An artificial ionophore based on a polyhydroxylated steroid dimer

The Na+ transporting properties of the first member of a new class of artificial ionophores, based on a C2-symmetric polyhydroxylated steroid dimer, are described.     

Interfacial adsorption of microorganisms and its effect on oxygen absorption by fermentation broths

Steady-state mass transfer experiments were done to determine whether certain strains of bacteria in homogeneous suspension will accumulate at the gas:liquid interface and thereby significantly increase the oxygen transfer rate to the suspension. In particular, with suspensions ofB. licheniformis, the measured transfer rates are as much as three times the rate expected for a uniform suspension. In contrast, suspensions ofM. luteus show no increase in oxygen flux above expected values. The fact thatB. licheniformis is motile, whereasM. luteus is not suggests that cell motility may play an important role in the accumulation process.

     

Determination of activation energies in the annealing of cobalt recoil atoms in single and double complexes

Activation energies for the recombination of point defects generated by the /n,/ reaction in Ba (CoEDTA)₂·4H₂O, t-(Co(en)₂(NO₂)₂)Cl and (Co(en)₂(NO₂)₂) (CoEDTA)·1H₂O, have been evaluated using isothermal step annealing. The reselts were interpreted in terms of compound structures. It is possible to relate the recoil atom behaviour between single complexes and the double one.     

Multivariate optimization of an axially-viewed inductively coupled plasma multichannel-based emission spectrometer for the analysis of environmental samples

An experimental design procedure was applied to optimize the operating conditions of an axially-viewed inductively coupled plasma emission spectrometer instruments equipped with echelle optics with cross dispersion and charge transfer device. The multivariate effect of carrier gas flow rate and r.f. power on several analytical figures was investigated and discussed. Both ultrasonic and pneumatic nebulization were used. For the final choice of the optimum, different criteria were taken into account, mainly plasma robustness, instrumental precision, analyte and background net emission, detection limits and signal-to-background ratios. It was found that the use of moderate power (1100W) and mean carrier gas flow rate (0.75 L/min) allows to obtain sufficient plasma robustness, satisfactory precision, and excellent signal-to-background ratios and limits of detection, favorable for ultratrace element determinations in environmental matrices.     

1,3-Diaxial steric effects and intramolecular hydrogen bonding in the conformational equilibria of new cis-1,3-disubstituted cyclohexanes using low temperature NMR spectra and theoretical calculations

The conformational equilibria of 3-X-cyclohexanol [X=F (1), Cl (2), Br (3), I (4), Me (5), NMe(2) (6) and MeO (7)] and of 3-X-methoxycyclohexane [X=F (8), Cl (9), Br (10), I (11), Me (12), NMe(2) (13) and MeO (14)] cis isomers were determined from low temperature NMR spectra and PCMODEL calculated coupling constants. The energy differences between aa and ee conformers were obtained from these data (DeltaG(J)(av) and DeltaG(PC)(av), respectively) and also by the additivity principle from data for the monosubstituted cyclohexanes (DeltaG(Ad)). H-1 and H-3 hydrogen vicinal coupling constants and DeltaG(J)(av) values showed that the diequatorial conformer is predominant in the conformational equilibrium of the compounds studied at low temperature. However, DeltaG(PC)(av) data show that compounds 6 and 7 constitute an exception, since they are almost equally populated by ee and aa at room temperature, due to stabilization of their aa conformer by an intramolecular hydrogen bond. DeltaG(Ad) values, obtained according to the additivity principle, show a better agreement for compounds 2 and 3, since the 1,3-diaxial steric effect is counterbalanced by the formation of an intramolecular hydrogen bond (IAHB). For the remaining compounds, DeltaG(Ad) values underestimate the energy differences, since the 1,3-diaxial steric effect, between X and OH or OCH(3), is absent in the monosubstituted compounds used as references. Moreover, the DeltaG(PC)(av), calculated from the coupling constants, obtained through the PCMODEL program, are rather smaller than the DeltaG(J)(av) values, since the program does not have parameters for the effect, observed in this report, of a substituent at gamma position on coupling constants values for the hydrogen under consideration.     

Quassinoids from Picrasma crenata

From woods of Picrasma crenata, a new stereoisomer dihydronorneoquassin was obtained together with others well knowns dihydronorneoquassin, parain, alpha-neoquassin, beta-neoquassin and quassin. The structures were determined by spectroscopic data and chemical evidence.     

Phase evolution of a barium tin 1,2-ethanediolato complex to barium stannate during thermal decomposition

The thermal behaviour of [Ba(C₂H₆O₂)₄][Sn(C₂H₄O₂)₃] used as a BaSnO₃ precursor, and its phase evolution during thermal decomposition are described. The initially formed transient barium tin oxycarbonate phase disintegrates into BaCO₃ and SnO₂, reacting subsequently to BaSnO₃. The existence of the intermediate oxycarbonate phase is evidenced by Fourier transformed infrared (FT-IR), X-ray powder diffraction (XRD) and electron energy loss spectroscopy (EELS (ELNES)) investigations.     

Conformational and stereoelectronic investigation in 1,2-difluoropropane: The gauche effect

The effect of attaching an additional fluorine atom at C-2 in 1-fluoropropane (FP), giving 1,2-difluoropropane (DFP), on its conformational equilibrium, is theoretically evaluated. This substitution causes critical implications on the conformer stabilities of DFP (TG, GT and GG conformations) and the steric and electrostatic interactions should favor the conformer with fluorine atoms trans. However, the gauche effect plays a major role in describing the energies balance in DFP, shifting the equilibrium towards the conformation in which the two fluorine atoms are gauche. The origin of this effect is discussed through an NBO analysis, which allows the evaluation of both classical and non-classical (hyperconjugation and bent bonds) interactions as the prevailing factors governing the conformational equilibrium of molecules containing the 1,2-difluoroethane fragment.     

Vilsmeier formylation of 5,10,15-triphenylcorrole: expected and unusual products

5,10,15-Triphenylcorrole (1) reacts with the Vilsmeier reagent (POCl(3)/DMF) to give the corresponding 3-formyl derivative 3 as the major product. The regioselectivity of the reaction was proven by X-ray crystallography and only traces of the 2-formyl isomer were observed. A more polar product is also observed and this compound becomes the major product when an excess of DMF is used for the preparation of the Vilsmeier reagent, while the formation of the 3-formyl isomer is almost completely suppressed. X-ray crystallography allowed us to identify this compound as the fully substituted N-ethane bridged derivative 4, formed from the attack of the Vilsmeier reagent at the inner core of the macrocycle. This compound is unique among porphyrinoid macrocycles, and further confirms the peculiarity of corrole chemistry.