The substitution of isoelectronic B-N units for C-C units in aromatic hydrocarbons produces novel heterocycles with structural similarities to the all-carbon frameworks, but with fundamentally altered electronic properties and chemistry. Since the pioneering work of Dewar some 50 years ago, the relationship between B-N and C-C and the wealth of parent all-carbon aromatics has captured the imagination of organic, inorganic, materials, and computational chemists alike, particularly in recent years. New applications in biological chemistry, new materials, and novel ligands for transition-metal complexes have emerged from these studies. This review is aimed at surveying activity in the area in the past couple of decades. Its organization is based on ring size and type of the all-carbon or heterocyclic subunit that the B-N analog is derived from. Structural aspects pertaining to the retention of aromaticity are emphasized, along with delineation of significant differences in physical properties of the B-N compound as compared to the C-C parent.Key words: boron-nitrogen heterocycles, aromaticity, organic materials, main-group chemistry.
There are numerous examples of animals or plants that synthesize extracellular high-performance skeletal biocomposites consisting of a matrix reinforced by nano sized crystalline domains. Cellulose and chitin are classical examples of these reinforcing elements, which occur as whisker-like microfibrils that are biosynthesized and deposited in a continuous fashion. In many cases, this mode of biogenesis leads to crystalline microfibrils that are almost defect-free, and whose axial physical properties therefore approach those of perfect crystals. During the last decade we have attempted to mimic biocomposites by blending cellulose or chitin whiskers from different sources with polymer matrices. Aqueous suspensions of such nano crystals can be prepared by acid hydrolysis of the substrate. The object of this treatment is to dissolve away regions of low lateral order so that the water-insoluble, highly crystalline residue may be converted into a stable suspensoid by subsequent vigorous mechanical shearing action. The resulting nano crystals occur as rod-like particles or whiskers, whose dimensions depend on the nature of the substrate. They are typically a few hundred nm long and between 5 and 20 nm in diameter. Starch can also be used as a source for the production of nano crystals. The constitutive nano crystals appear as platelet-like nano particles with a length ranging between 20 and 40 nm, a width ranging between 15 and 30 nm, and a thickness ranging between 5 and 7 nm. Since the first announcement of using cellulose whiskers as a reinforcing phase, they have been used extensively as model fillers in several kinds of polymeric matrices, including synthetic and natural ones. Casting mixtures of polysaccharide nano crystals and lattices led to the production of nano composite materials with drastically enhanced mechanical properties, especially at T > Tg of the matrix, by virtue of the formation of a whiskers network, even when the whisker volume fraction was only a few percent. The formation of this rigid network, resulting from strong interactions between whiskers, was assumed to be governed by a percolation mechanism. This hydrogen-bonded network induced a thermal stabilization of the composite up to 500 K, the temperature at which polysaccharides start to decompose. Any factors that perturb the formation of this percolating network directly affect the reinforcing effect of polysaccharide nano crystals. In addition to some practical applications, the study of these nano composite materials can help researchers understand such physical properties as the geometric and mechanical percolation effect.Key words: nano composites, polysaccharide, polymer, cellulose, nano crystal.Il existe de nombreux exemples d'animaux ou de plantes qui synthétisent des biocomposites haute performance constitués d'une matrice renforcée par des domaines cristallins de taille nano métrique. La cellulose et la chitine sont des exemples classiques de ces éléments de renfort qui se présentent sous la forme de microfibrilles biosynthétisées et arrangées de manière régulière. Ce mode de biogenèse conduit généralement à des microfibrilles cristallines pratiquement exemptes de défauts présentant donc des propriétés physiques axiales proches de celles du cristal parfait. Au cours de la dernière décennie, nous avons tenté de mimer ces biocomposites en mélangeant des whiskers de cellulose ou de chitine de différentes sources avec des matrices polymères. Des suspensions aqueuses de ces nano cristaux peuvent être préparées par hydrolyse acide du substrat. L'objectif de ce traitement est de dissoudre les régions de faible cohésion de sorte que le résidu insoluble fortement cristallin soit transformé en une suspension stable par une action mécanique de cisaillement vigoureuse. Les nano cristaux résultants se présentent sous forme de particules en bâtonnet ou whiskers, dont les dimensions dépendent de la nature du substrat. Leur longueur typique est de l'ordre de 100 nm et le diamètre varie entre 5 et 20 nm. L'amidon peut également être utilisé comme source pour la préparation de nano cristaux. Ces nano cristaux se présentent sous forme de plaquettes de longueur comprise entre 20 et 40 nm, de largeur située entre 15 et 30 nm et d'épaisseur comprise entre 5 et 7 nm.
Glycosaminoglycans (GAGs) are an important class of carbohydrates that serve critical roles in blood clotting, tissue repair, cell migration and adhesion, and lubrication. The variable sulfation pattern and iduronate ring conformations in GAGs influence their polymeric structure and nature of interaction. This study characterizes several heparin-like GAG disaccharides and tetrasaccharides using NMR and molecular dynamics simulations to assist in the development of parameters for GAGs within the GLYCAM06 force field. The force field additions include parameters and charges for a transferable sulfate group for O- and N-sulfation, neutral (COOH) forms of iduronic and glucuronic acid, and Δ4,5-unsaturated uronate (ΔUA) residues. ΔUA residues frequently arise from the enzymatic digestion of heparin and heparin sulfate. Simulations of disaccharides containing ΔUA reveal that the presence of sulfation on this residue alters the relative populations of 1 H 2 and 2 H 1 ring conformations. Simulations of heparin tetrasaccharides containing N-sulfation in place of N-acetylation on glucosamine residues influence the ring conformations of adjacent iduronate residues.
Topological indices are numerical parameters of a graph that characterize its topology and are usually graph invariant. In a QSAR/QSPR study, physicochemical properties and topological indices such as Randić, atom–bond connectivity (ABC), and geometric–arithmetic (GA) indices are used to predict the bioactivity of chemical compounds. Graph theory has found a considerable use in this area of research. In this paper, we study different interconnection networks and derive analytical closed results of the general Randić index (R α (G)) for α = 1, 1 2 , –1, − 1 2 only, for dominating oxide network (DOX), dominating silicate network (DSL), and regular triangulene oxide network (RTOX). All of the studied interconnection networks in this paper are motivated by the molecular structure of a chemical compound, SiO 4 . We also compute the general first Zagreb, ABC, GA, ABC 4 , and GA 5 indices and give closed formulae of these indices for these interconnection networks.
UV-vis and NMR spectroscopic techniques were employed to demonstrate the ability of the synthetic macrocyclic host cucurbituril (CB7) to solubilize and stabilize widely used fungicides and anthelmintic drugs of the benzimidazole family in water, namely, albendazole (ABZ), carbendazim (CBZ), thiabendazole (TBZ), fuberidazole (FBZ), and the parent benzimidazole (BZ). CB7 binds the protonated forms of these guests very strongly (e.g., K = 2.6 × 10 7 L/mol for ABZ) but their neutral forms significantly more weakly (e.g., K = 6.5 × 10 4 L/mol for ABZ), which reflects a complexation-induced increase of their pK a values by 2.6 units for ABZ, 2.5 units for CBZ, 4.0 units for TBZ, 3.8 units for FBZ, and 3.5 units for BZ. The absolute drug solubilities increased upon complexation from 0.003 to 0.300 mmol/L for ABZ, from 0.160 to 1.12 mmol/L for CBZ, from 0.110 to 1.11 mmol/L for TBZ, and from 0.25 to 0.75 mmol/L for FBZ (for BZ, the solubility enhancement was found to be insignificant). Complexation by CB7 further improves the photostability of the drugs and alters their photophysical properties.
A numerical quantity that characterizes the whole structure of a graph is called a topological index. The concept of Randić (R α ), atom−bond connectivity (ABC), and geometric−arithmetic (GA) topological indices was established in chemical graph theory based on vertex degrees. In this paper, we study a carbon nanotube network that is motivated by the molecular structure of a regular hexagonal lattice and determine R α , ABC, and GA indices for this important class of networks.
Certain organisms survive low temperatures using a range of physiological changes including the production of antifreeze proteins (AFPs), which have evolved to adsorb to ice crystals. Several of these proteins have been purified and shown to also inhibit the crystallization of clathrate hydrates. They have been found to be effective against structure II (sII) hydrates formed from the liquid tetrahydrofuran, sI and sII gas hydrates formed from single gases, as well as sII natural gas hydrates using a mixture of three gases, as assessed using a variety of instrumentation including stirred reactors, differential scanning calorimetry, nuclear magnetic resonance, Raman spectroscopy, and X-ray powder diffraction. For the most part, AFPs are equal to or more effective than the commercial kinetic hydrate inhibitor (KHI) polyvinylpyrolidone, even under field conditions where saline and liquid hydrocarbons are present. Enclathrated gas analysis has revealed that the adsorption of AFPs to the hydrate surface is distinct from tested commercial KHIs and results in properties that should make these proteins more valuable in some field applications. Efforts to overcome the difficulties of recombinant protein production are ongoing, but in silico models of AFP adsorption to hydrates may offer the opportunity to design commercial KHIs for hydrocarbon recovery and transport with all the attributes of these AFP ”green inhibitors”, including their benefits for human and environmental safety.
An efficient protocol for the synthesis of 2,4,6-triarylpyridines and 3-(2,6-diarylpyridin-4-yl)-1H-indoles by the one-pot pseudo four component condensation reaction of aldehydes with acetophenones and ammonium acetate in the presence of Ph 3 CCl under neutral and solvent-free conditions has been reported. Mechanistically, it is interesting that trityl chloride by in situ generation of trityl carbocation (Ph 3 C + ) promotes the reaction. In this work, seven products have been reported for the first time.
The one-step pi-extension of corannulene was achieved using a palladium-catalyzed C-H coupling reaction. The X-ray crystal structure and photophysical properties of the thus formed phenanthro[9,10-a]corannulene (1) were investigated, and the structural properties of 1 were examined by density functional theory calculations. In contrast to dibenzo[g,p]chrysene, the most stable structure of 1 was a butterfly-shaped structure, resulting from the bowl-shaped distortion of the corannulene moiety.
This paper gives a brief summary of zethrene chemistry after the pioneering works by Clar, Mitchell, Sondheimer, and Staab et al. In the past a few years, various synthetic methods and stabilizing strategies have been developed, which have made the synthesis of stable, vertically and laterally extended zethrenes practically achievable. One of the most important discoveries for this type of molecule is their open-shell diradical character, which is the origin of their unique optical, electronic, and magnetic properties. The fundamental structure–diradical character–physical property relationships were investigated, and these studies form the basis for the tailored design of more general open-shell singlet diradicaloids with controllable diradical character and physical properties.
An indole ring system is considered as a versatile scaffold in the pharmaceutical field. In this article, comparative QSAR modeling (2D-QSAR, 3D-QSAR; kNN-MFA and CoMSIA) was performed on some Knoevenagel-type cytotoxic indole derivatives to understand the structural requirements for the cytotoxic property of these compounds. The 2D-QSAR model was statistically significant and imparted high predictive ability (n Train = 30; R = 0.917; R A 2 = 0.801; c R p 2 = 0.757; Q 2 = 0.722; n Test = 9; R pred 2 = 0.799). A statistically significant 3D-QSAR kNN-MFA model (both with stepwise forward and simulated annealing model selection method) as well as a 3D-QSAR CoMSIA model was developed to identify the key chemical features associated with enhancing the cytotoxic activities of these indoles. The results suggest that the presence of bulky group in R position can cause better cytotoxic activities. Consequently, substitution with cyano group at X portion and cyano/ester/keto/sulphonyl features at Y position is favourable for the cytotoxicity. However, hydrophobic features in R′ region are unfavourable for the biological activity. The chemical and structural features identified from the study may provide important avenues to modulate the structure of these indoles to a desirable biological end point.
A quantitative description for the interaction of Cu(I) with poly(styrene-co-maleic anhydride) modified with 4-aminopyridine (denoted as CuI/SMI complex) is presented using density functional theory (DFT) and quantum theory of atoms in molecules (QTAIM) approaches. Topological analysis of electron density revealed the existence of effective interactions between Cu(I) ions and the nitrogen in the pyridine ring. Interestingly, the results also showed that there is considerable interaction between Cu(I) and the oxygen of the carbonyl motif in the SMI ligand. Thus, CuI/SMI was examined as a heterogeneous and recyclable catalyst in Hantzsch pyridine synthesis under solvent-free conditions, affording diverse 1,4-dihydropyridines (1,4-DHPs) in excellent yields with relatively short reaction times.
The novel 1,4-bis(2,6-dibromo-4-formylphenoxy)butane ( 3 ), prepared from 3,5-dibromobenzaldehyde ( 1 ), reacted with different hydrazines 4a–4d and active methylene containing compounds 9a–9d to give the corresponding bis(hydrazones) 5a–5d and bis(cinnamonitriles) 11a–11d , respectively. Both bis(2-cyanoacetic acid hydrazide) derivative 5d and bis(thioacrylamide) derivative 11a were taken as synthetic precursors for the synthesis of the target molecules bis(pyridine-2(1H)-thione) derivative 10 and 13a–13c and their bis(2-methylsulfanylpyridine) derivative 14 and 17a–17c . Another synthetic route was designed to prepare the target molecules 14 and 17a–17c in a better yield using pyridine-2(1H)-thione) derivative 15 and 19a–19c . Characterization of the newly prepared compounds via elemental analyses and spectral data are established.
Graph theory plays a vital role in modeling and designing any chemical structure or chemical network. Chemical graph theory helps in understanding the molecular structural properties of a molecular graph. The molecular graph consists of atoms called vertices and chemical bonds between atoms called edges. In this article, we study the chemical graphs of carbon graphite and crystal structure of cubic carbon. Moreover, we compute and give closed formulas of degree-based additive topological indices, mainly the first and second Zagreb indexes, general Randić index, atom bond connectivity index, geometric arithmetic index, fourth atom bond connectivity index, and fifth geometric arithmetic index of carbon graphite denoted by CG(m, n) for t levels, and crystal structure cubic carbon denoted for n levels.
We synthesized thiophene-functionalized pyrene, peropyrene, and teropyrene through a two- or four-fold alkyne annulation reaction promoted by triflic acid. All of the target compounds were fully characterized spectroscopically, and the structure of the peropyrene analogue was unambiguously confirmed by X-ray crystallography. A significant red shift in the absorption and emission properties as a function of extended conjugation was observed by UV–vis and fluorescence spectroscopy. This alkyne annulation strategy is a useful method for the conversion of smaller polycyclic aromatics to larger ones, such as nanographenes.
A reliable understanding of radiolysis processes in supercritical water (SCW) cooled reactors is required to ensure optimal water chemistry control. In this perspective, Monte Carlo track chemistry simulations of the radiolysis of pure, deaerated SCW at 400 degrees C by 2 MeV mono-energetic neutrons were carried out as a function of water density between 0.15 and 0.6 g/cm(3). The yields of hydronium ions (H3O+) formed at early time were obtained based on the G values calculated for the first three generated recoil protons. Combining our calculated G(H3O+) values with a cylindrical track model allowed us to estimate the concentrations of H3O+ and the corresponding pH values. An abrupt, transient, and highly acidic pH response ("acid spikes") was observed at early times around the "native" fast neutron and recoil proton trajectories. This intra-track acidity was found to be strongest at times of less than a few tens to a hundred of picoseconds, depending on the value of the density considered (pH similar to 1). At longer times, the pH gradually increased for all densities, finally reaching a constant value corresponding to the non-radiolytic, pre-irradiation concentration of H3O+, due to the autoprotolysis of water. Interestingly, the lower the density of the water, the longer the time required to reach this constant value. Because many in-core processes in nuclear reactors critically depend on the pH, the present work raises the question whether such highly acidic pH fluctuations, though local and transitory, could promote or contribute to corrosion and degradation of materials under proposed SCW-cooled reactor operating conditions.