Showing papers on "Solvent effects published in 1993"

Relation Between Bond-Length Alternation and Second Electronic Hyperpolarizability of Conjugated Organic Molecules

Abstract: The solvent dependence of the second hyperpolarizability, γ, of a variety of unsaturated organic compounds has been measured by third harmonic generation at 1907 nanometers. It is seen that the measured y is a function of solvent polarity. These solvent-dependent hyperpolarizabilities are associated with changes in molecular geometry from a highly bond-length alternated, polyene-like structure for a formyl-substituted compound in nonpolar solvents, to a cyanine-like structure, with little bond-length alternation, for a dicyanovinyl-substituted compound in polar solvents. By tuning bond-length alternation, γ can be optimized in either a positive or negative sense for polymethine dyes of a given conjugation length.

Coexisting stable conformations of gaseous protein ions.

Abstract: For further insight into the role of solvent in protein conformer stabilization, the structural and dynamic properties of protein ions in vacuo have been probed by hydrogen-deuterium exchange in a Fourier-transform mass spectrometer. Multiply charged ions generated by electrospray ionization of five proteins show exchange reactions with 2H2O at 10(-7) torr (1 torr = 133.3 Pa) exhibiting pseudo-first-order kinetics. Gas-phase compactness of the S-S cross-linked RNase A relative to denatured S-derivatized RNase A is indicated by exchange of 35 and 135 hydrogen atoms, respectively. For pure cytochrome c ions, the existence of at least three distinct gaseous conformers is indicated by the substantially different values--52, 113, and 74--of reactive H atoms; the observation of these same values for ions of a number--2, 7, and 5, respectively--of different charge states indicates conformational insensitivity to coulombic forces. For each of these conformers, the compactness in vacuo indicated by these values corresponds directly to that of a known conformer structure in the solution from which the conformer ions are produced by electrospray. S-derivatized RNase A ions also exist as at least two gaseous conformers exchanging 50-140 H atoms. Gaseous conformer ions are isometrically stable for hours; removal of solvent greatly increases conformational rigidity. More specific ion-molecule reactions could provide further details of conformer structures.

Polymer brushes at curved surfaces.

Abstract: The polymer adsorption theory of Scheutjens and Fleer is used to describe polymer brushes at spherical and cylindrical surfaces that are immersed in a low molecular weight solvent. The volume fraction profiles of such brushes is analyzed, special attention being paid to spherical brushes in athermal solvents. These are shown to generally consist of two parts: a power law-like part and a part that is consistent with a parabolic potential energy profile of the polymer segments

Dynamic Solvent Effects on Electron-Transfer Reactions

Abstract: Electron-transfer processes in solution are among the most important reactions in chemistry and biology. The huge number of redox reactions of transition metal ions and complexes, many preparatively important oxidations and reductions of organic compounds, photosynthesis, and metabolism are only a few examples where electron-transfer reactions play a pivotal role. This ubiquity, as well as their relative simplicity, makes them excellent models for the study on a molecular level of chemical reactions in solution. A particularly important question in chemical reaction dynamics in solution is the influence of the solvent on the reaction rate. In this context one distinguishes between static and dynamic solvent effects. Static effects refer to the stabilization of reactants, transition state, and products, that is, how the solvent affects the free energies of these species and the energy of activation. This interpretation of solvent effects on all kinds of chemical reactions is well established. A more recent development is the investigation of the influence of solvent dynamics on the rate of a reaction. The transfer of an electron is usually thought to be triggered by a fluctuation of the dielectric polarization in the surrounding solvent. The dynamics of such fluctuations is determined by the finite response time of the orientational polarization of the solvent. Under certain conditions this dielectric response time can become the rate-determining factor of the reaction. In this article I intend to give a review of these modern developments in the theory and experimental study of electron-transfer processes. We shall see that solvent dynamics may lead to a whole plethora of phenomena in reaction dynamics. The concepts needed for their description are not limited to electron transfer but bear relevance to many other chemical reactions in solution.

Solvent effects on the competitive .beta.-scission and hydrogen atom abstraction reactions of the cumyloxyl radical. Resolution of a long-standing problem

Abstract: Laser flash photolysis (LFP) techniques with detection in the infrared and in UV-visible regions of the spectrum have been used in combination with detailed product studies to assess solvent effects on the hydrogen abstraction and β-scission reactions of cumyloxyl radicals. The variation in the ratio of the products of these two competing processes in solvents with different polarities is due to the solvent's influence on β-scission

Toxicity of homologous series of organic solvents for the gram-positive bacteria Arthrobacter and Nocardia Sp. and the gram-negative bacteria Acinetobacter and Pseudomonas Sp.

Abstract: The toxicity of homologous series of organic solvents has been investigated for the gram-positive bacteria, Arthrobacter sp. and Nocardia sp., and the gram-negative bacteria, Acinetobacter sp. and Pseudomonas sp. The hydrophobicity of the solvent, expressed by its logP(octanol), proves to be a good measure for the toxicity of solvents in a two-phase system. The transition from toxic to nontoxic solvents occurs between logP(octanol) 3 and 5 and depends on the homologous series. No correlation has been found between the hydrophobicity of the substituent on the alkyl backbone of the solvent and the location of the transition point in toxicity. The logP(octanol), above which all solvents are nontoxic, is used to express the solvent tolerance of the bacteria. In general, the solvent tolerance of gram-negative bacteria is found to be slightly higher than that of gram-positive bacteria, but this does not hold for all homologous series of organic solvents investigated.Because the toxicity effects of organic solvents in a two-phase system can be ascribed to molecular as well as phase toxicity effects, molecular toxicity effects were investigated separately in a one-phase system with subsaturating amounts of organic solvent. The solvent concentration in the aqueous phase, at which 50% of the metabolic activity of the bacteria is lost, is used to express solvent toxicity. This concentration is found to be similar for the gram-positive Arthrobacter and the gram-negative Acinetobacter. Assuming the critical membrane concentration theory (G. J. Osborne et al. Enzyme Microb. Technol. 1990, 12: 281-291) to be valid, it can be concluded that differences in solvent tolerance between these two bacteria, cannot be ascribed to differences in response to molecular toxicity. Prediction of the toxicity of any solvent, using the critical membrane theory, appears to be possible in the case of alkanols or alkyl acetates. However, prediction of the toxicity of ethers appears to be impossible.

Hetero-Diels-Alder reaction transition structures: reactivity, stereoselectivity, catalysis, solvent effects, and the exo-lone-pair effect

Abstract: Transition structures for the Diels-Alder reactions of butadiene with formaldehyde, thioformaldehyde, formaldimine, N-methylformaldimine, diaxene, nitrosyl hydride, singlet oxygen, BH 3 -coordinated formaldehyde and formaldimine, and protonated formaldimine have been located with ab initio molecular orbital calculations using the 3-21G basis set. Activation energies have been evaluated with MP2/6-31G * calculations on these geometries. Formaldehyde and the imines are predicted to be comparable in reactivity to ethylene, whereas the other dienophiles are predicted to be more reactive

Hybridization of Block Copolymer Micelles

Abstract: : Block copolymers of styrene and methacrylic acid dissolved in water/ dioxane mixtures form spherical micelles with polystyrene cores and poly(methacrylic acid) shells. When two micellar solutions containing micelles made from two different copolymers are mixed, the micelles may hybridize. The hybridization was followed by the method of sedimentation velocity. The rate of hybridization was found to be a very sensitive function of the architecture of both copolymers and of the thermodynamic properties of the solvent mixture. In many instances this rate is imperceptible slow; it may be totally frozen when the solvent is very poor with respect to the micellar core. The analysis of the hybridization of numerous micellar pairs provided new insights into the dynamics and thermodynamics of unimer/micelle equilibria. (Author)

Solution and solid-state properties of hybrid linear-dendritic block copolymers

Abstract: The solution and solid-state properties of hybrid linear-dendritic polyether copolymers are investigated by SEC with coupled viscometric detection, polarized light microscopy, and X-ray analysis. The results obtained show that the block copolymers are able to form mono- and multimolecular micelles depending on the dendrimer generation and the concentration in methanol/water (good solvent for the linear blocks). Only monomolecular micelles are observed in THF (good solvent for the dendritic blocks). The linear blocks are able to crystallize in spherulites, axialites, or unique dendritic structures depending on the overall composition of the copolymers and the solvent used

Solvent variation inverts substrate specificity of an enzyme

Abstract: The substrate specificity of the serine protease subtilisin Carlsberg in the transesterification reaction of N-Ac-L-Ser-OEt and N-Ac-L-Phe-OEt with 1-propanol was examined in 20 anhydrous solvents. The serine substrate was strongly favored in some solvents, while the phenylalanine substrate was greatly preferred in others. A thermodynamic model was derived which correctly predicted the substrate specificity as a function of the solvent-to-water partition coefficients of the substrates and the substrate specificity of the enzyme-catalyzed hydrolysis of the esters in water. This model is independent of the substrates and and the substrate, so long as the latter is removed from the solvent in the transition state

Vibrational and rotational relaxation times of solvated molecular ions

Abstract: Infrared pump–probe and infrared polarization spectroscopy have been used to measure the vibrational relaxation times (T1) of the antisymmetric stretching mode and the reorientation times (TR) for N3−, NCS−, and NCO− in D2O and/or methanol. For N3−, experiments were also conducted in H2O and hexamethyl–phosphamide (HPMA) solutions. The rapid vibrational relaxation and slow reorientation observed demonstrate strong coupling between the ions and the solvents. Longer vibrational relaxation and shorter reorientation times measured for NCS− reveal weaker solvent interactions that may be due to the importance of the charge distribution and the form of the normal coordinate. A comparison of the T1 and TR times in different solvents permits a determination of the relative interaction strengths for the solvents investigated. The relatively weaker coupling of N3− in the aprotic solvent HMPA demonstrates the importance of hydrogen bonding in strong solvent interactions in ionic solutions. The experimental results ar...

Solvent effects on the mechanism and selectivities of asymmetric Diels-Alder reactions

Abstract: The electrostatic effect of the solvent on the reaction of cyclopentadiene with methyl acrylate has been studied with the help of a Self-Consistent Reaction Field model by means of ab initio computations. The effect on the reaction mechanism, endo/exo selectivity, and activation energy has been analyzed. Moreover, the computations allow a discussion of the solvent effect on the diastereofacial selectivity when chiral acrylates are used and a omparison with experimental data for the reaction of cyclopentadiene with (-)-menthyl acrylate. The results are in agreement with the experimentally observed increase of the endo/exo and diastereofacial selectivities as a function of solvent polarity, but hydrophobic effects and hydrogen-bond formation are necessary to account for changes in reaction rates

Structure-photophysics correlations in a series of 4-(dialkylamino)stilbenes: intramolecular charge transfer in the excited state as related to the twist around the single bonds

Abstract: 4-(Dimethylamino)stilbene and eight regioselectively bridged 4-(dialkylamino)stilbene derivatives have been synthesized and their solvatochromism, fluorescence quantum yields, and lifetimes measured versus the solvent polarity and the temperature. When the single bond connecting the dialkylanilino group to the ethylene group (bond 2) is bridged, strong fluorescence quenching is observed; when this bond is flexible, the fluorescence quenching is strongly reduced, and lifetime maxima at intermediate temperature indicate the involvement of a further TICT emitting state. Fluorescence quenching is reduced in polar solvents and suppressed when the double bond is included in a five-membered carbocyclic ring

Reconsideration of solvent effects calculated by semiempirical quantum chemical methods

Abstract: AM1 and PM3 semiempirical calculations are reported for the solvent effects on the tautomeric equilibria of 2-pyridone/2-hydroxypyridine and 4-pyridone/4-hydroxypyridine in the gas phase and solution. The solvent effects on the tautomeric equilibria were investigated by self-consistent reaction field (SCRF) theory implemented in the AMPAC and MOPAC program in two different ways: one in which all the solvent relaxation is included in the quantum mechanics and the total energy must be corrected for the solvent change in energy, method A; and a second in which the quantum mechanics directly includes this term, method B. The calculated (AM1, method A) tautomeric equilibrium constants (log K1) for 2-pyridone in the gas phase, cyclohexane, chloroform, and acetonitrile are −0.3, 0.3, 0.8, and 1.3, respectively, in good agreement with the experimental data (−0.4, 0.24, 0.78, and 2.17, respectively). For 4-pyridone/4-hydroxypyridine differences between calculated log K1 for the gas phase, chloroform and acetonitrile (−6.0, −2.6, and −1.2, respectively) and experimental data (< −1, 0.11, and 0.66, respectively) are larger but the experimental values are also less certain. The experimental acetonitrile data are disturbed by specific interactions. An extension of the SCRF for aqueous solutions is reviewed. © 1993 John Wiley & Sons, Inc.

Charge recombination reactions in photoexcited fullerene C60-amine complexes studied by picosecond pump probe spectroscopy

Abstract: Photoexcitation of complexes between fullerence C[sub 60] and organic amines in benzene solutions is known to result in charge separation (CS) and subsequent charge recombination (CR) reactions, which lead to varying yields of fullerence triplet formation. Picosecond flash photolysis studies are carried out on C[sub 60]-diphenylamine (DPA), C[sub 60]-triethylamine (TEA), C[sub 60]-diazabicyclooctane (DABCO), and C[sub 60]-triphenylamine (TPA) systems to find out mechanistic details of the triplet formation on CR by inducing heavy atom and polarity effects by using suitable solvents. It is found that in the case of C[sub 60]-DPA, C[sub 60]-TEA, and C[sub 60]-DABCO systems proton transfer from the amine cation to the C[sub 60] anion in the ion pair state dominates, leading to poor triplet yields, which improve in heavy atom containing solvents. In TPA, proton transfer is not possible and hence fullerene triplet yields are high. Increase of solvent polarity for this system results in decreased C[sub 60] triplet yields with a consequent increase in the ion dissociation yield. A suitable reaction scheme is proposed to explain the results obtained. 34 refs., 5 figs., 2 tabs.

Solvent-induced frequency shifts in the infrared spectrum of acetonitrile in organic solvents

Abstract: The solvent-induced frequency shifts (SIFS) for the C≡N stretching vibration (ν 2 ) in acetonitrile and deuterated acetonitrile have been determined in a wide variety of organic solvents. It is shown that this band is shifted to higher energies in the presence of solvents which are more stronger Lewis acids than acetonitrile itself. On the other hand, in the presence of solvents which are stronger Lewis bases, the ν 2 band is shifted to lower energies. This is taken to be evidence that the electronegative end of the Lewis base interacts with the methyl group in acetonitrile. Supporting evidence for this conclusion is reported on the basis of shifts in the symmetrical (ν 1 ) and antisymmetrical (ν 5 ) CD3 stretching bands

Crystallization of polymorphs : the effect of solvent

Abstract: The effect of solvent in crystallization of polymorphs has been studied using the drug sulphathiazole as a model compound. The solubilities of the four polymorphic forms of sulphathiazole were determined as a function of temperature in various solvents. Within the temperature ranges studied, the rank order of solubility of the polymorphs was the same in all solvent systems. On the basis of this knowledge of the temperature dependence of the solubilities, recrystallization experiments, in which the supersaturation was systematically varied, were carried out in an endeavour to isolate each of the polymorphic forms from each solvent system. These recrystallization experiments reveal that not all of the know polymorphic forms can be crystallized from any given solvent by varying the supersaturation. Indeed some solvents selectively favour the crystallization of a particular form of forms. The authors conclude that thermodynamic effects are not responsible for the selective behaviour of a solvent. A kinetic mechanism is proposed. It is considered that the solvent acts by selective adsorption to certain faces of some of the polymorphs, and thereby either inhibits their nucleation or retards their growth to the advantage of others.

Tautomeric equilibrium and hydrogen shifts of tetrazole in the gas phase and in solution

Abstract: High-level ab initio molecular orbital calculations, using basis sets up to 6-311+G(2d,2p) with electron correlation incorporated at the quadratic configuration interaction [QCISD(T)] level, have been used to study the tautomeric equilibrium and hydrogen shifts of tetrazole in the gas phase and in solution. The solvent effects were investigated by self-consistent reaction field (SCRF) theory. Consistent with experimental observations, the 1H-tetrazole (1)/2H-tetrazole (2) tautomeric equilibrium is calculated to be strongly influenced by the surrounding medium. 2H-Tetrazole is the energetically preferred tautomer in the gas phase. In a nonpolar solution, both the 1H and 2H forms are predicted to exist in comparable amounts. However, in a medium of high dielectric constant the more polar 1H tautomer is the dominant species. The calculated free energy changes for tautomerization of IH-tetrazole in the gas phase and in nonpolar (epsilon = 2) and polar (epsilon = 40) media are -7, 1, and 12 kJ mol-1, respectively. The molecular geometry, charge distribution, and vibrational frequencies of the polar 1H tautomer are found to be altered significantly in the presence of a solvent reaction field. Isomerization of 1 to 2, via a [1,2] hydrogen shift, requires an energy barrier of 207 kJ mol-1 in the gas phase. 5H-Tetrazole (3) is predicted to lie 82 kJ mol-1 above 1, due to its nonaromatic character. However, rearrangement of 3 to 1, via a [1,5] hydrogen shift, is inhibited by an activation barrier of 150 kJ mol-1. Conversely, the energy barrier for the rearrangement of 1 to 3 is 232 kJ mol-1, slightly larger than that required for the isomerization of 1 to 2. These results suggest that 3 is a good candidate for experimental observation. Inclusion of electron correlation leads to a drastic change in the molecular geometry of 3. At the MP2 level, an acyclic structure is predicted, while at the MP3 and QCISD levels the expected cyclic structure is found, The calculated molecular geometry of 1H-tetrazole at the MP2 level is found to differ significantly from the available solid-state structural data.

Donor numbers of anions in solution: the use of solvatochromic Lewis acid–base indicators

Abstract: The shift of the visible d–d absorption bands of [Cu(acac)(tmen)]+(acac = acetylacetonate, tmen =N,N,N′,N′-tetramethylethylenediamine) in various solvents resulting from the co-ordination of solvent molecules and anionic species X has been investigated. The results have been used to establish donor numbers (DNX, dce) of anions dissolved in 1,2-dichloroethane (dce) on the same scale as that already established for solvents. Apparent donor numbers for anions dissolved in various solvents other than dce (DNX, solv) are also given and used to show that the donor properties of anions are directly related to the acceptor number (ANsolv) of the solvent. This arises from competition between solvent and acceptor ion for the anion leading to a steady decrease in the effective donor number of the anion as ANsolv increases, culminating in an effective donor number of zero for all anions in the strong acceptor solvent water (ion pairing is at a minimum in water). The values of the anion donor numbers are related to some experimental results such as spectral, equilibrium, kinetic and thermodynamic data for selected systems. This method of establishing donor numbers for anions is shown to be reliable and easy to apply.

Proton and carbon-13 NMR investigations of lead zirconate titanate (Pb(Zr,Ti)O3) thin-film precursor solutions

Abstract: Solvent reactions, ligand substitutions, and the oligomer/polymer backbone structure are important factors in the solution preparation of ceramic films. In this study the authors have used H and C NMR spectroscopy to characterize solvent and ligand effects in precursor solutions used for the deposition of ferroelectric PZT (lead zirconate titanate) thin films. Solutions were prepared by a sequential precursor addition method from carboxylate and alkoxide precursors of the three cations, and the solvent, acetic acid, methanol, and water. The results indicate that acetic acid was a key component in the solution preparation process. As observed previously for single metallic component systems, its presence resulted in esterification reactions, leading in the present case to the formation of methyl, isopropyl, and n-butyl acetates. Second, acetic acid functioned as a chemical modifier, or chelating agent, replacing essentially all of the alkoxy ligands of the original precursors. Since alkoxy replacement appeared to be complete, we may describe the PZT species formed in solution as oxo acetate in nature. Finally, the solvent and ligand behavior of a solution prepared by an inverted mixing order was compared to the behavior of the solution prepared by a sequential precursor addition. The spectra for the two solutions weremore » similar, and only differences in the relative intensities of the ester and alcoholic resonances were observed. 29 refs., 5 figs., 3 tabs.« less

A study of solvent effects on hyperpolarizabilities: The reaction field model applied to acetonitrile

Abstract: The reaction field method has been applied in the calculation of the nonlinear optical properties of acetonitrile (CH3CN) in the liquid phase. Both self‐consistent field (SCF) and second order perturbation theory (MP2) methods are used to calculate the dipole moment, polarizability, first and second static hyperpolarizabilities. Based on previous gas‐phase studies which stress the importance of electron correlation in the calculation of hyperpolarizabilities, the MP2 results should be more reliable than the corresponding SCF results. The choice of a cavity radius is of considerable importance. For the two choices made, one based on the liquid density and the other on van der Waals’ parameters, the total second hyperpolarizability changed by as much as a factor of 3 and these calculated values bracket the two differing experimental results. The form of the reaction field factor suggests that the radius dependence should be less for acetonitrile in a solvent of reduced dielectric constant. Although this is true, as demonstrated by calculations involving acetonitrile dissolved in chloroform, significant dependence is still observed. The effect of using an ellipsoidal cavity as opposed to a spherical one is also examined. In this case the hyperpolarizability is less sensitive to the change in cavity parameters from those corresponding to the liquid density to those based on van der Waals’ radii. The hyperpolarizability determined is intermediate between the two values obtained with the spherical cavity and closer to one of the experimental values. Further work using more sophisticated solvation models is required in order to establish whether the reaction field model, with an appropriate choice of cavity parameters, can be used to investigate the hyperpolarizabilities of molecules in solution reliably.

Macromolecular engineering of polyactones and polyactides. 12. Study of the depolymerization reactions of poly(.epsilon.-caprolactone) with functional aluminum alkoxide end groups

Abstract: Ring-opening polymerization of e-caprolactone he been initiated with functional aluminum alkoxides in toluene at 25°C. Degradation of «living» polyester chains by transesterification reactions he been studied in relation to temperature, solvent polarity, average molecular weight, and structure of the initiator, i.e., the nature of the functional group and the number of alkoxides per Al atom. Degradation products, particularly cyclic oligomers, have been isolated by supercritical fluid extraction (SFE) and characterized by GC-MS. The effect of Lewis bases on transesterification reactions he been investigated and discussed in reference to the «coordination-insertion» mechanism