Solvent Sensitivity Research Articles

Solvent Effects on the Redox Properties of Cu Complexes Used as Mediators in Atom Transfer Radical Polymerization

Solvent effects on the redox properties of six Cu(I) complexes used as mediators in atom transfer radical polymerization (ATRP) have been studied using cyclic voltammetry. The six ligands used were tris[2-(dimethylamino)ethyl]amine, N-(n-propyl)-2-pyridylmethanimine, N,N,N',N',N'-pentamethyldiethylenetriamine, 1,1,4,7,10,10-hexamethyl-triethylenetetramine, 2,2'-bipyridine, and 1,4,8,11-tetraaza-1,4,8,11-tetramethylcyclotetradecan. The solvents used were DMSO, DMF, MeCN, MeOH, IP, and BuOH. Significant solvent effects were observed and quantitatively analyzed in terms of Kamlet-Taft relationships. The resulting Kamlet-Taft equations were found to successfully describe the solvent effects and could thus be used as tools for the design of ATRP in new solvents. The solvent sensitivity of the different ligands and the nature of the solvent effects are also discussed to some extent.

Solvent Effects on the Redox Properties of Thioethers

The one-electron reduction potential of the radical cations of thioanisole (1), benzyl methyl sulfide (2) and 2-hydroxyethyl benzyl sulfide (3) in water, formamide, acetonitrile, acetone, 1,1,1,3,3,3-hexafluoropropan-2-ol, methanol and 2-propanol was investigated by cyclic voltammetry. For comparison the one-electron reduction potentials in water were also measured using pulse radiolysis. The redox potential is strongly influenced by the nature of the solvent and the solvent sensitivity increases with charge localization. The present results have been used to evaluate solvent effects in view of the Kamlet-Taft relationship. The Kamlet-Taft expression quantitatively describes the solvent effects on the redox properties of 1-3 and gives the relative importance of the different solvent properties. The dominating contribution to the solvent effects is the solvent dipolarity/polarizability pi*, whereas alpha appears to be of minor importance. Furthermore, the relationship between the pi* and reduction potential of radical cations of 1-3 appear to be linear. It was also possible to find the same trend between the solvent dipole moment and peak potential of 1-3. These facts indicate that the nature of solvation is mainly nonspecific.

The Link between Sequence and Conformation in Protein Structures Appears To Be Stereochemically Established

In search of the link between sequence and conformation in protein structures, we perform molecular dynamics analysis of the effect of stereochemical mutation in end-protected octa-alanine Ac-Ala8-NHMe from poly-L to an alternating-L,D structure. The mutation has a dramatic effect, transforming the peptide from a condition of extreme sensitivity to one of extreme insensitivity to solvent. Examining the molecular folds of poly-L and alternating-L,D structure in atomistic detail, we find them to differ in the relationship between peptide dipolar interactions at the local and nonlocal levels, either conflicting or harmonious depending upon the chain stereochemistry. The stereochemical transformation of interpeptide electrostatics from a condition of conflict to one of harmony explains the long-standing puzzle of why poly-L and alternating-L,D peptides strongly differ in properties such as "stiffness" and solvent sensitivity. Furthermore, it is possible that poly-L stereochemistry is also the fulcrum of protein sensitivity to the effects of amino acid side-chain structures via dielectric arbitrations in interpeptide electrostatics. Indeed the evidence is accumulating that the amino acid side chains differing in alpha-helix and beta-sheet propensities also differ in their desolvating effects in the adjacent and nearest-neighbor peptides and thus possibly in the solvent screening of peptide dipolar interactions.

Primary Photoinduced Processes in Bimetallic Dyads with Extended Aromatic Bridges. Tetraazatetrapyridopentacene Complexes of Ruthenium(II) and Osmium(II)

The photophysics of the binuclear complexes [(phen)2M(tatpp)M(phen)2]4+, where M = Ru or Os, phen = 1,10-phenanthroline, and tatpp = 9,11,20,22-tetraazatetrapyrido[3,2-a:2'3'-c:3'',2''-l:2''',3''']pentacene, has been studied in acetonitrile and dichloromethane by femtosecond and nanosecond time-resolved techniques. The results demonstrate that complexes of different metals have different types of lowest excited state: a tatpp ligand-centered (LC) triplet in the case of Ru(II); a metal-to-ligand charge-transfer (MLCT) triplet state in the case of Os(II). The excited-state kinetics is strongly solvent-dependent. In the Ru(II) system, the formation and decay of the LC state take place, respectively, in 25 ps and ca. 5 ns in CH3CN and in 0.5 ps and 1.3 micros in CH2Cl2. These solvent effects can be rationalized on the basis of a thermally activated decay of the LC state through the upper MLCT state. In the Os(II) system, the formation and decay of the MLCT state take place, respectively, in 3.8 and 60 ps in CH3CN and in 0.5 and 4 ps in CH2Cl2. These effects are consistent with the solvent sensitivity of the MLCT energy, in terms of driving force and energy-gap law arguments. The relevance of these results for the use of ladder-type aromatic bridges as potential molecular wires is discussed.

Effect of Reaction Media on the Growth and Photoluminescence of Colloidal CdSe Nanocrystals

Using cadium oxide (CdO) as the Cd precursor and tri-n-octylphosphine selenide (TOPSe) as the Se source, TOP-capped and TOP/tri-n-octylphosphine oxide (TOPO)-capped CdSe nanocrystals were synthesized without the use of an acid. The synthetic approach involved the addition of a TOPSe/TOP solution into a CdO/TOP solution with or without TOPO at one temperature and subsequent growth at a lower temperature. The temporal evolution of the optical properties, namely, absorption and luminescence, of the growing nanocrystals was monitored in detail. A comprehensive examination on the control of the photoluminescence (PL) properties was performed by systematically varying the TOP/TOPO weight ratio of the reaction media. Surprisingly, a rational choice of 100% TOP or 80% TOP was found to produce "quality" nanocrystals when monitored under the present experimental conditions and growth-time scale. The term "quality" is mainly based on the sharp features and rich substructure exhibited in the absorption spectra of the growing nanocrystals, as well as the sharp features in the emission spectra with narrow full width at half-maximum (fwhm). There are two distinguishable stages of growth: an early stage (<5 min) and a later stage. TOP plays a major role in the control of a slow growth rate in the early stage, while TOPO controls slow growth in the later stage. The optical sensitivity of the growing nanocrystals when dispersed in nonpolar or polar solvents was studied, including two size-dependent parameters, namely, the solvent sensitivity (PL intensity) and nonresonant Stokes shift (NRSS). The insights gained from the present study enable a synthetic approach in which high-quality CdSe nanocrystals are achieved with high synthetic reproducibility.

Transcriptional analysis of the ostA/imp gene involved in organic solvent sensitivity in Escherichia coli.

Transcriptional initiation sites of the ostA gene involved in organic solvent sensitivity in Escherichia coli were found by primer extension analysis. Two transcriptional initiation sites were newly identified at -133 and -48 nucleotides from the initiation codon of ostA, but the previously reported sigmaE-dependent one at -227 could not be detected. No heat-inducible expression of ostA was observed by Northern blotting analysis, indicating that the contribution of sigmaE-dependent transcription was very small if any. SigmaD-dependent promoter-like sequences were found just upstream of the newly identified transcriptional initiation sites by computer-aided analysis. Deletion analysis of ostA-lacZ fusions demonstrated that these two promoters contributed almost equally to the constitutive expression of the ostA gene.

A kinetic study of 1,3-dipolar cycloadditions in micellar media.

The kinetics of the 1,3-dipolar cycloadditions (DC) of benzonitrile oxide with a series of N-substituted maleimides in micellar media have been investigated. Surfactants studied include anionic sodium dodecyl sulfate, cationic cetyltrimethylammonium bromide, and a series of nonionic alkyl poly(ethylene oxide) surfactants (CxEy). The kinetic data have been analyzed by using the pseudo-phase model for bimolecular reactions. Much larger micellar accelerations (up to a factor of 17) were observed for these reactions than was previously found for Diels-Alder (DA) reactions (J. Org. Chem. 2002, 67, 7369-7377). This is explained by the smaller solvent sensitivity of DC reactions, which causes the micellar rate constants to be much closer to the value of water (km/kw approximately 0.25-0.45 for DC reactions vs 0.02-0.05 for DA reactions). Further evidence is presented, that a water/1-propanol mixture ([H2O] ca. 15 M) is a fairly good mimic of the micellar reaction environment for these reactions. Isobaric activation parameters have been determined for the reaction in the micellar phase of C16E20, using micellar rate constants. They correspond well to values obtained for the aforementioned micelle mimic.

Separation and trace estimation of benzidine and its macromolecular adducts using supercritical fluid chromatography

A sensitive, rapid, selective and reproducible method has been developed to measure blood plasma levels of benzidine (BZ) and its acetylated metabolite, N-OH- N, N′-diacetylbenzidine (N-OH-DABZ), using supercritical fluid chromatography (SFC) for the first time. Benzidine and N-OH- N, N′-diacetylbenzidine were extracted from the plasma using ether. Separation was done on a Nucleosil ( 250 mm×4.6 mm) 10 μm, Nucleosil-RP-C 18 column with 7.4% (v/v) methanol-modified supercritical fluid carbon dioxide (2.5 ml min −1) as mobile phase. The column temperature was 45 °C and the outlet pressure was set at 8.83 MPa. The detection was done using a UV-Vis detector set at 280 nm. The limit of quantification was 0.10 ng ml −1 (BZ) and 0.14 ng ml −1 (N-OH-diacetylbenzidine) using 1 ml plasma specimen. The mean extraction recovery of BZ was found to be 98.6%. The SFC method was directly compared to a published HPLC–UV method. With respect to speed, organic solvent usage, sensitivity, specificity and accuracy, SFC was found to be superior. The method has been successfully used to estimate the BZ, N-OH-diacetylbenzidine levels in blood plasma of the animals who were administered 15 μg kg −1 body weight of benzidine. Further, this method has been also applied for the detection and quantification of benzidine DNA and hemoglobin adducts from the blood and tissue samples of the benzidine dosed animals.

Nitrile anions: solvent-dependent cyclizations.

Extensive cyclizations in hydrocarbon and polar solvents demonstrate a profound solvent sensitivity for intramolecular nitrile anion alkylations. S(N)i cyclizations enforce very precise steric constraints in the transition state, allowing correlation of the cyclization stereochemistry with the orbital orientation of the nitrile anion. Collectively the cyclizations suggest a continuum of nitrile anion transition states, varying from planar to fully pyramidal, that selectively cyclize to cis- and trans-decalins, respectively.

Oxo-Metal-Polyimide Nanocomposites. 2. Enhancement of Thermal, Mechanical, and Chemical Properties in Soluble Hexafluoroisopropylidine-Based Polyimides via the in Situ Formation of Oxo-Lanthanide(III)-Polyimide Nanocomposites

Tris(2,4-pentanedionato)lanthanide(III) polyhydrates were added as inorganic sol−gel precursors to DMAc solutions of the polyimides formed from 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride and 1,3-bis-(aminophenoxy)benzene, 6FDA/1,3(3)-APB and 6FDA/2,2-bis[4-(4-aminophenoxy]hexafluoropropane, 6FDA/4-BDAF. Thermal treatment of doped-resin films gave composite oxo-metal(III) nanocomposite membranes which were visually clear with lower linear coefficients of thermal expansion, increased moduli, and reduced solvent sensitivity. Glass transition temperatures and 10% weight loss temperatures in nitrogen were close to those of the parent polymers. Films were characterized by thermal, mechanical, X-ray, and transmission electron micrographic techniques.

Solvatochromism and piezochromism of dicyano-, tricyano-, and tetracyano-diimine-iron(II) complexes

The solvatochromism of several dicyano-bis-diimine-iron(II) complexes in various binary aqueous solvent mixtures has been established, at 25 °C. A solvent sensitivity scale has been developed for these complexes. The solvatochromic properties of two tricyano-terdentate ligand iron(II) complexes in several solvents have also been determined, as have those of a series of tetracyano-diimine complexes in DMSO–water media. These results have been analysed and systematic but varied trends of solvation were demonstrated. Piezochromic parameters for two complexes have been obtained and are discussed in the context of a solvatochromism/piezochromism correlation.

Defining a rob regulon in Escherichia coli by using transposon mutagenesis.

The Rob protein of Escherichia coli is a member of the AraC-XylS family of prokaryotic transcriptional regulators and is expressed constitutively. Deletion of the rob gene increases susceptibility to organic solvents, while overexpression of Rob increases tolerance to organic solvents and resistance to a variety of antibiotics and to the superoxide-generating compound phenazine methosulfate. To determine whether constitutive levels of Rob regulate basal gene expression, we performed a MudJ transposon screen in a rob deletion mutant containing a plasmid that allows for controlled rob gene expression. We identified eight genes and confirmed that seven are transcriptionally activated by normal expression of Rob from the chromosomal rob gene (inaA, marR, aslB, ybaO, mdlA, yfhD, and ybiS). One gene, galT, was repressed by Rob. We also demonstrated by Northern analysis that basal expression of micF is significantly higher in wild-type E. coli than in a rob deletion mutant. Rob binding to the promoter regions of most of these genes was substantiated in electrophoretic mobility shift assays. However, Mu insertions in individual Rob-regulated genes did not affect solvent sensitivity. This phenotype may depend on changes in the expression of several of these Rob-regulated genes or on other genes that were not identified. Rob clearly affects the basal expression of genes with a broad range of functions, including antibiotic resistance, acid adaptation, carbon metabolism, cell wall synthesis, central intermediary metabolism, and transport. The magnitudes of Rob's effects are modest, however, and the protein may thus play a role as a general transcription cofactor.

Electrochemical studies of poly(vinylferrocene) films in aqueous and non-aqueous electrolyte solutions

The electrochemical response of a conducting polymer, poly(vinylferrocene) (PVF), as a function of the electrolyte media, is reported. Cyclic voltammetry has been used to investigate the redox behaviour of the films spin-cast onto glass electrodes (ITO). The films were cycled in non-aqueous (propylene carbonate and tetramethylene sulfone) electrolyte solutions containing different salts: sodium, lithium and tetrabutylammonium perchlorates (NaClO4, LiClO4 and TBAClO4), tetrabutylammonium hexafluorophosphate and tetrafluoroborate (TBAPF6, TBABF4). Aqueous electrolyte solutions of NaClO4 and LiClO4 have also been studied for comparison with the reported literature data. Some of the criteria for reversible systems have been tested for the different electrolytes used. Cyclic voltammetry experiments have also been carried out on the same films, after 3 months, using electrolyte solutions in propylene carbonate and tetramethylene sulfone. The results show that the film has a ‘solvent sensitivity’ effect, and the charge density changes according to the solvent used. There is a reversible charge sensivity effect when the films are cycled between propylene carbonate- and tetramethylene sulfone-based electrolytes. © 2000 Society of Chemical Industry

Investigation of solvatochromism in the low-lying singlet states of dithienyl polyenes

To understand the low-lying singlet states of dithienyl polyenes, we investigated the solvatochromism of a series of α,ω-di(2-dithienyl 3,4-butyl) polyenes having n=1–5 double bonds. Absorption and emission spectra were collected in a series of aprotic solvents. The absorption energy dispersion effect sensitivity increased smoothly with n, reaching asymptotic behavior as n approached 5. The emission energy had less solvent sensitivity. The trends gave evidence for the existence of a 1 B ∗ u absorbing state and a 1 A ∗ g emitting state. We observed sensitivity of the absorbing and emitting states to solute–solvent electrostatic interactions, suggesting the dithienyl polyenes had a polar ground state conformation.

Widely differing stabilities of molecule-bridged cyanodiiron(III, II) species in non-aqueous solvents

The redox systems [(NC) 5Fe(μ-L)Fe(CN) 5]) 6−/5−/4− with L=pyrazine ( 1 n ), 4,4′-bipyridine ( 2 n ) or 1,2-bis(4-pyridyl)ethene ( 3 n ), and [(NC) 4Fe(μ-bmtz)Fe(CN) 4] 5−/4−/3− ( 4 n ) with bmtz=3,6-bis(2-pyrimidyl)-1,2,4,5-tetrazine were studied in acetonitrile and other aprotic solvents. The comproportionation constants K c for the mixed-valent diiron(III, II) intermediates vary from less than 10 for 3 5− via 10 6.5 for 1 5− to 10 14.1 for 4 3−. Intervalence charge transfer band maxima lie between 1270 nm ( 2 5−) and 2475 nm ( 1 5−). In comparison with previous results from aqueous solutions, the system 1 n in particular shows increased metal–metal interaction. UV–Vis and IR spectroelectrochemistry suggest a localized/delocalized borderline sitation for 1 5− in acetonitrile, with only slightly weaker metal–metal coupling than in the related Creutz–Taube ion. The results illustrate how the established strong solvent sensitivity of cyanoiron complexes can be exploited in mixed-valence chemistry.

Simultaneous and Simple Determination of Saccharin and Acesulfame K in Foods by GC-NPD

A rapid and simple method for the simultaneous determination of saccharin (SA) and acesulfame K (AK) in foods by GC-NPD was developed. A good extraction rate was obtained by using ethyl acetate as an extraction solvent and high sensitivity was obtained by using GC-NPD. Interfering substances were removed or reduced effectively by clean-up using an acidic alumina cartridge column.The detection limit was 5 ppm in samples, and SA and AK were confirmed by GC/MS. The proposed method was applied to 89 samples of imported foods, such as soft drinks, instant cocoa and confectionery.

Gradient-enhanced TOCSY experiments with improved sensitivity and solvent suppression

Gradient-enhanced versions of the homonuclear TOCSY experiment are described, with solvent suppression and sensitivity superior to that of a conventional TOCSY experiment. The pulse sequences are constructed by appending a WATERGATE module to a z-filtered TOCSY experiment. Pulsed-field gradients and appropriately phased selective rf pulses are used to maintain precise control of the water magnetization vector. Problems associated with radiation damping and spin-locking of the water magnetization are thus alleviated. The water magnetization is returned to equilibrium prior to each acquisition, which improves water suppression and minimizes signal losses due to saturation transfer.

Interfacial polycondensation. II. Fundamentals of polymer formation at liquid interfaces

If a solution of a fast-reacting diacid halide in a water-immiscible solvent is brought together with an aqueous solution of a diamine without stirring, a thin film of high polymer is formed at once at the interface. Polyurethanes and polyamides in. particular form tough films which can be grasped and pulled continuously from the interface as a folded rope of film. The unstirred interfacial polycondensation not only provides a dramatic demonstration of polymer formation but has permitted the observation of polymerization behavior and the study of the effects of many variables upon the process. The polymer-forming reactions proceed by nucleophilic displacement, and many have rate constants of at least 102–101 l. mole-1 sec.-1. Polymers derived from diamines were found to form in the organic solvent phase. Therefore the rate of polymer formation is controlled by the transfer of diamine from the aqueous phase. It is believed that the liquid interface does not have any beneficial orienting effect on the reactants but that it provides for the regulated flow of one reactant into an excess of the other. Furthermore, the aqueous phase provides for the removal of the interfering byproduct, hydrogen halide, from the polymerization site. Some of the interrelated variables which have been studied and which are discussed in relation to the physical mechanism are the solvent sensitivity of the polymer, partition coefficients of the reactants, reactant concentration, duration of the polymerization, and the addition of the monofunctional reactants, detergents, and salts. The formation of polyphenyl esters is discussed briefly.

FT-IR Solution Spectra of Propyl Sulfide, Propyl Sulfoxide, and Propyl Sulfone

FT-IR spectra were obtained of 0.5% volumetric solutions of propyl sulfide, propyl sulfoxide, and propyl sulfone in hexane, CCl4, CS2, and CHCl3 to assist in the assignment of FT-IR-PAS spectra of propyl sulfoxide sorbed within the structure of several peats and onto cellulose. The C-H stretching bands of all three compounds showed a 10-fold larger solvent sensitivity than did analogous aliphatic bands in a comparable series of n-alkanes in solution. The S=O stretching bands of alkyl sulfoxides are known to be quite sensitive to solvent environments because of donor-acceptor interactions involving the oxygen of the sulfoxide group and the slightly acidic protons of generalized organic acids; the expected decreases in frequency were seen with propyl sulfoxide and acidic solvents like chloroform. The frequencies of asymmetric and symmetric stretching vibrations of the -SO2 group of propyl sulfone both exhibit smaller decreases in Old, than does the sulfoxide S=O stretching vibration.

Temperature and Solvent Effects on the Luminescence Spectrum of C70: Assignment of the Lowest Singlet and Triplet States

The temperature, solvent, and concentration dependence of the fluorescence and phosphorescence spectra of C{sub 70} in glassy solutions have been examined. Spectra have been recorded over the temperature range 4-200 K. In addition, the AC Stark field modulated phosphorescence and the phosphorescence of {sup 13}C{sub 70} have been recorded and analyzed. The lowest triplet state is identified as a {sup 3}E{sub 1}` state and the vibronic structure consists primarily of Herzberg-Teller active e{sub 2}` modes. The intensity of the electronic origin is comparable to the vibronically induced intensity and is extraordinarily solvent sensitive. The solvent sensitivity exhibited by the spectra is shown to have the same origins as that observed in benzene and pyrene, but is several times greater in magnitude. Analysis of the spectra suggests that two electronic excited states contribute to the observed phosphorescence spectrum. 27 refs., 6 figs., 1 tab.