A unified orbital model of delocalised and localised currents in monocycles, from annulenes to azabora-heterocycles

Why are some (4n+2)π systems aromatic, and some not? The ipsocentric approach to the calculation of the current density induced in a molecule by an external magnetic field predicts a four‐electron diatropic (aromatic) ring current for (4n+2)π carbocycles and a two‐electron paratropic (antiaromatic) current for (4n)π carbocycles. With the inclusion of an electronegativity parameter, an ipsocentric frontier‐orbital model also predicts the transition from delocalised currents in carbocycles to nitrogen‐localised currents in alternating azabora‐heterocycles, which rationalises the differences in (magnetic) aromaticity between these isoelectronic π‐conjugated systems. Ab initio valence‐bond calculations confirm the localisation predicted by the naïve model, and coupled‐Hartree–Fock calculations give current‐density maps that exhibit the predicted delocalised‐to‐localised/carbocycle–heterocycle transition.     

Spektroskopisches und thermisches Verhalten von K4H2[S2I2O14]

The infrared andRaman spectra of the tittle compound have been recorded and an assignment for the internal vibrations of the H₂S₂I₂O ₁₄ ^4– polyanion is proposed. The thermal behaviour was investigated using TG-, DTA-, IR- and X-ray methods; it is shown that the polyanion breaks down after the release of one mole of H₂O, generating a mixture of -K₂SO₄ and I₂O₅.

     

Two New Antiferromagnetic Nickel(II) Complexes Bridged by Azido Ligands in the Cis Position. Effect of the Counteranion on the Crystal Structure and Magnetic Properties

Two new nickel(II) end-to-end azido-bridged compounds, cis-catena-[NiL(2)(&mgr;-N(3))](n)()(ClO(4))(n)().nH(2)O (1) and [Ni(2)L(4)(&mgr;-N(3))(2)](PF(6))(2) (2), were synthesized and characterized; L is 2-(aminoethyl)pyridine. The crystal structures of 1 and 2 were solved. Complex 1: monoclinic system, space group P2(1)/a, a = 8.637(2) ?, b = 18.9995(7) ?, c = 12.3093(7) ?, beta = 105.92(2) degrees, Z = 4. Complex 2: triclinic system, space group P&onemacr;, a = 9.139(7) ?, b = 10.124(3) ?, c = 12.024(2) ?, alpha = 70.407(14) degrees, beta = 84.19(2) degrees, gamma = 67.67(4) degrees, Z = 1. In the two complexes the nickel atom is situated in a similarly distorted octahedral environment. The two complexes are different; 1 is a one-dimensional helicoidal complex with the two L ligands and the two end-to-end azido bridges in a cis arrangement while complex 2 is a dinuclear system with two end-to-end azido bridges, indicating the extreme importance of the counteranion present (ClO(4)(-) for 1 and PF(6)(-) for 2). The magnetic properties of the two compounds were studied by susceptibility measurements vs temperature. The chi(M) vs T plot for 1 shows the shape for a weakly antiferromagnetically coupled nickel(II) one-dimensional complex without a maximum until 4 K. In contrast, for complex 2 the shape of the chi(M) vs T curve shows a maximum near 40 K, indicating medium antiferromagnetic coupling. From the spin Hamiltonian -J(ij)()S(i)()S(j)(), J values for 1 and 2 were less than -1 and -29.1 cm(-)(1), respectively. The magnetic behavior for 1 and 2 may be explained in terms of the overlap between magnetic orbitals, taking into account the torsion of the Ni(II) atoms and azido-bridging ligands in the two structures.     

Critical evaluation of global mechanisms of wood devolatilization

Thermogravimetric data on the devolatilization rate of beech wood are re-examined with the aim of incorporating the effects of high heating rates (up to 108 K min⁻¹) in the global kinetics. The mechanism consisting of three independent parallel reactions, first-order in the amount of volatiles released from pseudo-components with chief contributions from hemicellulose, cellulose and lignin, is considered first. It is found that the set of activation energies estimated by Gronli et al. [M.G. Gronli, G. Varhegyi, C. Di Blasi, Ind. Eng. Chem. Res. 41 (2002) 4201-4208] (100, 236 and 46 kJ mol⁻¹, respectively) for one slow heating rate results in very high deviations between predicted and measured rate curves. The agreement is significantly improved by a new set of data consisting of activation energies of 147, 193 and 181 kJ mol⁻¹, respectively. In this case, the overlap is reduced between the reaction rates of the three pseudo-components whose chemical composition is also modified. In particular, instead of a slow decomposition rate over a broad range of temperatures, the activity of the third reaction is mainly explicated along the high-temperature (tail) region of the weight loss curves. The performances of more simplified mechanisms are also evaluated. One-step mechanisms, using literature values for the kinetic constants, produce large errors on either the conversion time (activation energy of 103 kJ mol⁻¹) or the maximum devolatilization rate (activation energy of 149 kJ mol⁻¹). On the other hand, these parameters are well predicted by two parallel reactions, with activation energies of 147 and 149 kJ mol⁻¹.     

A simple and fast micromethod for the analysis of polychlorinated biphenyls in air by sorbent enrichment and ultrasound-assisted solvent extraction

A combination of sorbent enrichment and ultrasound-assisted solvent extraction has been used to determine polychlorinated biphenyls in air. Analytes were sampled by pumping a known volume of air through a porous polymer (Tenax TA). The enriched adsorbent was transferred into a glass vial, and ultrasound-assisted extraction of the analytes was then performed in n-hexane. Quantification was carried out by using gas chromatography coupled to tandem mass spectrometry. Breakthrough volume of the sampling step was studied, indicating that 10 m³ of air could be processed without losses of the most volatile compounds. Good recoveries (75–96%) were obtained, and limits of detection at the sub ng m^–3 were achieved for all the analytes. The proposed method is very simple and fast, avoiding the use of large solvent volumes and time-consuming preconcentration steps.     

Generation of new landomycins by combinatorial biosynthetic manipulation of the LndGT4 gene of the landomycin E cluster in S. globisporus

A 3 kb DNA fragment from the Streptomyces globisporus 1912 landomycin E (LaE) biosynthetic gene cluster (lnd) was completely sequenced. Three open reading frames were identified, lndGT4, lndZ4, and lndZ5, whose probable translation products resemble a glycosyltransferase, a reductase, and a hydroxylase, respectively. Studies of generated mutants from disruption and complementation experiments involving the lndGT4 gene allowed us to determine that LndGT4 controls the terminal L-rhodinose sugar attachment during LaE biosynthesis and that LndZ4/LndZ5 are responsible for the unique C11-hydroxylation of the landomycins. Generation of the novel landomycins F, G, and H in the course of these studies provided evidence for the flexibility of lnd glycosyltransferases toward their acceptor substrates and a basis for initial structure-activity relationships within the landomycin family of antibiotics.     

As, Bi, Sb and Sn determination in atmospheric particulate matter by direct solid sampling-hydride generation-electrothermal atomic absorption spectrometry

A novel, rapid and simple method by hydride generation-electrothermal atomic absorption spectrometry (HG-ETAAS) after direct As, Bi, Sb and Sn hydrides generation from untreated filters of atmospheric particulate matter (PM₁₀ and PM_2.5) was optimised. PM₁₀ and PM_2.5 were not subjected to any pre-treatment: circular portions between 0.28 and 6.28 cm² were directly placed into the reaction vessel of a batch mode generation system. A 2⁸ × 3/64 Plackett–Burman design was used as a multivariate strategy for the evaluation of the effects of several variables affecting the hydride generation, trapping and atomisation efficiencies. Trapping temperature was the most statistically significant variable for As, Bi and Sn. Atomisation temperature was also statistically significant for Sb determination. Optimum values of significant variables were selected by using univariate optimisation approaches. An aqueous calibration method was used throughout. The developed method has been found to be precise with relative standard deviations of 6.2, 5.3, 9.1 and 7.5% for 11 determinations in a filter sample containing 0.7, 1.0, 1.4 and 1.7 μg l⁻¹ for As, Bi, Sb and Sn, respectively. Results obtained by direct solid sampling-HG-ETAAS have been found statistically comparable with those obtained after conventional method based on an acid digestion followed to ICP-MS. Absolute detection limits were 37, 15, 30, and 41 ng l⁻¹ for As, Bi, Sb and Sn, respectively. Detection limits referred to the air volume sampled (in the range of 0.020–0.050 ng m⁻³) were low enough for the determination of several hydride-forming elements from PM₁₀ and PM_2.5 samples collected in a non-polluted suburban area of A Coruña (NW Spain).     

Synthesis of ynones by palladium-catalyzed acylation of terminal alkynes with acid chlorides

Phosphane-free oxime-derived palladacycle 2 is an efficient precatalyst for the copper-free acylation of terminal alkynes with different carboxylic acid chlorides in toluene in the presence of 3 equiv of TEA as base, giving the corresponding ynones in good yields. The coupling reaction can normally be performed under air or under inert atmosphere when very low catalyst loadings (10(-3) mol % Pd) (turnover numbers (TONs) up to 23,000, turnover frequencies (TOFs) up to 958 h(-1)) or sensitive carboxylic acid chlorides are used. In addition, Pd(OAc)(2) has been shown as an efficient catalyst for the ligandless process, although usually working under higher loading conditions. This new protocol allows one to perform the synthesis of ynones at 110 degrees C, at room temperature, or under microwave irradiation conditions.     

Vibrational molecular quantum computing: basis set independence and theoretical realization of the Deutsch-Jozsa algorithm

The phase of quantum gates is one key issue for the implementation of quantum algorithms. In this paper we first investigate the phase evolution of global molecular quantum gates, which are realized by optimally shaped femtosecond laser pulses. The specific laser fields are calculated using the multitarget optimal control algorithm, our modification of the optimal control theory relevant for application in quantum computing. As qubit system we use vibrational modes of polyatomic molecules, here the two IR-active modes of acetylene. Exemplarily, we present our results for a Pi gate, which shows a strong dependence on the phase, leading to a significant decrease in quantum yield. To correct for this unwanted behavior we include pressure on the quantum phase in our multitarget approach. In addition the accuracy of these phase corrected global quantum gates is enhanced. Furthermore we could show that in our molecular approach phase corrected quantum gates and basis set independence are directly linked. Basis set independence is also another property highly required for the performance of quantum algorithms. By realizing the Deutsch-Jozsa algorithm in our two qubit molecular model system, we demonstrate the good performance of our phase corrected and basis set independent quantum gates.     

Analysis of mono- and disaccharides in milk-based formulae by high-performance liquid chromatography with refractive index detection

A simple and reproducible method for the qualitative and quantitative analysis of free mono- and disaccharides (fructose, glucose, galactose, sucrose, lactulose and lactose) in milk-based formulae by high-performance liquid chromatography (HPLC) with refractive index (RI) detection was developed and validated. The method showed good linearity with determination coefficients exceeding 0.99. The limits of detection (DL) in these sugars were 0.17, 0.13, 0.06, 0.16, 0.05 and 0.25 mg/ml, respectively; and the limits of quantification (QL), 0.27, 0.24, 0.20, 0.26, 0.22 and 0.38 mg/ml. The relative standard deviations (R.S.D.s) for repeatability in fructose, sucrose, lactulose and lactose were 0.78, 0.99, 2.91 and 0.46 and the R.S.D.s for reproducibility were 4.8, 6.15, 7.04 and 2.49, respectively. Recoveries in all sugars were between 93 and 113%.