CIS 1,2-functionalization of cyclohexane using selenium intermediates

The trans 1,2-phenylseleno acetate, acetamide, alcohol and nitrile of cyclohexane may be oxidized at selenium by halogens and the phenylseleno moiety displaced by halide to give high yields of 1,2 halide-containing products with cis geometry.     

Negative ion chemical ionization mass spectra of (η6-arene)Cr(CO)3

The negative ion chemical ionization mass spectra, with ammonia and methane as reagent gases, of the (η⁶-arene)Cr(CO)₃ complexes, where the arene is C₆H₅COCH₃, C₆H₅COC₂H₅, C₆H₅COC₃H₇, C₆H₅COC(CH₃)₃, 2-CH₃C₆H₄COC₃H₇, C₆H₅COOCH₃, C₆H₅CH₃, 1,3,5-(CH₃)₃C₆H₃ and C₆H₅CH₂COC₂H₅, are reported. Similar behaviour is observed with the two reagent gases, but ammonia shows a much higher abundance for the ions produced by reactions of [NH₂]^? with sample molecules. The compounds containing the C₆H₅CO group display an abundant [M]^{? ˙}, whereas the other compounds exhibit [M? H]^? as base peak, produced by ion/molecule reactions. A comparison of the negative ion chemical ionization mass spectra of the (η⁶-arene)Cr(CO)₃ complexes with those of the corresponding ligands shows the strong electron withdrawing power of the Cr(CO)₃ group in the gas phase.     

Ethanol production from steam-explosion pretreated wheat straw

Bioconversion of cereal straw to bioethanol is becoming an attractive alternative to conventional fuel ethanol production from grains. In this work, the best operational conditions for steam-explosion pretreatment of wheat straw for ethanol production by a simultaneous saccharification and fermentation process were studied, using diluted acid [H₂SO₄ 0.9% (w/w)] and water as preimpregnation agents. Acid-or water-impregnated biomass was steam-exploded at different temperatures (160–200°C) and residence times (5, 10, and 20 min). Composition of solid and filtrate obtained after pretreatment, enzymatic digestibility and ethanol production of pretreated wheat straw at different experimental conditions was analyzed. The best pretreatment conditions to obtain high conversion yield to ethanol (approx 80% of theoretical) of cellulose-rich residue after steam-explosion were 190°C and 10 min or 200°C and 5 min, in acid-impregnated straw. However, 180°C for 10 min in acid-impregnated biomass provided the highest ethanol yield referred to raw material (140 L/t wheat straw), and sugars recovery yield in the filtrate (300 g/kg wheat straw).     

Designing generalized statistical ensembles for numerical simulations of biopolymers

Conformational properties of polymers, such as average dihedral angles or molecular alpha-helicity, display a rather weak dependence on the detailed arrangement of the elementary constituents (atoms). We propose a computer simulation method to explore the polymer phase space using a variant of the standard multicanonical method, in which the density of states associated to suitably chosen configurational variables is considered in place of the standard energy density of states. This configurational density of states is used in the Metropolis acceptance/rejection test when configurations are generated with the help of a hybrid Monte Carlo algorithm. The resulting configurational probability distribution is then modulated by exponential factors derived from the general principle of the maximal constrained entropy by requiring that certain average configurational quantities take preassigned (possibly temperature dependent) values. Thermal averages of other configurational quantities can be computed by using the probability distributions obtained in this way. Moments of the energy distribution require an extra canonical sampling of the system phase space at the desired temperature, in order to locally thermalize the configurational degrees of freedom. As an application of these ideas we present the study of the structural properties of two simple models: a bead-and-spring model of polyethylene with independent hindered torsions and an all-atom model of alanine and glycine oligomers with 12 amino acids in vacuum.     

On the growth and shape of sodium taurodeoxycholate micellar aggregates: a spin-label and quasielastic light scattering investigation

Electron spin resonance (ESR) and quasielastic laser scattering (QELS) measurements have been carried out on sodium taurodeoxycholate (NaTDC) micellar aqueous solutions. Computer simulation of the ESR line shape has been used to quantitatively analyze the rotational dynamics of the cholestan-spin label (CSL) dissolved by the NaTDC micellar aggregates as a function of temperature and NaCl concentration. The local reorientation of CSL has been accounted for motionally-averaged g- and A-tensors assuming fast oscillation around the spin-probe long molecular axis. The overall Brownian tumbling of CSL-micelle complexes has been modeled by an axially symmetric rotational tensor. Good agreement with experimental spectra is obtained. Best-fit rotational parameters and QELS data suggest that, in the circumstance of large aggregation, NaTDC micelles have cylindrical shape and micellar growth occurs along the cylinder axis.     

Molecular-level control of the photoluminescence from PPV nanostructured films

The fabrication of varied molecular architectures in layer-by-layer (LbL) films is exploited to control the photoluminescence (PL) of poly(p-phenylene vinylene) (PPV) in an unprecedented way. This was achieved by controlling the F?rster energy transfer between PPV layers (donors) and layers of a commercial azodye, Brilliant Yellow (BY) (acceptors). Energy transfer was controlled by inserting spacer layers of inert polymers between PPV and BY layers and by photoaligning the BY molecules via trans-cis-trans isomerization. The PPV/BY LbL films displayed polarized PL whose intensity could be varied almost continuously by changing the time of photoalignment, which was carried out by impinging a linearly polarized laser light simultaneously to the PL experiments. For PPV/BY films with no spacer layers, PL was completely quenched, but its intensity increased with the number of spacing layers. Further increase in PL was obtained by photoaligning the BY molecules perpendicularly to the PPV molecules. This minimizes energy transfer, since F?rster processes are directional, dipole-dependent resonant transfers. Energy transfer is also controlled by imparting a preferential orientation of the PPV chains on PPV/BY LbL films deposited onto flexible Teflon substrates that may be stretched.     

Reaction of singlet oxygen with some benzylic sulfides

Product distribution, total quenching rate (k_T), and rate of chemical reaction (k_r) with singlet oxygen have been determined for some alkyl, benzyl, α-methylbenzyl, and cumyl sulfides. Their contributions depend on the steric hindering around the sulfur atom. In protic solvents, the sulfoxide is the main product via a hydrogen-bonded persulfoxide. In apolar solvents, intramolecular α-H abstraction leads to oxidative C-S bond cleavage, with varying efficiency. The behavior of sulfides is compared to that of alkenes and amines.     

Sorption equilibrium of methanol on new composite sorbents “CaCl<Subscript>2</Subscript>/silica gel”

This paper presents experimental data on methanol sorption on new composite sorbents which consist of mesoporous silica gels and calcium chloride confined to their pores. Sorption isobars and XRD analysis showed the formation of a solid crystalline solvate CaCl₂⋅2MeOH at low methanol uptake, while at higher uptake the formation of the CaCl₂–methanol solution occurred. The solution confined to the silica pores showed the sorption properties similar to those of the CaCl₂–methanol bulk solution. Calorimetric and isosteric analyses showed that the heat of methanol sorption depends on the methanol uptake, ranging from 38±2 kJ/mol for the solution to 81±4 kJ/mol for the solid crystalline phase CaCl₂⋅2MeOH. The above mentioned characterizations allowed the evaluation of the methanol sorption and the energy storage capacities, clearly showing that the optimal applications of these new composite sorbents are the methanol removal from gaseous mixtures, heat storage and sorption cooling driven by low temperature heat.     

The synthesis and reactivity of the heptanuclear dianion [Os7(CO)20]2− including the synthesis and structural characterizations of the compounds [Os7(CO)20(AuPEt)3 2] and [Os8(CO)20(η6-C6H6]

Reduction of the heptaosmium cluster [Os₇(CO)₂₁] With [Et₄N][NH₄) gives the cluster dianion [Os₇(CO)₂₀]^2–,1, in high yield. The reaction of the dianion with [AuPR ₃Cl] (R=Et or Ph) in the presence of TlPF₆ forms [Os₇((CO)₂₀(AuPR ₃)₂] [R=Et (2a);R = Ph(2b)] in 80% yield, while the corresponding reaction with (Os(C₆H₆)(CH₃CN)₃]²⁺ gives [Os₈(CO)₂₀ ( ⁶-C₆H₆)] (3) in reasonable yield (ca. 30%). The dianion,1, and the clusters2 and3 have been fully characterized by bout spectroscopic and crystallographic methods. The crystal structure of the [Ph₄P]⁺ salt of1 shows that the metals in the anion adopt a capped octahedral geometry, with all twenty carbonyl ligands in terminal sites. The metal core geometry in2a is best described as a tricapped octahedron, and is based on the structure of the dianion1 with two adjacent octahedral faces capped by the Au atoms of the two AuPEt₃ groups. In a similar fashion, the geometry of3 is related to that of1 with the addition of an Os(C₆H₆) unit capped to a triangular face, to give a bicapped octahedral framework.