A computational study of halomethyllithium carbenoid mixed aggregates with lithium halides and lithium methoxide

Density functional theory calculations were used to examine the formation of lithium halide and lithium alkoxide mixed aggregates with halomethyllithium carbenoids. These mixed aggregates may be the important intermediates in carbenoid reactions where lithium halides are formed as byproducts, or when the mixture has been exposed to small amounts of air. The calculations showed that in the gas phase and in THF solution, mixed dimers, trimers, and tetramers may coexist with free lithium carbenoids, depending on the lithium salt. The calculations also indicated that mixed aggregates may influence the activation free energies of cyclopropanation reactions of lithium carbenoids.     

A computational study of lithium methoxide mixed aggregates with alkyllithiums

The formation of alkyllithium-lithium methoxide mixed aggregates was modeled with the B3LYP density functional method. In the gas phase there was little tendency to form mixed dimers or trimers. Mixed tetramer formation was more energetically favorable, particularly for tert-butyllithium. THF solvation favored the formation of methyllithium and ethyllithium mixed tetramers, but not those of sec-butyllithium and tert-butyllithium. The potential for resolution of chiral alkyllithiums by mixed aggregate formation with enantiomerically pure lithium alkoxides was examined.     

Mixed Aggregates of 1-Methoxyallenyllithium with Lithium Chloride

A combined computational and ¹³C NMR study was used to investigate the formation of mixed aggregates of 1-methoxyallenyllithium and lithium chloride in tetrahydrofuran (THF) solution. The observed and calculated chemical shifts, as well as the calculated free energies of mixed aggregate formation (MP2/6-31+G(d)), are consistent with the formation of a mixed dimer as the major species in solution. Free energies of mixed dimer, trimer, and tetramer formation were calculated by using the B3LYP and MP2 methods and the 6-31+G(d) basis set. The two methods generated different predictions of which mixed aggregates will be formed, with B3LYP/6-31+G(d) favoring mixed trimers and tetramers in THF solution, and MP2/6-31+G(d) favoring mixed dimers. Formation of the sterically unhindered mixed dimers is also consistent with the enhanced reactivity of these compounds in the presence of lithium chloride. The spectra are also consistent with some residual 1-methoxyallenyllithium tetramer, as well as small amounts of higher mixed aggregates. Although neither computational method is perfect, for this particular system, the calculated free energies derived using the MP2 method are in better agreement with experimental data than those derived using the B3LYP method.     

Ketone enolization with lithium dialkylamides: the effects of structure,solvation, and mixed aggregates with excess butyllithium

The effects of lithium dialkylamide structure, mixed aggregate formation, and solvation on the stereoselectivity of ketone enolization were examined. Of the lithium dialkylamides examined, lithium tetramethylpiperidide (LiTMP) in THF resulted in the best enolization selectivity. The stereoselectivity was further improved in the presence of a LiTMP-butyllithium mixed aggregate. The use of less polar solvents reduced the enolization stereoselectivity. Ab initio calculations predict LDA and LiTMP to form mixed cyclic dimers in ethereal solvents. The calculations also predict LiTMP-alkyllithium mixed aggregates to competitively inhibit the formation of less stereoselective LiTMP-lithium enolate mixed aggregates.     

Study of the formation of dye-induced premicellar aggregates and its application to the determination of quaternary ammonium surfactants

Formation of dye-induced mixed premicellar aggregates from binary surfactant solutions is proposed for the determination of alkyltrimethylammonium surfactants at the muM level. The Coomassie Brilliant Blue G (CBBG) dye, negatively charged, induces the formation of cationic surfactant aggregates at concentrations far below the cmc. The role of CBBG in the formation of premicelles was studied by using pyrene as a fluorimetric probe. Formation of CBBG-cationic surfactant aggregates of well-defined stoichiometries that depend on the total surfactant concentration added is demonstrated. Also, the influence of analytical parameters affecting the concentration at which a given aggregate is formed was studied. Linear calibrations for alkyltrimethylammonium surfactants were obtained by using different cationic surfactants as titrants; therefore, the previously derived measurement parameter for mixed micelles is applicable to premicellar aggregates as well.     

Phase Behavior in Mixtures of Cationic Dimeric and Anionic Monomeric Surfactants

The formation of mixed aggregates has been investigated in the mixture of oppositely charged surfactants vastly differing in molecular geometry and size. The systems considered is mixture of the cationic gemini surfactant, ethanediyl-1,2-bis(dodecyldimethylammonium bromide), and anionic surfactant, sodium dodecyl sulfate. Various mixed nano- and microaggregates (micelles, vesicles, thin lamellar sheets, and tubules) were formed depending on bulk composition and total surfactant concentration. Two types of aggregates were found in precipitate, the tubules as prevailing aggregates on the gemini-rich side, and vesicles as prevailing aggregates on the SDS-rich side. The tubules formation was ascribed to mutual influence of specific structure of cationic dimeric surfactant and electrostatic interactions at the bilayer/solution interface. The proposed mechanism involved the formation of lamellar sheets, which rolled-up into tubules.     

Self-assembly of [Et,Et]-bacteriochlorophyll cF on highly oriented pyrolytic graphite revealed by scanning tunneling microscopy

The chlorosomal light-harvesting antennae of green phototrophic bacteria consist of large supramolecular aggregates of bacteriochlorophyll c (BChl c). The supramolecular structure of (3(1)-R/S)-BChl c on highly oriented pyrolytic graphite (HOPG) and molybdenum disulfide (MoS2) has been investigated by scanning tunneling microscopy (STM). On MoS2, we observed single BChl c molecules, dimers or tetramers, depending on the polarity of the solvent. On HOPG, we observed extensive self-assembly of the dimers and tetramers. We propose C=O...H-O...Mg bonding networks for the observed dimer chains, in agreement with former ultraviolet-visible and infrared spectroscopic work. The BChl c moieties in the tetramers are probably linked by four C=O...H-O hydrogen bonds to form a circle and further stabilized by Mg...O-H bondings to underlying BChl c layers. The tetramers form highly ordered, distinct chains and extended two-dimensional networks. We investigated semisynthetic chlorins for comparison by STM but observed that only BChl c self-assembles to well-structured large aggregates on HOPG. The results on the synthetic chlorins support our structure proposition.     

Mixed aggregates of dilithiodiamines with alkyllithiums and lithium enolates

The formation of mixed aggregates of N,N′-dilithiodiamines with alkyllithiums and lithium enolates was investigated. Enolization of 3-pentanone with the dilithium salt of N,N′-dimethyl-1,3-propanediamine generated both the E and Z enolates and the E/Z ratio changed in the presence of a lithium enolate or excess butyllithium. The formation of mixed aggregates was modeled with the B3LYP DFT method and it was found that mixed aggregate formation is energetically favorable. The infrared spectra of dilithio-N,N′-dimethyl-1,3-propanediamine in the presence of excess butyllithium or lithium enolate are consistent with the formation of mixed aggregates.     

The Self-Recognition and Self-Assembly of Folic Acid Salts in Isotropic Water Solution

The self-assembly of alkaline folates in isotropic water solutions, with or without added salts, has been investigated by small-angle neutron-scattering, circular-dichroism, and NMR techniques. The assembled species are chiral, cylindrical aggregates of finite length, composed of stacked tetramers; each tetramer is formed by Hoogsteen-bonded folate residues. The assembly process is more efficient in the presence of an excess of Na^I ions, leading to longer aggregates with stronger tetramer-tetramer-tetramer interaction. In pure water, the rods are shorter and the tetramer-tetramer interaction weaker. Association between folates can be detected by circular-dichroism spectroscopy starting from a concentration of 6· 10^?4 mol l^?1, well below the critical concentration for the formation of the cholesteric mesophase (ca. 0.5 mol l^?1).     

Role of aggregates in Claisen acylation reactions of imidazole, pyrazole, and thioesters with lithium enolates in THF

[formula: see text] Although phenyl esters react with both monomers and dimers or tetramers of two lithium enolates in THF, the reactions of phenyl thiobenzoates are relatively much faster with the monomers. Similarly, imidazole esters react primarily with the monomers but pyrazole esters react with monomers and aggregates. The results are rationalized by a mechanism in which coordination with two lithium cations within an enolate aggregate is required for the reaction of aggregates to compete with monomers.     

Reduction of pyracylene by alkali metals: Mechanism and formation of aggregates

Five reaction stages have been identified in the reduction of pyracylene to its dianion. The same stages were also observed in the photochemical oxidation starting with the dianion. The formation of dimers of anions and of mixed valence aggregates is discussed.     

Osmotic coefficients and activity coefficients of lodic acid at high concentrations

Isopiestic vapor pressure measurements have been used to determine the osmotic coefficients of aqueous solutions of iodic acid at molalities from 0.1 to 17 mole-kg^?1 at 25°C. The isopiestic standards were solutions of sodium chloride and solutions of sulfuric acid. Because of the corrosive nature of iodic acid, platinum cups were used. Stoichiometric activity cofficients of iodic acid were derived by a Gibbs-Duhem integration. The activity coefficients for solutions of molality greater than 0.5 mole-kg^?1 cannot be accounted for in terms of the two equilibria, namely, the acidic dissociation of iodic acid and formation of the ion H(IO₃) ₂ ^? , shown by Pethybridge and Prue to explain adequately the behavior in dilute solutions. The activity coefficient is unexpectedly small in concentrated solutions, suggesting the formation of neutral aggregates of iodic acid. The presence of dimers and tetramers, or alternatively trimers and tetramers, can explain the observed results up to a molality of 7 mole-kg^?1.     

Steric factor in ligand displacement of zirconium alkoxides dissolved in alcohols

The present study indicates EXAFS direct structural data obtained for zirconium ethoxide, iso- and n-propoxide are alike those for zirconium n-butoxide complexes. This is evident for agreement of structural units of abovementioned alkoxides complexes. Our group developed structural model of n-butoxide earlier. Six interconnected tetramers assemble a structural unit of the alkoxides under study. The tetramers build of zirconium atoms in the highly flattened pyramid corners. Four zirconium atoms are bonded in series by double bridges through oxygen atoms of alkoxide ligands. Two of the four zirconium atoms are bias bonded by single ligand bridges. The distances between pairs of zirconium atoms inside the tetramer are 3.3 and 3.5 Å. Tetramers are linked together also by a single ligand bridges. The distance between zirconium atoms of the two neighbor tetramers is 3.9 Å. Primary particles (tetramer sextet) form aggregates in a solution. Possible shapes of the anisotropically sized aggregates is a large diameter cylinder with small height (a disk), or a small diameter cylinder with large height (a rod). Primary particles composed through ligands. The distance between the neighbor zirconium atoms of different primary particles is 4.8 or 5.1 Å depending on the coordination nature and the neighboring particles number.     

Mixed vesicle formation on a ternary surfactant system: didodecyldimethylammonium bromide/dodecylethyldimethylammonium bromide/water

The formation of mixed aggregates has been investigated on a ternary system consisting of two cationic surfactants with similar polar heads and two and/or one 12 carbon atom hydrophobic tail, respectively, didodecyldimethylammonium bromide and dodecylethyldimethylammonium bromide and water. The study has been carried out by means of conductivity, zeta potential, and cryogenic transmission electronic microscopy (cryo-TEM) experiments on the very diluted region. A variety of mixed aggregates, microaggregates, vesicles, and micelles has been found, depending on system composition and total surfactant concentration. Mixed critical microaggregate concentration and mixed critical vesicle concentration have been determined from conductivity data. Furthermore, zeta potential and cryo-TEM experiments allow for the characterization of the aggregates/solution interface and of the shape and size of the aggregates. This experimental evidence has also been analyzed in terms of the theoretical packing parameter, P.     

A computational study of lithium enolate mixed aggregates

Ab initio calculations were performed to examine the formation of mixed dimer and trimer aggregates between the lithium enolate of acetaldehyde (lithium vinyloxide, LiOV) and lithium chloride, lithium bromide, and lithium amides. Gas-phase calculations showed that in the absence of solvation effects, the mixed trimer 2LiOV.LiX is the most favored species. Solvation in ethereal solvents was modeled by a combination of specific coordination of dimethyl ether ligands on each lithium and "dielectric solvation" (DSE, dielectric solvation energies), immersion of each molecule in a cavity within a continuous dielectric having the dielectric constant of THF at room temperature. DSE is less important for aggregates (reduced dipoles or quadrupoles) than monomers (dipoles) and is also reduced for the coordinatively solvated species. Both solvation terms reduce the exothermicity of aggregation. In many cases, lithium salts that are three- rather than four-coordinate have significant populations at room temperature. The strongly basic lithium amides prefer mixed aggregates with weaker bases than homoaggregates. The computational results are consistent with the limited experimental data available.     

Investigation of aggregation in solvent extraction of lanthanides by acidic extractants (organo-phosphorus and naphthenic acid)

Three acidic extractants (Ⅰ) di(2-ethylhexyl) phosphoric acid (HDEHP),(Ⅱ) 2-ethylhexyl phos-phonic acid mono-2-ethylhexyl ester (KEHPEHE) and (Ⅲ) naphthenic acid were employed in preparing the samples for the characterization of the coordination structure of lanthanlde-extractant complexes and the physicochemical nature of aggregates formed in the organic diluent of the solvent extraction systems.Photo correlation spectroscopy (PCS) re-suits on the aggregates formed by the partially saponified HDEHP in n -heptane showed that the hydrodynamic radius of the aggregates was comparable to the molecular dimensions of HDEHP.The addition of 2-octanol into the diluent,by which the mixed solvent was formed,increased the dimensions of the corresponding aggregates.Aggregates formed from the ianthamde ions and HDEHP in the organic phase of the extraction systems were found very unstable.In the case of naphthenic acid,PCS data showed the formation of w/o microemulsion from the saponified naphthenic acid in the mix     

Role of microenvironment in the mixed Langmuir-Blodgett films

This paper reports the pi-A isotherms and spectroscopic characteristics of mixed Langmuir and Langmuir-Blodgett (LB) films of nonamphiphilic carbazole (CA) molecules mixed with polymethyl methacrylate (PMMA) and stearic acid (SA). pi-A isotherm studies of mixed monolayer as well as the remarkable change in collapse pressure of the mixed monolayer isotherms definitely show that CA is incorporated into PMMA and SA matrices. However, CA is stacked in the PMMA/SA chains and forms microcrystalline aggregates, as is evidenced from the scanning electron micrograph picture. The nature of these aggregated species in the mixed LB films has been revealed by UV-vis absorption and fluorescence spectroscopic studies. The presence of two different kinds of band systems in the fluorescence spectra of the mixed LB films have been observed. This may be due to the formation of low-dimensional aggregates in the mixed LB films. Intensity distribution of different band systems is highly sensitive to the microenvironment of two different matrices as well as also on the film thickness.     

Electrochemical, microscopic, and spectroscopic characterization of prevesicle nanostructures and vesicles on mixed cationic surfactant systems

Several experimental techniques (conductivity, zeta potential, transmission electronic microscopy, and steady-state fluorescence spectroscopy) have been used to study the formation of mixed colloidal aggregates consisting of a cationic double-chain surfactant, di-dodecyldimethylammonium bromide (di-C12DMAB), and a single-chain alkyltrimethylammonium bromide with 10 and/or 14 carbon atoms (decyltrimethylammonium bromide, C10TAB, and/or tetradecyltrimethylammonium bromide, C14TAB). Special interest has been devoted to the prevesicle domain, within which the formation of aggregated nanostructures was first reported in our laboratory. For that purpose, studies have been carried out on the very dilute region by means of conductivity experiments, confirming the existence of two critical aggregation concentrations in that concentration domain: the so-called mixed critical aggregate concentration, CAC, and the mixed critical vesicle concentration, CVC. By carrying out TEM experiments on negatively stained samples, we were surprised to find a number of aggregates without a clear aggregation pattern and with a variety of sizes and shapes at concentrations below CAC, where only monomers were expected. However, the nanoaggregates found at concentrations between CAC and CVC, also by TEM microscopy, show a clear and ordered "fingerprint"-like aggregation pattern similar to the liquid-crystalline phases reported for DNA-liposome complexes and/or DNA packed with viral capsids. Finally, at total surfactant concentrations above CVC, the aggregates were confirmed, by means of cryo-TEM micrographs and zeta potential measurements, to be essentially unilamellar spherical vesicles with a medium polydispersity and a net-averaged surface density charge of around 12 x 10(-3) C m(-2). The fluorescence emission of two probes, TNS (anionic) and PRODAN (nonionic), allows for the analysis of the micropolarity and microviscosity of the different microenvironments present in aqueous surfactant solutions where the above-mentioned vesicle and prevesicle aggregates are present.     

Clustering of nucleosides in the presence of alkali metals: Biologically relevant quartets of guanosine,deoxyguanosine and uridine observed by ESI-MS/MS

Electrospray ionization (ESI) mass spectra of nucleosides, recorded in the presence of alkali metals, display alkali metal ion-bound quartets and other clusters that may have implications for understanding non-covalent interactions in DNA and RNA. The tetramers of guanosine and deoxyguanosine and also their metaclusters (clusters of clusters), cationized by alkali metals, were observed as unusually abundant magic number clusters. The observation of these species in the gas phase parallels previous condensed-phase studies, which show that guanine derivatives can form quartets and metaclusters of quartets in solution in the presence of metal cations. This parallel behavior and also internal evidence suggest that bonding in the guanosine tetramers involves the bases rather than the sugar units. The nucleobases thymine and uracil are known to form magic number pentameric adducts with K+, Cs+ and NH4+ in the gas phase. In sharp contrast, we now show that the nucleosides uridine and deoxythymidine do not form the pentameric clusters characteristic of the corresponding bases. More subtle effects of the sugars are evident in the fact that adenosine and cytidine form numerous higher order clusters with alkali metals, whereas deoxyadenosine and deoxycytidine show no clustering. It is suggested that hydrogen bonding between the bases in the tetramers of dG and rG are the dominant interactions in the clusters, hence changing the ribose group to deoxyribose (and vice versa) generally has little effect. However, the additional hydroxyl group of RNA nucleosides enhances the non-selective formation of higher-order aggregates for adenosine and cytidine and results in the lack of highly stable magic number clusters. Some clusters are the result of aggregation in the course of ionization (ESI) whereas others appear to be intrinsic to the solution being examined.