Switching of global minima of novel germylenic reactive intermediates via halogens (X): C2GeH2 vs. C2GeHX at ab initio and DFT levels

For 30 C₂GeHX germylenic isomers, one cyclic structure, X-germacyclopropenylidene, and three acyclics are considered, which include: ethynyl-X-germylene, X-vinylidenegermylene, and (X-ethynyl)germylene (X = H, F, Cl, and Br). The global minimum among six isomeric C₂GeH₂ (where X = H), is found to be cyclic, aromatic, singlet germacyclopropenylidene. In contrast, among the 24 corresponding halogermylenes, C₂GeHX (where X = F, Cl, and Br), the global minima switch to acyclic, singlet ethynylhalogermylenes, at eight reasonably high ab initio and DFT levels. The direct resonance interaction between X and the divalent center Ge in the singlet acyclic ethynylhalogermylene structures, is claimed to justify switching of the calculated global minima in the halo derivatives. GIAO-NICS calculations indicate that the X-germacyclopropenylidene isomer is more aromatic for X = H than X = F, Cl, or Br. The angle ∠XGeC bending potential energy curves show the singlet and triplet ethynylgermylene crossing at ≈146°, for X = H.     

Ultrasound-assisted emulsification microextraction using low density solvent for analysis of toxic nitrophenols in natural waters

An ultrasound-assisted emulsification microextraction (USAEME) based on low-density solvents was successfully applied for the extraction and pre-concentration of four toxic nitrophenols in water samples. The extracted analytes were analyzed by high-performance liquid chromatography-UV detection. The important parameters influencing the extraction efficiency were studied and optimized utilizing two different optimization methods: one variable at a time (OVAT) and central composite design (CCD). The results showed that the emulsification process can be completed in a few seconds using low-density solvents, but almost 10–20?min is necessary for high-density solvents. Under the optimum conditions (extraction solvent, 1-octanol; extraction solvent volume, 40?µL; sample pH, 3.0; salt concentration, 20% (w/v) NaCl; extraction temperature, 40 (±3)°C), limits of detection of the method were in the range of 0.25 to 1?µg?L^?1 and the repeatability and reproducibility of the proposed method, expressed as relative deviation, varied in the range of 2.2–4.2% and 4.7–6.9%, respectively. Linearity was found to be in the range of 1 to 200?µg?L^?1 and the preconcentration factors (PFs) were between 77 and 175. The relative recoveries of the four nitrophenols from water samples at spiking level of 10.0?µg?L^?1 were in the range of 92.0 to 115.0%.     

Homogeneous Liquid–Liquid Microextraction for Determination of Organochlorine Pesticides in Water and Fruit Samples

A fast and effective preconcentration method for extraction of organochlorine pesticides (OCPs) was developed using a homogeneous liquid–liquid extraction based on phase separation phenomenon in a ternary solvent (water/methanol/chloroform) system. The phase separation phenomenon occurred by salt addition. After centrifugation, the extraction solvent was sedimented in the bottom of the conical test tube. The OCPs were transferred into the sedimented phase during the phase separation step. The extracted OCPs were determined using gas chromatography–electron capture detector. Several factors influencing the extraction efficiency were investigated and optimized. Optimal results were obtained at the following conditions: volume of the consolute solvent (methanol), 1.0 mL; volume of the extraction solvent (chloroform), 55 μL; volume of the sample, 5 mL; and concentration of NaCl, 5 % (w/v). Under optimal conditions, the preconcentration factors in the range of 486–1,090, the dynamic linear range of 0.01–100 μg L^?1, and the limits of detection of 0.001–0.03 μg L^?1 were obtained for the OCPs. Using internal standard, the relative standard deviations for 1 μg L^?1 of the OCPs in the water samples were obtained in the range of 4.9–8.6 % (n = 5). Finally, the proposed method was successfully applied for extraction and determination of the OCPs in water and fruit samples.     

Halogen switching of azacarbenes C2NH ground states at ab initio and DFT levels

Relative stabilities and singlet–triplet energy differences are calculated for 24 C₂NX azacarbenes (where X is H, F, Cl, and Br). Three skeletal arrangements are employed including azacyclopropenylidene, [(imino)methylene]carbene, and cyanocarbene. Halogens appear to alternate the electronic ground states of C₂NH azacarbenes, from triplet to singlet states, at MP3/6‐311++G**, B1LYP/6‐311++G**, B3LYP/6‐311++G**, MP2/6‐311++G**, MP4(SDTQ)/6‐311++G**, QCISD(T)/6‐311++G**, CCSD(T)/6‐311++G**, CCSD(T)/cc‐pVTZ, G1, and G2 levels of theory. The aromatic characters of singlet cyclic azacyclopropenylidenes are measured using GIAO–NICS calculations. Linear correlations are found between the B3LYP/6‐311++G** calculated LUMO–HOMO energy gaps (ΔE_{HOMO ‐ LUMO}) of the singlet carbenes versus their corresponding singlet–triplet energy separations (ΔE). Electrophilic characters are found for all singlet azacarbenes in their addition reactions to alkenes with the highest electrophilicity being exhibited for X = F. © 2008 Wiley Periodicals, Inc. Heteroatom Chem 19:377–388, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20442     

Novel silicon nanorings: Persilacyclacenes at DFT

Structures, energies, and aromatic characters are compared and contrasted for a series of [n]persilacyclacenes with n = 6–12: Si₂₄H₁₂, Si₂₈H₁₄, Si₃₂H₁₆, Si₃₆H₁₈, Si₄₀H₂₀, Si₄₄H₂₂, and Si₄₈H₂₄, respectively, at B3LYP levels (n, number of fused benzenoid rings). These are a brand of silicon nanorings that bear a resemblance to the shortest zig‐zag silicon nanotubes (SiNTs), henceforth referred to as SiNRs. The NBO results show nearly sp²‐hybridization for virtually all Si atoms of our SiNRs. This is in contrast to most reports where sp³‐hybridization is proposed for typical SiNTs. Comparison between the optimized SiNRs and their corresponding planar (polyacenic) forms shows longer bond lengths for the former, due to their curvatures. Except for sterically hindered Si₂₄H₁₂ (n = 6), all even SiNRs (n = 8, 10, and 12), are more aromatic than the odd ones (n = 7, 9, and 11). Such a higher aromaticity is witnessed inside, outside, and on the surface of the scrutinized SiNRs. Also, except for Si₄₄H₂₂ (n = 11), the energy gaps (ΔE_HOMO?LUMO) for the odd set of SiNRs, as well as the even set, appear inversely proportional to their corresponding diameters, per se. Except for the sterically hindered SiNRs with n = 6 or 7, all the even SiNRs enjoy a higher stability (aromaticity) and conductivity for showing lower ΔE_HOMO?LUMO than the odd ones. Evidently, the ideal diameter for persilacyclacenes (SiNRs) studied is from 0.92 to 1.42 nm, corresponding to n = 8–12, respectively. Higher than 1.42 nm causes structural disorders while lower than 0.92 brings about bond localization due to the high‐steric effects. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Taguchi OA16 orthogonal array design for the optimization of cloud point extraction for selenium determination in environmental and biological samples by tungsten-modified tube electrothermal atomic absorption spectrometry

Orthogonal array design (OAD) was applied for the first time to optimize cloud point extraction (CPE) conditions for Se(IV) determination by electrothermal atomic absorption spectrometry (ETAAS) in environmental and biological samples. Selenium was reacted with o-phenylenediamine to form piazselenol in an acidic medium (pH 2). Using Triton X-114, as surfactant, piazselenol was quantitatively extracted into small volume (about 30 μL) of the surfactant-rich phase after centrifugation. Five relevant factors, i.e. surfactant concentration, pH, ionic strength, equilibrium time and temperature were selected and the effects of each factor were studied at four levels on the extraction efficiency of Se(IV) and optimized. The statistical analysis revealed that the most important factors contributing to the extraction efficiency are ionic strength, pH and percent of surfactant. Based on the results obtained from the analysis of variance (ANOVA), the optimum conditions for extraction were established as: pH 6; vial temperature = 50 °C; extraction time = 7 min and 0.3% (w/v) of Triton X-114. The method was permitted to obtain a detection limit of 0.09 ng mL⁻¹ and two linear calibration ranges from 0.6 to 1.0 and 1.0 to 80.0 ng mL⁻¹ Se. The precision (%RSD) of the extraction and determination for the six replicates of Se at 20 ng mL⁻¹ was better than 3.6% and the enrichment factor of 63.5 was achieved. The studied analyte was successfully extracted and determined with high efficiency using cloud point extraction method in water and biological matrices.     

Developments in hollow fiber based liquid-phase microextraction: principles and applications

Hollow fiber liquid-phase microextraction (HF-LPME) offers an efficient alternative to classical techniques for sample preparation and preconcentration. Features include high selectivity, good enrichment factors, and improved possibilities for automation. HP-LPME relies on the extraction of target analytes from aqueous samples into a supported liquid membrane (SLM) sustained in the pores of the wall of a porous hollow fiber, and then into an acceptor phase (that can be aqueous or organic) in the lumen of the hollow fiber. After extraction, the acceptor solution is directly subjected to a chemical analysis. HP-LPME can be performed in either the 2- or 3-phases mode. In the 2-phase mode, the organic solvent is present both in the porous wall and inside the lumen of the hollow fiber. In the 3-phase mode, the acceptor phase can be aqueous and this results in a conventional 3-phase system compatible with HPLC or capillary electrophoresis. Alternatively, the acceptor solution is organic and this represents a 3-phase extraction system with two immiscible organic solvents that is compatible with all common analytical instruments. In HP-LPME methods based on the use of SLMs, the mass transfer occurs by passive diffusion, and high extraction yields as well as efficient extraction kinetics are obtained by applying a pH gradient. In addition, active transport can be performed by using carrier or applying an electrical potential across the SLM. Due to high analyte preconcentration, excellent sample clean-up, and low consumption of organic solvent, HF-LPME has a large application potential in areas such as drug analysis and environmental monitoring. This review focuses on the fundamentals of extraction principles, technical implementations, and future trends in HF-LPME.

Ab initio and DFT energetics of silylenic X–CNSi (X=H, F, Cl, and Br)

Singlet–triplet energy splitting for 24 silylenic reactive intermediates, X–CNSi (where X=H, F, Cl and Br), are compared and contrasted at 11 levels of theory: B1LYP/6-31++G**, B3LYP/6-31++G**, B1LYP/6-311++G**, B3LYP/6-311++G**, MP3/6-31G*, MP3/6-311++G**, MP2/6-31+G**, MP2/6-311++G**, MP4 (SDTQ)/6-311++G**, QCISD(T)/6-311++G** and CCSD(T)/6-311++G**. Each X-substituted silylenic species may either be singlet (s) or triplet (t), with one of the following three structures: 3-X-2-aza-1-silacyclopropenylidene (1_s-X, 1_t-X); [(X-imino)methylene]silylene (2_s-X, 2_t-X); and X-cyanosilylene (3_s-X, 3_t-X). For all X–CNSi species studied, orders of singlet–triplet energy separations (ΔE_s-t,X), appear as a function of electro-negativity (F>Cl>Br>H). For the six H–CNSi isomers (X=H), stability order is: 3_s-H>1_s-H>2_t-H>3_t-H>2_s-H>1_t-H. Likewise, stability order for the six isomers with X=F, is: 3_s-F>3_t-F>1_s-F>1_t-F>2_s-F>2_t-F. For X=Cl, the order of stability is: 3_s-Cl>1_s-Cl>3_t-Cl>2_t-Cl>1_t-Cl>2_t-Cl. Finally, the order of stability for six isomers of Br–CNSi is: 3_s-Br>3_t-Br>1_s-Br>2_s-Br>2_t-Br>1_t-Br. The lowest energy minimum, among all 24 species scrutinized, appears to be the singlet acyclic 3_s-X. Triplet silylene 2_t-H is suggested to be more stable than its corresponding 2_s-H at MP3, MP2 and DFT levels of theory. Comparisons between relative stabilities; multiplicities and geometrical parameters of 1–3 are discussed.