bases; X = ClO4, PF6)

Two Schiff bases N,N′-(bis(pyridin-2-yl)benzylidene)propane-1,3-diamine (pbpd) and N,N′-(bis(pyridin-2-yl)formylidene)butane-1,4-diamine (pfbd) have been prepared and used to synthesize copper(II) complexes. Four complexes of the type [Cu(L)(N₃)]X (1–4) [L = pbpd; X = ClO₄ (1); L = pbpd; X = PF₆ (2); L = pfbd; X = ClO₄ (3); L = pfbd; X = PF₆ (4)] have been synthesized and characterized on the basis of microanalytical, spectroscopic, magnetic, electrochemical, luminescence and other physicochemical properties. Two representative complexes of the series, 2 and 3, have been characterized by single crystal X-ray diffraction measurements which reveal that in each complex the copper(II) ion assumes a distorted trigonal bipyramidal environment through coordination of the metal centre by two pyridine N atoms and two imine N atoms of the Schiff base with the fifth position occupied by a N atom of a terminal . They display intraligand ¹(π–π^*) fluorescence at room temperature and intraligand ³(π–π^*) phosphorescence in glassy solutions (MeOH at 77 K). A band (492 nm) observed for the complexes in their solid-state emission spectra is an excimeric emission arising due to an aromatic π–π interaction. Electrochemical electron transfer study reveals Cu^II–Cu^I reduction in methanolic solutions.