Tailoring approach for exploring electron densities and electrostatic potentials of molecular crystals

Experimental and theoretical studies of electron densities and the corresponding derived entities such as electrostatic potentials have been the primary means of understanding the chemical nature and electronic properties of crystalline substances. Conventional crystal calculation methods such as the embedded cluster models are capable of performing calculations on small and medium-sized molecules, while periodic ab initio methods can treat crystals with up to 200 atoms per unit cell. A linear scaling method, viz. the molecular tailoring approach, has recently been developed for obtaining ab initio quality one-electron properties. In the present study, the molecular tailoring approach is employed to generate electron density, electrostatic potential and interaction density maps with the ibuprofen crystal as a test case. The interaction density and electrostatic potential maps produced in the present work succinctly bring out the actual crystalline environment around a given reference molecule by including the interactions with atoms in its neighborhood. The results obtained from the molecular tailoring approach may thus be expected to enhance our understanding of the environment in the crystalline material with reasonably small computational effort.Contribution to the Jacopo Tomasi Honorary Issue     

Asymmetric synthesis of 2,3-dihydrofurans and of unsaturated bicyclic tetrahydrofurans through alpha-elimination and migratory cyclization of silyloxy alkenyl aminosulfoxonium salts. Generation and intramolecular O,Si-bond insertion of chiral disubstituted beta-silyloxy alkylidene carbenes

Treatment of the bis(allylsulfoximine)titanium complexes derived from the beta-methyl-substituted acyclic allylic sulfoximines 13a and 13b with aldehydes gave with high selectivities the corresponding sulfoximine-substituted homoallylic alcohols which were isolated as the silyl ethers 15a-h. Methylation of sulfoximines 15a-h afforded the aminosulfoxonium salts 5a-h which upon treatment with LiN(H)tBu gave in high yields the enantio- and diastereomerically pure silyl-substituted 2,3-dihydrofurans 4a-h. Treatment of the titanium complexes derived from the cyclic allylic sulfoximines 17a, 17b, and ent-17c with p-MeOC(6)H(4)CHO delivered with high selectivities the corresponding sulfoximine-substituted cyclic homoallylic alcohols which were isolated as the silyl ethers 18a, 18b, and ent-18c, respectively. Methylation of sulfoximines 18a, 18b, and ent-18c furnished the aminosulfoxonium salts 8a, 8b, and ent-8c, respectively, whose treatment with LiN(H)t-Bu gave the enantio- and diastereomerically pure fused bicyclic 2,3-dihydrofurans 6a, 6b, and ent-6c, respectively, in good yields. It is proposed that the 1-alkenyl aminosulfoxonium salts 5a-h, 8a, 8b, and ent-8c react with the base under alpha-elimination and formation of the acyclic and cyclic beta-silyloxy alkylidene carbenes 2a-h, 7a, 7b, and ent-7c, respectively, which then undergo a 1,5-O,Si-bond insertion and 1,2-silyl migration. The cyclic aminosulfoxonium salts 8a, 8b, and ent-8c upon treatment with 1,8-diazabicyclo[5.4.0]-7-undecene did not undergo an alpha-elimination but suffered a novel migratory cyclization with formation of the enantio- and diastereomerically pure bicyclic tetrahydrofurans 9a, 9b, and ent-9c, respectively. It is proposed that the 1-alkenyl sulfoxonium salts 8a, 8b, and ent-8c are isomerized to the allylic aminosulfoxonium salts 10a, 10b, and ent-10c, respectively, which then suffer an intramolecular substitution of the (dimethylamino)sulfoxonium group by the silyloxy group followed by a desilylation. The syntheses of the 2,3-dihydrofurans 4a-h, 6a, and 6b and of the tetrahydrofurans 9a and 9b are accompanied by the formation of sulfinamide 16 of >or=98% ee, which can be converted via sulfoxide 28 of >or=98% to the starting sulfoximine 11 of >or=98% ee.     

Facile reduction of aromatic nitro/azido functionality on solid support employing Al/NiCl2·6H2O and Al/NH4Cl: synthesis of pyrrolo[2,1-c][1,4]benzodiazepines

An efficient and mild method for the reduction of aromatic nitro and azido groups on solid support using Al/NiCl₂·6H₂O and Al/NH₄Cl is described. This solid phase reduction technique has been applied towards the synthesis of DNA binding pyrrolo[2,1-c][1,4]benzodiazepine antitumour antibiotics.     

Spectrophotometric determination of palladium(II) with 2-diethylaminoethanethiol hydrochloride and the simultaneous determination of rhodium and palladium

2-Diethylaminoethanethiol hydrochloride is proposed for spectrophotometric determination of palladium(II). The sensitivity of the reaction is 0.0085 smg/cm² and the yellow colored complex shows absorption maxima at 258 mμ and 303 mμ. Color development is slow in the cold but complete after heating for 10–15 min. The optimum pH range is 3.5 to 5.5 and the system adheres to Beer's law between 0.2 and 16.8 p.p.m. of palladium. The average and maximum relative standard deviations were 0.60% and 1.40% respectively. Interferences due to other platinum metals were studied and a procedure is suggested for the simultaneous determination of rhodium and palladium.     

A new kinetic model for bulk polymerization of vinyl chloride based on two-phase hypothesis

A kinetic model has been developed for the bulk polymerization of vinyl chloride using Talamini's hypothesis of two-phase polymerization and a new concept of kinetic solubility which assumes that rapidly growing polymer chains have considerably greater solubility than the thermodynamic solubility of preformed polymer molecules of the same size and so can remain in solution even under thermodynamically unfavourable conditions. It is further assumed that this kinetic solubility is a function of chain length. The model yields a rate expression consistent with the experimental data for vinyl chloride bulk polymerization and moreover is able to explain several characteristic kinetic features of this system. Application of the model rate expression to the available rate data has yielded 2.36 × 10⁸l mol^?1 sec^?1 for the termination rate constant in the polymer-rich phase; as expected, this value is smaller than that reported for homogenous polymerization by a factor of 10–30.     

Synthesis and binding studies of new bis-calix[4]arenes containing aromatic and heteroaromatic units

A series of new bis-calix[4]arenes containing different aromatic and heteroaromatic moieties have been synthesized. The complexing behavior of these bis-calix[4]arenes have been studied towards different metal ions and it has been found that these bis-calix[4]arenes bind silver ions selectively over other metal ions. The complexation has been studied by liquid-liquid extraction and by NMR and IR spectroscopy.     

Synthesis and characterization of monomers and polymers of acrylated phosphonate esters derived from glycerol,D-mannitol,D-sorbitol,pentaerythritol, and dipentaerythritol

Phosphonic acid ester derivatives of glycerol, D -mannitol, D -sorbitol, pentaerythritol, and dipentaerythritol have been synthesized by transacetalation reactions with diethyl 2,2-diethoxyethylphosphonate. These phosphonated derivatives of polyols and carbohydrates have been esterified to from the corresponding methacrylates. All these compounds have been characterized on the basis of their elemental analysis and spectroscopic (infrared including FT–IR, ¹H-, ¹³C-, ³¹P-NMR, and mass) methods. Transacetalation reactions with dialkyl 2,2-dialkoxyethylphosphonate lead to the synthesis of 1,3-dioxane derivatives in the case of D -mannitol, pentaerythritol, and dipentaerythritol, whereas a mixture of both 1,3-dioxane and 1,3-dioxolane derivatives is obtained with D -sorbitol and glycerol. The methacrylates of phosphonylated polyol derivatives show the capacity to dissolve and interact with metal salts such as bismuth bromide and uranyl nitrate. Some of the polymers obtained from these monomers have been characterized on the basis of their spectral and thermal (differential scanning calorimetry) properties.     

Tractable molecular theory of transport of Lennard-Jones fluids in nanopores

We present here a tractable theory of transport of simple fluids in cylindrical nanopores, which is applicable over a wide range of densities and pore sizes. In the Henry law low-density region the theory considers the trajectories of molecules oscillating between diffuse wall collisions, while at higher densities beyond this region the contribution from viscous flow becomes significant and is included through our recent approach utilizing a local average density model. The model is validated by means of equilibrium as well nonequilibrium molecular dynamics simulations of supercritical methane transport in cylindrical silica pores over a wide range of temperature, density, and pore size. The model for the Henry law region is exact and found to yield an excellent match with simulations at all conditions, including the single-file region of very small pore size where it is shown to provide the density-independent collective transport coefficient. It is also shown that in the absence of dispersive interactions the model reduces to the classical Knudsen result, but in the presence of such interactions the latter model drastically overpredicts the transport coefficient. For larger micropores beyond the single-file region the transport coefficient is reduced at high density because of intermolecular interactions and hindrance to particle crossings leading to a large decrease in surface slip that is not well represented by the model. However, for mesopores the transport coefficient increases monotonically with density, over the range studied, and is very well predicted by the theory, though at very high density the contribution from surface slip is slightly overpredicted. It is also seen that the concept of activated diffusion, commonly associated with diffusion in small pores, is fundamentally invalid for smooth pores, and the apparent activation energy is not simply related to the minimum pore potential or the adsorption energy as generally assumed.     

Solution-based chemical synthesis of boehmite nanofibers and alumina nanorods

This article reports an easy chemical method of synthesizing boehmite nanofibers by a modified sol-gel process involving aluminum isopropoxide precursor. Nanorods of gamma-alumina have been successfully prepared after dehydration of the viscous sol at 600 degrees C for 4 h in air. The boehmite nanofibers and gamma-alumina nanorods were characterized by X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy for surface chemistry and functional groups, scanning electron microscopy, high-resolution transmission electron microscopy with selected area electron diffraction, and energy-dispersed spectroscopy for morphology and structure identification. The length of the boehmite nanofibers was found to be more than 10 mum with a crystalline lattice structure. The mechanism of formation of the boehmite nanofibers included the preferential growth along the longitudinal axis due to interaction between the solvent molecules and the surface OH- groups of hydrogen bonds. It is also suggested that the boehmite nanofibers may have formed due to the inherent instability of the planar structure of the boehmite lattice. The diameter of the gamma-alumina nanorods was found to be less than 10 nm with a varying length in the range of 50-200 nm. Boehmite to gamma-Al2O3 transformation was attributed to the loss of water molecules by internal condensation of protons and hydroxyl ions.     

Novel solid-phase extraction and preconcentration technique coupled with ICP-AES for the determination of Cr(III), Ni(II), and Zn(II) in various water samples

Newly synthesized 2-propylpiperidine-1-carbodithioate (2-PPC) was used for the extraction of Cr(III), Ni(II), and Zn(II) from various water samples. In the present investigation, the use of a syringe loaded with sorbent for the separation and enrichment of Cr(III), Ni(II), and Zn(II) prior to their determination by inductively coupled plasma-atomic emission spectrometry (ICP-AES) was proposed to substitute the batch and column techniques. The described method was compared with the column technique with respect to fastness, simplicity, recovery, and risk of contamination. The syringe was loaded with 1.0 g of sorbent in order to retain the analyte elements. Next, 7.0 mL of sample solution (pH 5.0 ± 0.2) was drawn into the syringe in 15 s and discharged over 15 s. Then, an eluent (3.0 M HCl) was drawn into the syringe and ejected back to desorb the analyte elements. At the optimum conditions, the percentage recoveries of Cr(III), Ni(II), and Zn(II) were in the range of 94.50 to 99.62% with a standard deviation (S.D.) of 0.03%. The elements could be concentrated by drawing and discharging several portions of sample successively and eluting only one time. The detailed study of various interferences proved the method to be highly selective. The risk of contamination is less than that with the column technique. The method was successfully applied to the determination of Cr(III), Ni(II), and Zn(II) in spiked and natural water samples. The results obtained are in good agreement with those obtained by the reported methods at the 95% confidence level. The text was submitted by the authors in English.