Investigation of substituent effects of chalcones by 13C n.m.r. spectroscopy

¹³C chemical shifts for 23 para- and meta-substituted chalcones of the types 1 and 2 have been determined. The aromatic shieldings are compared with previous results for other aromatic derivatives. Correlations of the ¹³C chemical shifts of vinyl carbons and carbonyl carbons as well as ring carbons with Hammett σ parameters, π electron densities and the reactivity parameters of Swain and Lupton provide a consistent picture of electronic effects transmitted through the carbon framework of the compounds studied.     

Syntheses and properties of aromatic polyamides and polyimides based on N-phenyl-3,3-bis[4-(p-aminophenoxy) phenyl] phthalimidine

N-Phenyl-3,3-Bis[4-(p-aminophenoxy)phenyl] phthalimidine ( II ) was used as a monomer with various aromatic dicarboxylic acids and tetracarboxylic dianhydrides to synthesize polyamides and polyimides, respectively. The diamine II was derived by a nucleophilic substitution of N-phenyl-3,3-bis(4-hydroxyphenyl) phthalimidine with p-chloronitrobenzene in the presence of K₂CO₃ and then hydro-reduced. Polyamides IV _a-g having inherent viscosities of 0.55–1.64 dL/g were prepared by the direct polycondensation of the diamine II with various aromatic diacids using triphenyl phosphite and pyridine as condensing agents. All the aromatic polyamides were amorphous and readily soluble in various polar solvents such as N,N-dimethylacetamide (DMAc), N,N-dimethylformamide, dimethylsulfoxide, and N-methyl-2-pyrrolidone (NMP). Transparent and flexible films of these polymers could be cast from the DMAc solutions. These aromatic polyamides had glass transition temperatures in the range of 293–319°C and 10% weight loss occurred up to 480°C. The polyimides were synthesized from diamine II and various aromatic dianhydrides via the two-stage procedure that included ring-opening polyaddition in DMAc to give poly(amic acid)s, followed by thermal or chemical conversion to polyimides. Most of the aromatic polyimides obtained by chemical cyclization were found to be soluble in NMP, m-cresol, and o-chlorophenol. These polyimides showed almost no weight loss up to 500°C in air or nitrogen atmosphere. © 1994 John Wiley & Sons, Inc.     

Mechanistic aspects on molecular structure formation of polymeric networks from diisocyanates with amidine compounds

Polymeric networks are produced by step‐growth polyaddition and co‐polyaddition reactions of 1‐ethylimidazoline in combination with various diisocyanates. Five aromatic, two aliphatic diisocyanates and a polyurethane prepolymer are used as particular reactant in N,N‐dimethylformamide as solvent at room temperature. Obviously, 1‐ethylimidazoline can serve as trifunctional monomer, which enables a crosslinking reaction with diisocyanates. Molecular structure elements of the polymeric networks were studied by solid state 13C‐NMR spectroscopy revealing that detailed molecular structure formations are determined whether aromatic or aliphatic diisocyanates are used. Quantum chemical calculations were used as supporting method to elucidate the complex reaction cascades. Hence, it can be shown that beside 3:1 stoichiometric structures 2:1 based structures are formed as well. These structures are observed as kinetically controlled products only when aromatic diisocyanate monomers are used. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 977–985     

N(a)-Demethylseredamin,ein neues Alkaloid aus Rauwolfia sumatrana; absolute Konfiguration von Seredamin. 140. Mitteilung über Alkaloide

From Rauwolfia sumatrana Jack (Apocynaceae) the two new indole alkaloids serpentinine and N_(a)-demethylseredamine were isolated. By chemical and spectroscopic evidence and by correlation experiments the latter was shown to have structure 2 . the exact position of the aromatic methoxyl group was established by an investigation of the brominated derivative 12 . The aromatic ORD. Cotton effect of N_(a)-acetyl-N_(a)-demethyl-seredamine ( 5 ) and its O-acetyl derivative 4 as well as the ketone Cotton effect of purpeline ( 9 ) suggest that all the alkaloids of the seredamine group have the same absolute stereochemistry as ajmaline ( 1 ). This was confirmed by a direct chemical correlation of seredamine ( 7 ) with ajmaline.     

The Chemical Behavior of Multibridged [2n] Cyclophanes

Multibridged [2_n] cyclophanes comprise a novel class of aromatic compounds, in which two benzene rings are connected by three up to a maximum of six ethano bridges. Compared to classical nonannelated arenes, their most distinct chemical property is the ease with which, particularly the higher-bridged homologs, take part in addition reactions (Diels-Alder additions, hydrogenations, and ionic additions). On the other hand, the typical regenerative behavior of aromatic molecules is not fully suppressed in the [2_n]cyclophanes as evidenced by such typical electrophilic aromatic substitution reactions as bromination, Friedel-Crafts acylation, and nitration. Besides these reactions in which the benzene rings are either destroyed or retained, reactions at and with the ethano bridges, e.g. their cleavage, isomerization, and functionalization can also occur.     

Syntheses and properties of aromatic polyamides and polyimides derived from 9,9-bis[4-(p-aminophenoxy)phenyl]fluorene

9,9-Bis[4-(p-aminophenoxy)phenyl]fluorene ( II ) was used as a monomer with various aromatic dicarboxylic acids and tetracarboxylic dianhydrides to synthesize polyamides and polyimides, respectively. The diamine II was derived by a nucleophilic substitution of 9,9-bis(4-hydroxyphenyl)fluorene with p-chloronitrobenzene in the presence of K₂CO₃ and then hydro-reduced. Polyamides IV _a-g having inherent viscosities of 0.73–1.39 dL/g were prepared by the direct polycondensation of the diamine II with various aromatic diacids using triphenyl phosphite and pyridine as condensing agents. All the aromatic polyamides were amorphous and readily soluble in various polar solvents such as N,N-dimethylacetamide, N,N-dimethylformamide, dimethylsulfoxide, and N-methyl-2-pyrrolidone. Transparent and flexible films of these polymers could be cast from the DMAc solutions. These aromatic polyamides had glass transition temperatures in the range of 283–309°C and 10% weight loss occurred up to 460°C. The polyimides were synthesized from diamine II and various aromatic dianhydrides via the two-stage procedure that included ring-opening poly-addition in DMAc to give poly(amic acid)s, followed by thermal or chemical conversion to polyimides. The poly(amic acid)s had inherent viscosities of 0.62–1.78 dL/g, depending on the dianhydrides. Most of the aromatic polyimides obtained by chemical cyclization were found to be soluble in NMP. These polyimides showed almost no weight loss up to 500°C in air or nitrogen atmosphere. © 1993 John Wiley & Sons, Inc.     

13C NMR studies of reduced porphyrin compounds. Aromatic delocalization pathways

¹³C NMR spectra of ms-tetraphenylchlorins, new aminoalkyl- and hydroxy-pyrroline substituted ms-tetra-phenylchlorins, and ms-tetraphenylisobacteriochlorins are presented and discussed. Significant changes in the chemical shifts of the α-pyrrole, α-pyrroline and meso skeletal carbons are found in chlorins and isobacteriochlorins in comparison to porphyrins. In contrast, the chemical shifts of the β-pyrrole carbons are almost unaffected by the structural modifications in chlorin and isobacteriochlorin. The chemical shifts of the α-pyrrole carbons in the various chlorins and isobacteriochlorins are strongly affected by the substitutents of the pyrroline ring, or by the introduction of an additional pyrroline ring in isobacteriochlorins. The results show that most of the electron density is concentrated in the unreduced part of the molecule, i.e. in the pyrrole rings and, especially, on the α-pyrrole carbons and is transferred or removed through the aromatic pathway by substituents on the pyrroline ring. These observations are supporting evidence that the α-pyrrole, α-pyrroline and meso-carbons are in the aromatic pathway and favour the proposal of a 16 atom dianion as the preferred delocalization pathway in chlorins and isobacteriochlorins.     

An Amphoteric Switch to Aromatic and Antiaromatic States of a Neutral Air‐Stable 25π Radical

Ever since the discovery of the trityl radical, isolation of a stable and neutral organic radical has been a synthetic challenge. A (4n+1)π open‐shell configuration is one such possible neutral radical but an unusual state between aromatic (4n+2)π and antiaromatic (4n)π electronic circuits. The synthesis and characterization of an air‐ and water‐stable neutral 25π pentathiophene macrocyclic radical is now described. It undergoes reversible one‐electron oxidation to a 24π antiaromatic cation and reduction to a 26π aromatic anion, thus confirming its amphoteric behavior. Structural determination by single‐crystal X‐ray diffraction studies revealed a planar configuration for the neutral radical, antiaromatic cation, and aromatic anion. In the solution state, the cation shows the highest upfield chemical shift ever observed for a 4nπ system, while the anion adhered to aromatic nature. Computational studies revealed the delocalized nature of the unpaired electron as confirmed by EPR spectroscopy.     

The reactive states of ions: Antiaromatic seven-membered N-substituted-1H-azepines and isomeric aromatic substrates

Comparison of the electron impact and chemical ionization mass spectra of N-substituted-1H-azepines and isomeric aromatic substrates reveals significant differences. The data indicate that the substituted azepincs examined are stable to electron impact at low temperatures and retain their antiaromatic 8π electron seven-membered cyclic system prior to their fragmentation. The electron impact data are supported by low electron-volt and metastable scan techniques. Electron impact or chemical ionization induced transformation of azepines to aromatic substrates similar to the thermal transformation of neutral analogues is not observed at low temperatures. Chemical ionization mass spectra of N-ethoxycarbonyl-1H-azepines and N-phenylcarbamates were complicated owing to thermal decomposition reactions at high temperatures.     

Substituted rod-like aromatic polyamides: Synthesis and structure-property relations

The synthesis and structure-property relations of a number of novel substituted paralinked aromatic homopolyamides and copolyamides are described. The synthesis of the polyamides was carried out by polycondensation of activated N,N'-bis-(trimethylsilyl) substitued aromatic diamines and aromatic diacid chlorides. In order to improve the solubility and to lower melting temperatures, novel arylsubstituted terephthalic acids moieties, such as p-terphenyl-2,5-dicarboxylic acid and o-terphenyl-2,5-dicarboxylic acid, were used in combination with substituted and noncoplanar diamines. Depending on the chemical structure, polyamides with very high solubility (up to 40% w/w) in polar aprotic solvents such as N,N-dimethylacetamide without the addition of inorganic salts were obtained. Lyotropic liquid crystalline behavior was observed for the first time in polyamides which contain noncoplanar biphenylene units.     

Nuclear magnetic resonance spectra of carbanions—VI. Cumyl-, α-methylbenzyl- and benzyl carbanions

The proton magnetic resonance spectra of the three title carbanions have been observed in THF with potassium as a counter ion. The ortho-protons in the α-methylbenzyl carbanion are nonequivalent at room temperature. This shows that the α-carbon in this carbanion is in the near-sp² configuration. The aromatic proton chemical shifts of the benzylpotassium obtained here are at higher shielding than those of benzyllithium reported previously by Sandel and Freedman. This seems to arise from the different ionic nature of the bonds between carbon and metal in the carbanions.     

Computational NMR Spectra of o-Benzyne and Stable Guests and Their Hemicarceplexes

The incarceration of o-benzyne and 27 other guest molecules within hemicarcerand 1 , as reported experimentally by Warmuth, and Cram and co-workers, has been studied by density functional theory (DFT). The ¹H NMR chemical shifts, rotational mobility, and conformational preference of the guests within the supramolecular cage were determined, which showed intriguing correlations of the chemical shifts with structural parameters of the host–guest system. Furthermore, based on the computed chemical shifts reassignments of some NMR signals are proposed. This affects, in particular, the putative characterization of the volatile benzyne molecule inside a hemicarcerand, for which our CCSD(T) and KT2 results indicate that the experimentally observed signals are most likely not resulting from an isolated o-benzyne within the supramolecular host. Instead, it is shown that the guest reacted with an aromatic ring of the host, and this adduct is responsible for the experimentally observed signals.     

Determination of aromaticity indices of thiophene and furan by nuclear magnetic resonance spectroscopic analysis of their anilides

A series of m‐ and p‐substituted anilides of benzoic acid, 2‐thienoic acid, and 2‐furoic acid were prepared and their ¹H and ¹³C nmr spectroscopic characteristics were examined. In general, good correlations were observed between the chemical shifts of proton and carbon signals of the acyl aromatic rings and the Hammett σ. Plots of the chemical shift values of the carbonyl carbons of the benzanilides against those of the 2‐thienamides and 2‐furamides gave an excellent correlation and the values of the slopes are 0.79 and 0.52, respectively, in dimethyl sulfoxide‐d₆. The slopes could be considered as a set of aromaticity index.     

Influence of NMR Data Completeness on Structure Determinations of Homodimeric Proteins

The structure determination of homodimeric proteins by NMR using conventional NOESY experiments is still challenging due to the degeneracy of the chemical shifts in the identical monomers, which causes ambiguity in the NOE assignments. Residues involved in the interface between two monomers provide essential intermolecular NOEs for the structure determinations of homodimeric proteins. Hence NMR data, such as NOE peak lists and chemical shift assignments of these interface residues, play a crucial role for the successful structure determination of homodimeric proteins. This paper extends our previous report (Lin, Y.‐J.; Kirchner, D. K.; Güntert, P. J. Magn. Reson.­ 2012 , 222, 96) and investigates the influence of incomplete NOESY peak lists combined with incomplete ¹H chemical shift assignments of the interface residues on the structure determination of homodimeric proteins using the program CYANA. Data incompleteness was simulated by random omission of both NOESY cross peaks and interface ¹H chemical shifts. Our results for three proteins with different percentages of interface residues reveal that the algorithm can tolerate about 40–50% NOESY peak omission with complete interface chemical shift assignments, which indicates that partial NOESY peak omission does not cause severe problems when the interface chemical shifts are completely assigned. Combining NOESY peak omission with incomplete interface chemical shift assignments, the tolerance for interface chemical shift omission decreases with the extent of omitted NOESY peaks. The tolerance for unassigned interface side chain, methyl and aromatic chemical shifts is affected more strongly by NOESY peak omission than that for the omission of general interface ¹H chemical shifts including the backbone. In general about 10–30% peaks omission is tolerated in conjunction with missing chemical shift assignments. If more NOESY peaks are omitted calculations gradually become unstable and tend not to tolerate any missing interface chemical shifts. A large amount of omitted NOESY peaks, for instance 30% omission in our calculations, could decrease the tolerance for missing aromatic or methyl interface ¹H chemical shifts to as few as 2–4 missing chemical shifts, suggesting that complete aromatic and methyl ¹H chemical shift assignments are important when the NOESY peak data is significantly incomplete. Finally, for homodimeric proteins with a low percentage of interface residues, our results reveal that the omission of NOESY peaks, even at an extent of only 10%, can result in no tolerance against the omission of interface ¹H chemical shifts, suggesting that the completeness of both interface ¹H chemical shift assignments and NOESY peaks are important for the successful structure determination of proteins with a small homodimer interface.     

D6h‐Au42 Isomer: A Golden Aromatic Toroid Involving Superatomic π‐Orbitals that Follow the Hückel (4n+2)π rule

Recently, it has been shown that the superatom concept is intimately connected to relevant tools of great chemical significance, such as the Lewis structure model and the VSEPR theory, which has been employed to understand hybridized and dimeric‐like molecules. This suggests a potential rational construction of superatomic clusters mimicking more complex structures. Here, we extend another well‐employed concept to the superatomic clusters, to construct a novel Au₄₂ isomer with resemblance to cyclic aromatic molecules. It is shown that the Hückel (4n+2)π rule is ready to be applied, predicting aromatic behavior latterly supported by the favorable evaluation of the induced shielding cone formation. The D₆h isomer of Au₄₂ described here exhibits inherent characteristics mimicking aromatic hydrocarbon rings, displaying π‐superatomic orbitals and related properties. This new cluster is the first member of the superatomic clusters family to exhibit an aromatic π‐electron system.     

Impact of gas chromatography and mass spectrometry combined with gas chromatography and olfactometry for the sex differentiation of Baccharis articulata by the analysis of volatile compounds

The Baccharis genus has more than 400 species of aromatic plants. However, only approximately 50 species have been studied in oil composition to date. From these studies, very few take into consideration differences between male and female plants, which is a significant and distinctive factor in Baccharis in the Asteraceae family. Baccharis articulata is a common shrub that grows wild in south Brazil, northern and central Argentina, Bolivia, Paraguay and Uruguay. It is considered to be a medicinal plant and is employed in traditional medicine. We report B. articulata male and female volatile composition obtained by simultaneous distillation–extraction technique and analyzed by gas chromatography with mass spectrometry. Also, an assessment of aromatic differences between volatile extracts was evaluated by gas chromatography with olfactometry. The results show a very similar chemical composition between male and female extracts, with a high proportion of terpene compounds of which β‐pinene, limonene and germacrene D are the main components. Despite the chemical similarity, great differences in aromatic profile were found: male plant samples exhibited the strongest odorants in number and intensity of aromatic attributes. These differences explain field observations which indicate differences between male and female flower aroma, and might be of ecological significance in the attraction of pollinating insects.     

Determination of aromaticity indices of thiophene and furan by nuclear magnetic resonance spectroscopic analysis of their phenyl esters

A series of m‐ and p‐substituted phenyl benzoates, 2‐thienoates, and 2‐furoates were prepared and their ¹H and ¹³C nmr spectroscopic characteristics were examined. In general, good correlations were observed between the chemical shift values of protons and carbons of the acyl aromatic rings and the Hammett σ. Plots of the chemical shift values of the carbonyl carbons of the benzoates against those of the 2‐thienoates and 2‐furoates gave an excellent correlation and the values of the slopes are 0.85 and 0.75, respectively, in dimethyl sulfoxide‐d₆ and 0.90 and 0.78, respectively, in chloroform‐d. The values could be considered as a set of aromaticity indices.     

Magnetic resonance study of diastereomeric interactions of nitroaromatic sulfxides with alcohols in solution

The synthesis of optically active (1R,2R)- and (1S,2R)-2-deuterio-2,3-dihydro-2-methyl-6-nitrobenzothiophene-1-oxide (3 and 4) is described. Several racemic aryl methyl carbinols in the presence of 3 or 4 give different chemical shifts for enantiomeric CH, CH₃ and OH protons. These chemical shift differences are larger than those observed in analogous sulfoxide–alcohol pairs without nitro groups. Quinine and cinchonidine cause observable non-equivalence of methylene and aromatic protons in racemic 4. The magnitude of the diastereomeric shift difference is sensitive to temperature, solvent, concentration, nature of the π system and steric effects.     

Synthesis of soluble and thermally stable polyimide from new diamine bearing N-[4-(9H-carbazol-9-yl)phenyl] formamide pendent group

A new aromatic diamine monomer, N-(4-(9H-carbazol-9-yl)phenyl)-3,5-diaminobenzamide, was successfully prepared in four steps using carbazole as starting material and polymerized with three aromatic tetracaboxylic acid dianhydrides via the conventional two-stage synthesis including the polyaddition and chemical cyclodehydration to produce a series of the aromatic polyimides. The polyimides were characterized by FT-IR, ¹H NMR, and ¹³C NMR spectroscopy, differential scanning calorimetric (DSC) and thermo gravimetric analysis (TGA) analysis. The polyimides with inherent viscosities in the range of 0.38–0.46 dL/g showed excellent solubility in various solvents such as N-methyl-2-pyrrolidinone (NMP), N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), pyridine and dioxane. DSC showed the glass transition temperatures (T_g) in the range of 277–288 °C. TGA showed that all polymers were stable, with 10% weights loss recorded above 524 °C in air atmosphere. Preliminary tests on films of the polyimides indicate that the materials are brittle.     

Pyrolysis/gas chromatography/mass spectrometry of asphaltene fractions

Two n-heptane-precipitated asphaltene samples, characterized by elemental analysis and nuclear magnetic resonance spectrometry, were fractionated according to relative molecular size by gel permeation chromatography (GPC). Both the whole asphaltene samples and their fractions were analysed by pyrolysis/gas chromatography/mass spectrometry. The data obtained from the pyrograms (average side-chain length, aromaticity index, sulphur compounds vs. aliphatic compounds, presence of SO₂ and CO₂) demonstrated that, in the case of asphaltenes, GPC fractionation results in the separation of different chemical structures ranging from lower molecular mass, highly aromatic and polar compounds to higher molecular mass, less polar and aromatic compounds.