Are They Linear,Bent, or Cyclic? Quantum Chemical Investigation of the Heavier Group 14 and Group 15 Homologues of HCN and HNC

The singlet potential‐energy surface (PES) of the system involving the atoms H, X, and E (the (H, X, E) system) in which X=N–Bi and E=C–Pb has been explored at the CCSD(T)/TZVPP and BP86/TZ2P+ levels of theory. The nature of the X? E bonding has been analyzed with charge‐ and energy‐partitioning methods. The calculations show that the linear isomers of the nitrogen systems lin ‐HEN and lin ‐HNE are minima on the singlet PES. The carbon compound lin ‐HCN (HCN=hydrogen cyanide) is 14.9 kcal mol^?1 lower in energy than lin ‐HNC but the heavier group 14 homologues lin ‐HEN (E=Si–Pb) are between 64.8 and 71.5 kcal mol^?1 less stable than the lin ‐HNE isomers. The phosphorous system (H, P, E) exhibits significant differences concerning the geometry and stability of the equilibrium structures compared with the nitrogen system. The linear form lin ‐HEP of the former system is much more stable than lin ‐HPE . The molecule lin ‐HCP is the only minimum on the singlet PES. It is 78.5 kcal mol^?1 lower in energy than lin ‐HPC , which is a second‐order saddle point. The heavier homologues lin ‐HPE , in which E=Si–Pb, are also second‐order saddle points, whereas the bent ‐HPE structures are the global minima on the PES. They are between 10.3 (E=Si) and 36.5 kcal mol^?1 (E=Pb) lower in energy than lin ‐HEP . The bent ‐HPE structures possess rather acute bending angles H‐P‐E between 60.1 (E=Si) and 79.7° (E=Pb). The energy differences between the heavier group 15 isomers lin ‐HEX (X=P–Bi) and the bent structures bent ‐HXE become continuously smaller. The silicon species lin ‐HSiBi is even 3.1 kcal mol^?1 lower in energy than bent ‐HBiSi . The bending angle H‐X‐E becomes more acute when X becomes heavier. The drastic energy differences between the isomers of the system (H, X, E) are explained with three factors that determine the relative stabilities of the energy minima: 1) The different bond strength between the hydrogen bonds H? X and H? E. 2) The electronic excitation energy of the fragment HE from the X ²Π ground state to the ⁴Σ^? excited state, which is required to establish a E≡X triple bond in the molecules lin ‐HEX . 3) The strength of the intrinsic X? E interactions in the molecules. The trends of the geometries and relative energies of the linear, bent, and cyclic isomers are explained with an energy‐decomposition analysis that provides deep insight into the nature of the bonding situation.     

Synthesis and characterization of Pd nanoparticle-modified magnetic Sm<Subscript>2</Subscript>O<Subscript>3</Subscript>–ZrO<Subscript>2</Subscript> as effective multifunctional catalyst for reduction of 2-nitrophenol and degradation of organic dyes

In this study, Pd nanoparticle-modified magnetic Sm₂O₃–ZrO₂ material (Pd–Fe₃O₄–Sm₂O₃–ZrO₂) as multifunctional catalyst was fabricated and used for catalytic reduction of 2-nitrophenol compound, degradation of methylene blue and rhodamine B dyes, which are toxic pollutants. The magnetic material was used for the first time as a catalyst for the reduction and degradation studies. Pd nanoparticle-modified magnetic Sm₂O₃–ZrO₂ catalyst was prepared using the deposition–precipitation methods and were characterized by X-ray diffraction, scanning electron microscopy, atomic absorption spectrometry, Raman spectroscopy and BET surface analyzer. The Pd nanoparticle-modified magnetic Sm₂O₃–ZrO₂ material can lead to high catalytic activity for the reduction of 2-nitrophenol and degradation of rhodamine B and methylene blue with >?95% conversion within ~?2 and 80 s even when the content of Pd in it is as low as 5.8 wt%.     

Synthesis and characterization of CeO<Subscript>2</Subscript>–graphene composite

The synthesis and characterization of graphite oxide (GO), graphene (GS), and the composites: GS–CeO₂ and GO–CeO₂ are reported. This synthesis was carried out by mixing aqueous solutions of CeCl₃·7H₂O and GO, which yields the oxidized composite GO–CeO₂. GO–CeO₂ was hydrothermally reduced with ethylene glycol, at 120 °C, yielding the reduced composite GS–CeO₂. GO, GS ,and the composites with CeO₂ were characterized by CHN, TG/DTG, BET, XRD, SEM microscopy, FTIR, and Raman spectroscopy. The estimation of crystallite size of CeO₂ anchored on GO and on GS by Raman, XRD, and SEM agreed very well showing diameters about 5 nm. The role of particles of CeO₂ coating carbon sheets of GO and GS was discussed.     

Comparison of sorption of Pb<Superscript>2+</Superscript> and Cd<Superscript>2+</Superscript> on Kaolinite clay and polyvinyl alcohol-modified Kaolinite clay

Kaolinite clay obtained from Ubulu-Ukwu, Delta state in Nigeria was modified with polyvinyl alcohol (PVA) reagent to obtain PVA-modified Kaolinite clay adsorbent. Scanning Electron Microscopy (SEM) of the PVA-modified adsorbent suggests that Kaolinite clay particles were made more compact in nature with no definite structure. Modification of Kaolinite clay with PVA increased its adsorption capacity for 300 mg/L Pb²⁺ and Cd²⁺ by a factor of at least 6, i.e., from 4.5 mg/g to 36.23 mg/g and from 4.38 mg/g to 29.85 mg/g, respectively, at 298 K. Binary mixtures of Pb²⁺ and Cd²⁺ decreased the adsorption capacity of Unmodified Kaolinite clay for Pb²⁺ by 26.3% and for Cd²⁺ by 0.07%, respectively. In contrast, for PVA-modified Kaolinite clay, the reductions were up to 50.9% and 58.5% for Pb²⁺ and Cd²⁺, respectively. The adsorption data of Pb²⁺ and Cd²⁺ onto both Unmodified and PVA-modified Kaolinite clay adsorbents were found to fit the Pseudo-Second Order Kinetic model (PSOM), indicating that adsorption on both surfaces was mainly by chemisorption and is concentration dependent. However, kinetic adsorption data from both adsorbent generally failed the Pseudo-First order Kinetic model (PFOM) test. Extents of desorption of 91% Pb²⁺ and 94% Cd²⁺ were obtained, using 0.1 M HCl, for the Unmodified Kaolinite clay adsorbent. It was found that 99% Pb²⁺ and 97% Cd²⁺, were desorbed, for PVA-modified Kaolinite clay adsorbents within 3 min for 60 mg/L of the metal ions adsorbed by the adsorbents.     

Synthesis and photocatalytic activity of mesoporous SiO<Subscript>2</Subscript>–TiO<Subscript>2</Subscript>

Mesoporous SiO₂–TiO₂ was synthesized by the sol–gel method using Si(OC₂H₅)₄, Ti(OC₂H₅)₄, and stearyltrimethylammonium chloride. By using acetylacetone as the capping agent of Ti(OC₂H₅)₄, homogeneous SiO₂–TiO₂ composite was obtained. Spherical mesoporous SiO₂–TiO₂ was also synthesized by the sol–gel method using W/O emulsion under microwave irradiation. The specific surface area of these mesoporous SiO₂–TiO₂ materials decreased when the Ti/Si molar ratio was higher than 0.1, which indicated that Ti was homogeneously distributed in mesoporous SiO₂ matrix at Ti/Si ≦ 0.1. The photocatalytic activity of mesoporous SiO₂–TiO₂ materials was investigated by the degradation of methylene-blue in water under UV light irradiation. Mesoporous SiO₂–TiO₂ was effective for the adsorption–decomposition of methylene-blue.     

Comparison of the Active Species in the RF and Microwave Flowing Discharges of N<Subscript>2</Subscript> and Ar–20 %N<Subscript>2</Subscript>

We report a detailed comparison between RF and microwave (HF) plasmas of N₂ and Ar–20 %N₂ as well as in the corresponding afterglows by comparing densities of active species at nearly the same discharge conditions of tube diameter (5–6 mm), gas pressure (6–8 Torr), flow rate (0.6–1.0 slm) and applied power (50–150 W). The analysis reveals an interesting difference between the two cases; the length of the RF plasma (~25 cm) is measured to be much longer than that of HF (6 cm). This ensures a much longer residence time (10^?2 s) of the active species in the N₂ RF plasma [compared to that (10^?3 s) of HF], providing a condition for an efficient vibrational excitation of N₂(X, v) by (V–V) climbing-up processes, making the RF plasma more vibrationally excited than the HF one. As a result of high V–V plasma excitation in RF, the densities of the vibrationally excited N₂(X, v > 13) molecules are higher in the RF afterglow than in the HF afterglow. Destruction of N₂(X, v) due to the tube wall is estimated to be very similar between the two system as can be inferred from the γ_v destruction probability of N₂(X, v > 3–13) on the tube wall (2–3 × 10^?3 for both cases) obtained from a comparison between the density of N₂(X, v > 3–9) in the plasmas to that of the N₂(X, v > 13) in the long afterglows. Interestingly enough, densities of N-atoms and N₂(A) metastable molecules in the afterglow regions, however, are measured to be very similar with each other. The measured lower density of N₂ ⁺ ions than expected in the HF afterglow is rationalized from a high oxygen impurity in our HF setup since N₂ ⁺ ions are very sensitive to oxygen impurity .     

Liquid-phase synthesis and physicochemical properties of xerogels,nanopowders and thin films of the CeO<Subscript>2</Subscript>–Y<Subscript>2</Subscript>O<Subscript>3</Subscript> system

Low-agglomerated xerogels, ultrafine oxide powders with particle sizes of 12–20 nm, and uniform thin films with particle sizes of 8–14 nm are prepared in the CeO₂–Y₂O₃ system using liquid-phase low-temperature methods, namely via coprecipitation of hydroxides and cocrystallization of salts, sol—gel technology. A comparative characterization of the prepared xerogels and nanopowders is performed using a set of physicochemical analytical methods. A dependence of phase composition, microstructure, and particle size on synthetic parameters is elucidated.     

Preparation of SiO<Subscript>2</Subscript>–ZrO<Subscript>2</Subscript> xerogel and its application for the removal of organic dye

SiO₂–ZrO₂ xerogel was prepared via a sol–gel method followed by ambient pressure drying. The xerogel was characterized by X-ray diffraction, thermal analysis, fourier transform infrared spectroscopy, scanning electron microscopy, and nitrogen adsorption/desorption analysis. The results showed that the SiO₂–ZrO₂ xerogel was amorphous and possessed a three-dimensional network structure with a narrow distribution of pore size. Its specific surface area reached up to 525.6?m²/g after 600?°C heat treatment, with a pore volume of 1.16?cm³/g and an average pore size of 8.5?nm. In order to explore the potential application of the SiO₂–ZrO₂ xerogel for the removal of organic dyes, its adsorption capacity was studied by removal of Rhodamine B (RhB) from aqueous solution through batch experiments. The results showed that the adsorption process of RhB onto SiO₂–ZrO₂ xerogel was slightly promoted under acidic conditions and significantly inhibited under strong alkaline conditions. And adsorption equilibrium can be achieved in 30?min. The kinetic data of the adsorption were analyzed using pseudo-first-order and pseudo-second-order models. The results indicated that the pseudo-second-order model described the adsorption mechanism better. The sorption behavior was evaluated by Langmuir and Freundlich isotherm models. The results suggested that the Langmuir model could accurately describe the experimental data and the adsorption capacity q_max was 177.7?mg/g. Thermodynamic analysis revealed that the adsorption of RhB onto the SiO₂–ZrO₂ xerogel was both spontaneous and exothermic in nature. Thus, the as-prepared SiO₂–ZrO₂ xerogel might be used as an adsorbent for wastewater treatment, especially for the removal of dyes.

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Retention studies of 2′-2′-difluorodeoxycytidine and 2′-2′-difluorodeoxyuridine nucleosides and nucleotides on porous graphitic carbon: Development of a liquid chromatography–tandem mass spectrometry method

The development of a method for the separation of 2′-2′-difluorodeoxycytidine (gemcitabine, dFdC), 2′-2′-difluorodeoxyuridine (dFdU) and their mono-, di- and triphosphates using a porous graphitic carbon column (Hypercarb), without ion-pairing agent, is described. The retention of dFdC and dFdU could be controlled with an organic modifier (acetonitrile, CH₃CN) and the retention of the anionic nucleotides with an eluting ion (bicarbonate). Separation of all analytes was achieved using a 0–25 mM ammonium bicarbonate gradient in CH₃CN–H₂O (15:85, v/v). Under these conditions, however, very long re-equilibration times were required. Injection of an acidic solution (100 μL 10% formic acid in H₂O, v/v; 2.65 M) after running a gradient directly restored the separation capabilities of the column. Still, separation between the analytes slowly deteriorated over a period of months. These problems were solved by preconditioning the column with a pH buffered hydrogen peroxide (H₂O₂) solution (0.05% H₂O₂ in CH₃CN–H₂O (15:85, v/v), pH 4) before starting an analytical run. The oxidation of the stationary phase with H₂O₂ prevented its slow reduction, which most likely caused the decreasing retention times. The analytes were detected using tandem mass spectrometry.     

Particulate sol–gel synthesis and optical and electrical properties of CeO<Subscript>2</Subscript>/TiO<Subscript>2</Subscript> nanocomposite

CeO₂/TiO₂ nanocomposite was synthesized by particulate sol–gel method. The X-ray diffractogram shows the presence of cubic CeO₂ and anatase TiO₂ in the composite. The high resolution scanning electron micrographs reveal the nanoparticulate nature of the prepared composite. The composite absorbs UV light and exhibits near-band gap emission corresponding to TiO₂ and deep level emission due to crystal defects. The Nyquist plot displays two semicircular arcs indicating the material heterogeneity. The physicochemical characteristics of the synthesized nanocomposite are in favour of its application as an ingredient of sunscreen formulations; under UV light the photocatalytic activity of CeO₂/TiO₂ composite, tested through the degradation of rhodamine B, is very much less than that by pristine anatase TiO₂. Reduced adsorption of moisture by the nanocomposite is a possible reason for the observed very low photocatalytic activity.     

Review of the thermodynamic and transport properties of EuBr<Subscript>2</Subscript>–RbBr binary system

Differential Scanning Calorimetry was used to study phase equilibrium in EuBr₂–RbBr binary system. It was established that this system includes two eutectics and three stoichiometric compounds. First of them, Rb₂EuBr₄, decomposes peritectically at 778 K. Second one, RbEuBr₃, undergoes the solid–solid phase transition at 732 K and melts incongruently at 852 K. Third compound, RbEu₂Br₅, melts congruently at 888 K. The composition and temperature values of eutectics were determined as x(EuBr₂) = 0.316; T _eut = 776 K and x(EuBr₂) = 0.797; T _eut = 859 K. Mixing enthalpy was measured by direct calorimetry on the whole composition range. The minimum of the mixing enthalpy occurs around the composition x(EuBr₂) ≈ 0.4. The electrical conductivity of liquid mixtures was also investigated over the whole composition range and measured down to temperatures below solidification. The specific conductance (liquid phase) plotted against the mole fraction of EuBr₂ shows a broad minimum at x(EuBr₂) ~ 0.6. The activation energy for conductivity changes with temperature. Results obtained are discussed in terms of possible complex formation.     

Hydrothermal synthesis and thermodynamic properties of 2ZnO · 3B2O3 · 3H2O

The important zinc borate of 2ZnO · 3B₂O₃ · 3H₂O has been synthesized and characterized by means of chemical analysis, XRD, FT-IR, and DTA–TG techniques. The molar enthalpies of solution of H₃BO₃(s) in HCl · 54.561H₂O, of ZnO(s) in the mixture of HCl · 54.561H₂O and calculated amount of H₃BO₃, and of 2ZnO · 3B₂O₃ · 3H₂O(s) in HCl · 54.604H₂O were measured, respectively. With the use of the standard molar enthalpies of formation for ZnO(s), H₃BO₃(s), and H₂O(l), the standard molar enthalpy of formation of ?(5561.7 ± 4.9) kJ · mol^?1 for 2ZnO · 3B₂O₃ · 3H₂O(s) was obtained. Thermodynamic properties of this compound were also calculated by a group contribution method.     

Structure and thermal decomposition of Cs<Subscript>2</Subscript>HPO<Subscript>4</Subscript> · 2H<Subscript>2</Subscript>O

The thermal transformations of disubstituted cesium orthophosphate crystal hydrate under heating in air up to 400°C have been studied. The dehydration process occurs in two stages with the loss of 0.6 water molecules at 60?100°C and 1.4 water molecules at 100?160°C. Anhydrous Cs₂HPO₄ is stable up to 300°C and is completely converted into cesium pyrophosphate Cs₄P₂O₇ at 330°C. The structure of Cs₂HPO₄ · 2H₂O has been determined. The compound crystallizes in monoclinic space group P2₁/c and has the unit cell parameters a = 7.4761(5) Å, b = 14.2125(8) Å, c = 7.9603(6) Å, β = 116.914(5)°, V = 754.20(9) ų, and Z = 4 at?123°C. An earlier unknown polymorph of Cs₄P₂O₇ has been found. According to X-ray powder diffraction data, hexagonal space group Р6₃ has been proposed for the formed pyrophosphate.     

Oligomers of β2- and of β3-Homoproline: What are the Secondary Structures of β-Peptides Lacking H-Bonds?

To study the role of H-bonds in stabilizing β-peptidic secondary structures, we have synthesized β-oligopeptides (up to the octadecamer 12 ) consisting of β²- and β³-homoproline, i.e., β-peptides lacking amide protons. The enantiomer purity of the building block β²-homoproline (nipecotic acid, 4 ) was determined by HPLC analysis of the N-(2,4-dinitrophenyl) derivative 5 on a Chiralcel-OD column (cf. Fig. 2). The CD spectra of the all-(S)-β²- and all-(S)-β³-HPro-containing β-peptides display novel and intensive CD patterns which may be indicative of a secondary structure (cf. Fig. 3). It is noteworthy that a distinct CD pattern was observed with the β³-HPro derivatives containing as few as three residues ( 7a ). The crystal structure of a N-deprotected β³-HPro-tripeptide 7c is presented (cf. Figs. 4 and 5), and a model for the structure of β-peptides consisting of β³-HPro is discussed (cf. Figs. 6 and 7).     

Preparation of ZrO<Subscript>2</Subscript>–SiO<Subscript>2</Subscript> mixed oxide by combination of sol–gel and alcohol-aqueous heating method and its application in tetrahydrofuran polymerization

Without using any acid or base catalyst, complexing agent or zirconium alkoxides, ZrO₂–SiO₂ mixed oxide with the ZrO₂ content of 50 mol% was prepared by combination of sol–gel and alcohol-aqueous heating method using zirconyl nitrate and tetraethoxysilane as starting materials. The structural and surface acidic properties were characterized by FT-IR, XRD, NH₃-TPD and pyridine adsorption FTIR. Compared with another mixed oxide with the same ZrO₂ content prepared by mechanical grinding, the obtained ZrO₂–SiO₂ mixed oxide was homogeneously mixed in molecular level. The existed Zr–O–Si hetero-linkages strongly retarded the ZrO₂ particle growth. The obtained mixed oxide maintained amorphous phase until it was calcined at 1,173 K for 3 h when crystallization of tetragonal zirconia took place. NH₃-TPD and pyridine adsorption FTIR showed that both Brønsted and Lewis acidity were largely developed in the mixed oxide and most of the acidic sites belonged to the medium acidity. Because of the existence of abundant medium acidity, the mixed oxide showed catalytic activity for tetrahydrofuran polymerization. Furthermore, the produced poly tetramethylene ether glycol had moderate average molecular weight around 2,000. Neither the pure oxides nor the mixed oxide prepared by the mechanical grinding presented catalytic activity for this reaction.     

The synthesis of a lanthanum metal–organic framework and its sensitivity electrochemical detection of H<Subscript>2</Subscript>O<Subscript>2</Subscript>

A lanthanum metal–organic framework, [La(BTC)(H₂O)(DMF)] (H₃BTC = 1, 3, 5-benzenetricarboxylic acid), was synthesized under mild hydrothermal conditions. The synthesized [La(BTC)(H₂O)(DMF)] was characterized by scanning electron microscopy in combination with energy dispersive X-ray spectroscopy (SEM/EDS), transmission electron microscopy (TEM), X-ray diffraction (XRD), and fourier transform infrared spectroscopy (FT-IR). Its electrochemical properties and electrocatalytic activity towards H₂O₂ reduction in acidic media were studied by cyclic voltammetry (CV) and amperometric current–time response. The [La(BTC)(H₂O)(DMF)] modified electrode shows good electrochemical behavior and performs well electrocatalytic activity towards hydrogen peroxide (H₂O₂) reduction at ca. ?0.7 V. The modified electrode displays a linear range from 5 μM to 2.67 mM and a limit of detection of 0.73 μM to H₂O₂. The [La(BTC)(H₂O)(DMF)] modified electrode also possesses good selectivity and stability. Thus, [La(BTC)(H₂O)(DMF)] will be a promising material for non-enzymatic H₂O₂ sensor.     

Proton conductivity and spectral data of Cs<Subscript>2</Subscript>HPO<Subscript>4</Subscript> · 2H<Subscript>2</Subscript>O

The Cs₂HPO₄ · 2H₂O single crystals synthesized from an aqueous solution containing equimolar amounts of H₃PO₄ and Cs₂CO₃ were studied by impedance and IR spectroscopy, X-ray diffraction analysis, and differential scanning calorimetry (DSC). The IR spectra were analyzed in accordance with the structural data, and the absorption bands were assigned. The proton conductivity was studied at temperatures in the range 20–250°C. The conductivity of dehydrated Cs₂HPO₄ was low, ~10^–5–10^–9 S cm^–1 at 90–250°C with an activation energy of conductivity E _a = 1.1 eV at 130–250°C. The processes determining the character of the temperature dependence of conductivity were consistent with the DSC and thermogravimetry data. According to these data, dehydration of the crystalline hydrate Cs₂HPO₄ · 2H₂O starts at 60°C and occurs in three stages, forming Cs₂HPO₄ · 1.5H₂O below 100°C; anhydrous Cs₂HPO₄ at t > 160°C, which is stable up to 300°C; and Cs₄P₂O₇ above 330°C.     

The character of interactions of CO,H<Subscript>2</Subscript>, and CH<Subscript>3</Subscript>OH with the surface of γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and Cu-Zn oxide catalyst

The thermal desorption of CO, H₂, and CH₃OH from the surface of Katalco-58 industrial catalyst for the synthesis of methanol and γ-Al₂O₃ was studied. Weak interaction of the gases with the surface of samples was observed over the temperature range 75–400°C. The desorption of the gases obeyed the second-order Wigner-Polyani equation. The desorption energies of the gases were calculated. The mechanism of dimethyl ether synthesis was studied.     

Synthesis and properties of siRNAs containing 5′-amino-2′,5′-dideoxy-2′α-fluororibonucleosides

Short interfering RNAs (siRNAs) containing P3′→N5′ phosphoramidate linkages were successfully synthesized by introducing 2′-deoxy-2′-fluororibonucleoside and 5′-amino-2′,5′-dideoxy-2′α-fluororibonucleoside in succession. It was found that the introduction of 5′-amino-2′,5′-dideoxy-2′α-fluororibonucleosides into siRNAs improved the nuclease-resistant properties of the siRNAs without loss of their silencing efficacy.