Influence of anionic and cationic reverse micelles on nucleophilic aromatic substitution reaction between 1-fluoro-2,4-dinitrobenzene and piperidine

The nucleophilic aromatic substitution (S(N)Ar) reaction between 1-fluoro-2,4-dinitrobenzene and piperidine (PIP) were studied in two different reverse micellar interfaces: benzene/sodium 1,4-bis(2-ethylhexyl) sulfosuccinate (AOT)/water and benzene/benzyl-n-hexadecyl dimethylammonium chloride (BHDC)/water reverse micellar media. The kinetic profiles of the reactions were investigated as a function of variables such as surfactant and amine concentration and the amount of water dispersed in the reverse micelles, W0 = [H2O]/[surfactant]. In the AOT system at W0 = 0, no micellar effect was observed and the reaction takes place almost entirely in the benzene pseudophase, at every AOT and PIP concentration. At W0 = 10, a slight increment of the reaction rate was observed at low [PIP] with AOT concentration, probably due to the increase of micropolarity of the medium. However, at [PIP] > or = 0.07 M the reaction rates are always higher in pure benzene than in the micellar medium because the catalytic effect of the amine predominates in the organic solvent. In the BHDC system the reaction is faster in the micellar medium than in the pure solvent. Increasing the BHDC concentration accelerates the overall reaction, and the saturation of the micellar interface is never reached. In addition, the reaction is not base-catalyzed in this micellar medium. Thus, despite the partition of the reactants in both pseudophases the reactions effectively take place at the interface of the aggregates. The kinetic behavior can be quantitatively explained taking into account the distribution of the substrate and the nucleophile between the bulk solvent and the micelle interface. The results were used to evaluate the amine distribution constant between the micellar pseudophase and organic solvent and the second-order rate coefficient of S(N)Ar reaction in the interface. A mechanism to rationalize the kinetic results in both interfaces is proposed.     

Intermediate coupling model description of55Fe and57Fe

The properties of the negative parity states of⁵⁵Fe and⁵⁷Fe are investigated in the framework of the intermediate coupling model. In the model, a neutron or a quasineutron is coupled to anharmonic vibrations of the core. Anharmonicities of the vibrations are estimated through the observed properties of the core. Energy levels, spectroscopic factors and electromagnetic properties have been calculated. The results of the present calculations are also compared with available experimental results and other theoretical results. The model reasonably accounts for many of the properties of the low-lying states.     

Distribution of amines in water/AOT/n-hexane reverse micelles: influence of the amine chemical structure

The distribution of different aliphatic and aromatic amines: n-butylamine (n-BA), isobutylamine (i-BA), tert-butylamine (t-BA), piperidine (PIP), N,N-dimethylaniline (DMA) and N-methylaniline (MA) in water/sodium 1,4-bis(2-ethylhexyl)sulfosuccinate(AOT)/n-hexane reverse micelles was investigated by steady-state fluorescence measurements. The partition constants were measured by an indirect method based on the effect that amine partitioning exert on the bimolecular rate of the reaction between a microphase incorporated fluorophore (Ru(bpy)2+(3)) and the quencher, (Fe(CN)3-(6)). For MA, that can act as a quencher of the fluorophore a direct method was used. The results show that primary amines have larger partition constants than the secondary ones. For tertiary amines the distribution constants were practically negligible. Laser flash photolysis experiments confirmed that tertiary amines, both aliphatic and aromatic, are not incorporated to the micellar pseudophase. The effect of the amine structure on the partition constant was analyzed through linear solvation free energy relationships (LSER) using solute parameters and compared with those obtained for alcohols. Hydrogen bond interactions with the AOT polar heads appear to be the main driving force for the distribution of amines between the organic and micellar pseudophases, whereas the size of the alkyl or aromatic group tends to hinder it.     

Electronic spectroscopy of the E<--X transition of NO-Kr and shielding/penetration effects in Rydberg states of NO-Rg complexes

We report the results of a (2+1) resonance-enhanced multiphoton ionization (REMPI) study of the E2Sigma+(4ssigma) Rydberg state of NO-Kr. We present an assignment of the two-photon spectrum based on a simulation, and discuss it in the context of previously-reported spectra of NO-Ne and NO-Ar. In addition, we report on spectra in the region of the vNO=1 level of the E, F and H' 4s and 3d Rydberg states of NO-Rg (Rg=Ne-Kr). Since the NO vibrational frequency is affected by electron donation from the rare-gas (Rg) atom to the NO+ core, as well as by the penetration of the Rydberg electron, the fundamental NO-stretch frequency reflects the interactions in the complex. The results indicate that the 4s Rydberg state has a strong interaction between the NO+ core and the Kr atom, as was the case for NO-Ar and NO-Ne. For the 3d Rydberg states, although penetration is not as significant as for the 4s Rydberg states, it does play an important role, with subtle angular effects being notable.     

On the investigation of the bilayer functionalities of 1,2-di-oleoyl-sn-glycero-3-phosphatidylcholine (DOPC) large unilamellar vesicles using cationic hemicyanines as optical probes: a wavelength-selective fluorescence approach

The behavior of the cationic hemicyanines trans-4-[4-(dimethylamino)-styryl]-1-methylpyridinium iodide (HC) and 4,(4-(dihexadecylamino)styryl-N-methyl-pyridinium iodide (DIA) were studied in large unilamellar vesicles (LUV) of 1,2-di-oleoyl-sn-glycero-3-phosphatidylcholine (DOPC) using absorption, emission, depolarization and time resolved spectroscopies. Also, thorough spectroscopic studies were performed in homogeneous media to investigate the different interactions that the dyes can experience with its microenvironment. These results help us to comprehend the dye performance under different media and, consequently find interesting features of the DOPC membrane properties. The studies in homogeneous media analyzed by the Kamlet and Taft's solvatochromic comparison method demonstrate, for the first time, that the cationic hemycianines undergo specific interactions with the medium through the solvents ability to donate an electron pair as measured by the beta parameter. Thus, the absorption bands shifts bathochromically with beta while, the emission band shifts hypsochromically. In addition, for the relaxed hemicyanines the 00 energy, nu00, is invariant with the solvent properties. The results in LUV of DOPC show that, DIA undergoes a strong association with the vesicle bilayer while HC partitions between the water and the bilayer pseudophases. To monitor directly the microenvironment and dynamics around HC and DIA inside the DOPC bilayer, we use the wavelength-selective fluorescence approach, which is based on the red edge effect in fluorescence spectroscopy, in addition with the nu00 energy of the hemicyanines. The results show that the fluid state of the DOPC bilayer resembles the microenvironment of sodium bis (2-ethylhexyl) sulfosuccinate (AOT) reverse micelles at W=[H2O]/[AOT] below 10 where there is no free water forming the water pool. Moreover, it is demonstrated for the first time, that the region of the bilayer close to the polar head of DOPC is a powerful electron donor environment.     

Kinetics of reactions catalyzed by enzymes in solutions of surfactants

The effect of surfactants, both in water-in-oil microemulsions (hydrated reverse micelles) and aqueous solutions upon enzymatic processes is reviewed, with special emphasis on the effect of the surfactant upon the kinetic parameters of the process. Differences and similarities between processes taking place in aqueous and organic solvents are highlighted, and the main models currently employed to interpret the results are briefly discussed.     

On the possibility that cyclodextrins' chiral cavities can be available on AOT n-heptane reverse micelles. A UV-visible and induced circular dichroism study

The formation of reverse micelles (RMs) of sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT) in n-heptane including two different beta-cyclodextrin (beta-CD) derivatives (hydroxypropyl-beta-CD, hp-beta-CD, and decenyl succinyl-beta-CD, Mod-beta-CD) is reported. Both cyclodextrins can be incorporated into AOT RMs in different zones within the aggregate, while beta-CD cannot. Using UV-vis and induced circular dichroism (ICD) spectroscopy and different achiral molecular probes (some azo dyes, p-nitroaniline and ferrocene), it was possible to determine that Mod-beta-CD is located with its cavity at the oil side of the AOT RM interface, while for hp-beta-CD the cavity is inside the RM water pool. Among the molecular probes used, methyl orange (MO) was the only one which gave the ICD signal when dissolved in the AOT RMs with hp-beta-CD, so a detailed study of MO behavior in homogeneous media was also performed to compare with the microheterogeneous media. The solvatochromic behavior of the dye depends not only on the polarity of the media but also on other specific solvent properties. A Kamlet-Taft analysis shows that the MO absorption spectrum shifts to longer wavelength with an increase in the solvent polarity-polarizability (pi*) and the hydrogen donor ability (alpha) of the medium. MO appears to be almost 3 times more sensitive to the pi* parameter than to the alpha parameter. In addition, from the MO absorption spectral changes with the hp-beta-CD concentration, the association equilibrium constants in pure water (K11W) and inside the RMs (K11RM) were computed. The results show that K11W is almost 10 times larger than the value inside the RMs. The latter can be explained considering that MO resides anchored to the RM interface through hydrogen bond interaction with the hydration bound water. This study shows for the first time that the cyclodextrin chiral cavity is available for a guest in an organic medium such as the RMs; therefore, we have created a potentially powerful nanoreactor with two different confined regions in the same aggregate: the polar core of the RMs and the chiral hydrophobic cavity of cyclodextrin.