Showing papers on "Solvation published in 1980"

Solvation forces in simple dense fluids. I

Abstract: The grand canonical ensemble Monte Carlo method is used to calculate the density profile of a simple dense liquid, under conditions close to the vapor line, between two solid bodies and also the solvation force between the solids due to the simple fluid. The force is large compared with the van der Waals force at moderate surface separations, h, but is an oscillatory function of h. At small values of h the solvation force is strongly repulsive.

Properties of swollen polymer networks. Solvation and swelling of peptide-containing resins in solid-phase peptide synthesis

Abstract: Peptide-resins containing linear peptides up to 6000 molecular weight-were prepared by stepwise solid-phase synthesis of the repeating model sequence Leu-Ala-Gly-Val-oxymethylphenylacetic acid on copoly(styrene-l% divinylbenzene) resin beads. Initial substitutions of 0.22 and 0.95 mmol/g of polystyrene were used and the resulting peptide-resins contained 11-81% peptide. Representative samples were taken after the addition of each model sequence unit and the diameters of the dry beads were measured by direct microscopic examination. The volume of 1 g of the unsubstituted dry beads was 1.0 mL and was found to increase by more than fivefold as the peptide content approached 80%. Similar measurements of the samples in CH2Cl2 or DMF showed that the swollen volumes of 1 g of the unsubstituted beads were 6.2 and 3.3 mL, respectively. The volumes of the swollen peptide-resins showed a dramatic increase in the course of the synthesis, and at 80% peptide the volumes/g of polystyrene were 12 mL in CH2Cl2 and 28 mL in DMF. There was no indication that the upper limits of swelling of the peptide-containing resin had been reached. The solvation properties of the cross-linked polymer network and the pendant peptide chains mutually affect one another and at high loadings of peptide in a solvent such as DMF the peptide component has a dominating influence on the swelling of the peptide-resin beads. The swelling of the unsubstituted resin is due to a decrease in free energy from solvation of the polystyrene and, at equilibrium, is balanced by the elastic restraining force resulting from deformation of the loosely cross-linked polymer network. The increased swelling of the peptide-resin can then be attributed to the additional net decrease in free energy from solvation of the linear peptide chains, which is counteracted by an increase in the elastic restraining force arising from further deformation of the loosely cross-linked network structure of the polymer support. No such additional counterforce is expected to arise from deformation of the linear peptide chain imposed by the expansion of the polymeric support. In either CH2Cl2 or DMF, the space available for peptide chain growth within the swollen resin beads is not a limiting factor in solid-phase peptide synthesis. After the synthesis of the 60-residue model there was actually more space within the bead for chain growth than at the beginning of the synthesis. The results of this study allow a rational choice of the level of loading of peptide on the resin and of an appropriate protocol for the synthesis of a particular peptide.

Computational Aspects for Large Chemical Systems

Abstract: 1.0 Introduction.- 1.1 Statement of the Problem.- 1.2 Definition of Chemical Complexity.- 1.3 On the Upper Limit of Quantum Chemical Computations.- 1.4 A General Method for Simulations of a Complex Chemical System.- 2.0 Complexity Because of the "Size" of the Largest Molecule in the System.- 2.1 Comments on Conformational Analyses for a Single Molecule.- 2.2 A New Method for Protein-Substrate Interaction Simulations..- 2.2.1 Macrodeformations.- 2.2.2 Microdeformations.- 2.3 Further Improvements for Enzymatic Reaction Simulations.- 3.0 Analyses of Chemical Bonds.- 3.1 Introduction.- 3.2 Bond Energy Analysis.- 3.3 One-Center Energies and the Molecular Orbital Valance State.- 3.4 Two-Center Bond Energy: Benzene.- 3.5 Orbital and Electron Energies.- 3.6 MOVS and Hybridization.- 3.7 Bond Energy Analysis: A "New" Formalism.- 3.8 Chemical Formulae From the Bonded Atom Pairs Analysis.- 3.9 Definition of Atoms and Molecules.- 3.10 BEA and Reaction Surface.- 3.11 BAP and Reaction Surface.- 3.12 Bond Energy Analysis and Vibrational Analysis.- 4.0 Atom-Atom Pair Potentials.- 4.1 Preliminary Comments.- 4.2 Atomic Classes for Atoms in Molecules.- 4.3 Determination of Two-Body Pair Potentials.- 4.4 Pair Potentials and Ab Initio Computations.- 4.5 Minimal Basis Set and Basis Set Superposition Error.- 4.6 The Dispersion Energy.- 4.7 Three and Many Body Corrections.- 5.0 Complexity Because of the Number of Components in the Chemical System.- 5.1 Liquid Water.- 5.2 Ion Water Clusters: Two Body Potentials.- 5.3 Ionic Solutions: Effective Two Body Potentials.- 5.4 Ionic Solutions: n-Body Correction.- 5.5 Energy Maps and Water Structure In Solutions.- 5.6 Monte Carlo Simulation of the Interaction Between Glycine and the Corresponding Zwitterion.- 5.7 Serine and the Corresponding Zwitterion.- 5.8 Enzyme-Water Interaction in Solution: A Preliminary Study on Lysozyme.- 5.9 The Water Structure in the Active Cleft of Human Carbonic Anhydrase/B.- 5.10 Contour Maps for the Molecular Fragments of DNA.- 5.11 Monte Carlo Simulations for Bases and Base-Pairs in Nucleic Acids.- 5.12 Solvation of B-DNA Double Helix at T=300 DegreesK.- 5.13 Solvation of Na+/-B-DNA at 300 DegreesK.- 5.14 Conclusion.- 6.0 References.

On the quantum mechanical treatment of solvent effects

Abstract: A new formalism is proposed for incorporating solvent effects into the quantum mechanical description of molecular electronic states. In contrast to existing methods, it does not lead to a non-linear effective hamiltonian, while both the solvent/solvent and the solute/solvent interactions are treated self-consistently. It also accounts more accurately for the solute's electrical field than the usual dipole approximation. Although formally treated on the Hartree-Fock level, the method incorporates dispersion interactions between solute and solvent.

Cycloaddition mechanism and the solvent dependence of rate

Abstract: The dependence of the rate constant on solvent polarity is an important mechanistic criterion which should always be used in conjunction with other diagnostics tool. The slow step of 2 + 2 cycloadditions of tetracyanoethylene (TCNE) with enol ethers [1], thioenol ethers [2] and trans-fixed 1,3-dienes [3] is the formation of a zwitterionic intermediate. The log k2 values are linear functions of the Dimroth-Reichardt parameter ET. The rate accelerations of TCNE cycloadditions in going from cyclohexane to acetonitrile amount to 29,000 for anethole, 10,800 for 1-ethoxyisobutene, 2,600 for butyl vinyl ether, 17,000 for ethyl 1-propenyl sulfide and 54,000 for verbenene. The violation of stereo-specificity becomes greater with increasing solvent polarity. In contrast, the additions of TCNE to anthracene [4] and related Diels-Alder reactions show only a minute solvent dependence. The same is true for 1,3-dipolar cycloadditions of diazoalkanes [5], phenyl azide [6], C-phenyl-n-methylnitrone and azomethine imines to various dipolarophiles. These concerted cycloadditions are characterized by early transition states, i.e., there is hardly any change of solvation energy during the activation process. A dichotomy of reaction paths was found for diazocarbonyl compounds and enamines. Whereas dialkylaminocyclohexenes produce cycloadducts, the corresponding enamines containing the cyclopentene ring undergo azo coupling furnishes enamino-hydrazones via a zwitterionic intermediate. The contrasting dependence of rate on solvent polarity supports different mechanism [7]. Dimethylketene combines with N-isobutenylprrolidine to give a 3-pyrrolidinocyclobutanone as 1:1 adduct and a δ-methylene-δ-lactone as 2:1 adduct. The dependence of the product ratio on the concentration of dimethylketene allows to disent angle a concerted pathway and a reaction via a zwitterionic intermediate which can be intercepted by a second molecule of dimethylketene (see formula Scheme) [8]. The different solvent dependencies of the two reaction branches confirm the mechanistic divergence.

Hydrophobic solvation of nonspherical solutes

Abstract: The theory of hydrophobic effects presented by Pratt and Chandler is generalized to include nonpolar solutes which are distinctly aspherical. The theory is used to study the solvation of simple aspherical hydrocarbon solutes in liquid water. The radial solvation of each component of diatomiclike solutes is studied as a function of their separation, or bond length. From these results it is found that when the bond length is large enough that one water molecule can fit between the apolar pair, the radial solvation of each is the same as that when the bond length approaches infinity. The solvation of the various sites of the homologous series methane, ethane, propane, and n‐butane is also studied, and effects of the geometrical structure of the solutes on their solvation is discussed.

On the Correlation of Total and Partial Enthalpies of Ion Solvation and the Relationship to the Energy Barrier to Nucleation

Abstract: The energetics of ion clustering for the full range of conditions extending from the isolated ion-single molecule cluster in the gas phase, to the bulk condensed state, is examined. Employing concepts taken from the Thomson equation a relationship is developed which correlates the gas phase enthalpies for successive clustering reactions, with the single-ion heats of solvation. The success of the correlation enables the prediction of the heat of solvation of a given ion in a solvent such as water or ammonia from the gas phase determination of the enthalpies for the attachment of only five ligands to a single ion. Employing this new relationship, the contribution of cluster bonding to the energy barrier to nucleation can thereby be readily evaluated for any size cluster.

Macroscopic model for solvated ion dynamics

Abstract: A macroscopic treatment of solvated ion dynamics is developed and applied to calculate the limiting (zero concentration) conductance of cations in several aprotic solvents. The theory is based on a coupled set of electrostatic and hydrodynamic equations for the density, flow, and polarization fields induced in the polar solvent by a moving ion. These equations, which are derived by the Mori projection technique, include crucial local solvent structure (ion solvation) effects through solvent compressibility, and local constitutive parameters. If solvent structure is suppressed, the equations reduce to those derived previously by Onsager and Hubbard [J. B. Hubbard and L. Onsager, J. Chem. Phys. 67, 4850 (1977)]. The macroscopic equations are approximately decoupled into electrostatic and hydrodynamic parts. The decoupled equations are solved assuming a step density, viscosity, and dielectric constant model for the local solvent structure and dynamics. This yields analytic expressions for the viscous, ζV, an...

Monte Carlo simulations of water clusters around Zn++ and a linear Zn++⋅CO2 complex

Abstract: As a step towards the elucidation of the reaction mechanism of the enzyme carbonic anhydrase, we have considered the solvation of Zn++ and a linear Zn++⋅CO2 complex, using Monte Carlo techniques. The simulated temperature of the system is 300 K. The coordination numbers for the first and second solvation shell are computed as 6 and 16, respectively, for the case of Zn++. When placed in the first solvation shell the CO2 molecule displaces one water molecule, apparently without altering the remaining water structure in the first shell.

Energy gaps in organic semiconductors derived from electrochemical data

Abstract: The band gap, EG, of an organic solid can be estimated by adding the magnitudes of oxidation and reduction potentials of the compound in solution, plus a correction to the ion solvation energy which allows for the different dielectric constants of solvent and solid. Values of EG for 30 organic solids have been estimated. The auto-ionization model, previously used for aromatic hydrocarbons, is applied to dyes and chlorophylls. The values of EG are used to interpret various existing results on photoconduction and semiconduction in dyes and chlorophylls.

Polymer‐solvent interactions: Dextrans in water and DMSO

Abstract: Enthalpies of solution, dilution and mixing of several dextran fractions with different molecular weights (MW) and low MW carbohydrates were determined in water and dimethyl sulfoxide at 25°C. The enthalpies of solution of dextran increase in the oligomer range up to a MW of about 1000; for higher MWs the enthalpies of solution are almost constant. The enthalpies of dilution decrease strongly with increasing MW and remain constant for MWs higher than 2 000. The interaction parameter was found to depend on the concentration of the solute in both solvents. From the experimental results it can be concluded that dimethyl sulfoxide is a better solvent for glucose and dextran than water. In dilute solutions glucose interacts considerably more strongly with the solvent than dextran. The conformational properties of dextran are similar in both solvents; for MWs above 2 000 the degree of solvation is found to be independent from the MW.

Internal association in solid phase peptide synthesis. Synthesis of cytochrome C residues 66–104 on polyamide supports

Abstract: Solvation of peptide–resins in solid phase synthesis may be increased by appropriate choice of side-chain protecting groups, with enhanced efficiency in amino-acid incorporation.

Current problems in the localization and solvation of excess electrons in glasses

Abstract: Recent advances and limitations on knowledge of electron localization and solvation are highlighted. The kinetics and mechanism of electron localization remain unknown. The physical interpretation of localization time is clarified. The minimum potential well depth for electron localization appears to be approx. 0.4 eV. A possible explanation of why electron solvation appears to ignore hydrogen bonding in liquids is advanced in terms of preferential selection of lower energy molecular reorientation pathways by analogy to recent results in glasses. New experiments on the molecular details of electron solvation in mixed methyltetrahydrofuran (MTHF)-methanol matrices are reported which argue against a tunneling mechanism from an MTHF environment to a methanol cluster and which support solvation shell conversion by a stepwise mechanism. Results on the electronic structure of presolvated electrons in various polar matrices imply that bound-continuum transitions generally predominate. The implications of this for the trapping potential are discussed. A rationale is given for the different energy level structure that is obtained when two OH dipoles occur in one solvent molecule. Current information on the geometrical structure of solvated electrons is surveyed, and new models of the solvated electron geometry in methanol, MTHF, and 3-methylpentane glasses are presented.

Updating the Molecular–Mechanical Force Field for Organic Halides

Abstract: In extension of previous molecular mechanical studies of organic halides, we examine three pathways to enlarge the scope of the force field. These are: adaptation of the constants to a new formulation of the basic force field; substitution of the monopole for the dipole approximation, in evaluating the electrostatic strain; estimation of solvation energies and conformational equilibria in solution. The structures studied are mono- and non-geminal dihaloalkanes, and monohaloketones (fluorides, chlorides, bromides). We discuss the quantitative separation of steric and electrostatic effects in halocyclohexanones, and identity the minimum-energy rotamers in haloacetones.

Solvent isotope effects upon the thermodynamics of some transition-metal redox couples in aqueous media

Abstract: The effects of substituting D/sub 2/O for H/sub 2/O as solvent upon the formal potentials of a number of transition-metal redox couples containing aquo, ammine, and simple chelating ligands have been investigated with the intention of evaluating the importance of specific solvation factors in the thermodynamics of such couples. The solvent liquid junction formed between H/sub 2/O and D/sub 2/O was shown to have a negligible effect on the measured formal potentials. Substantial solvent isotope effects were observed for a number of these systems, particularly for couples containing aquo ligands. The effects of separately deuterating the ligands and the surrounding solvent were investigated for some ammine couples. Possible origins of the solvent isotope effects are discussed in terms of changes in metal-ligand and ligand-solvent interactions. It is tentatively concluded that the latter influence provides the predominant contribution to the observed effects for aquo couples arising from increases in the extent of hydrogen bonding between the aquo ligands and surrounding solvent when D/sub 2/O replaces H/sub 2/O. The implications of these results in unraveling the solvent isotope effects upon the kinetics of simple redox reactions are also considered.

Studies in ion solvation in non-aqueous solvents and their aqueous mixtures. Part 20.—Enthalpies of transfer of alkali-metal halides in the methanol + water system from enthalpies of dilution

Abstract: Enthalpies of transfer, ΔH0t, of the alkali-metal chlorides and of sodium bromide and sodium iodide from water to methanol + water mixtures over the whole range of solvent composition have been determined by measuring heats of dilution of concentrated aqueous solutions. Greater precision is achieved than when heats of solution are measured. A submarine calorimeter was used and the concentrated solutions injected with a dispensing pipette. This gave precise results rapidly.Results for sodium chloride obtained by measuring heats of solution were in good agreement with those from heats of dilution. There is fair agreement with the classical work of Slansky except for Li+Cl–.Comparison with ΔG0t and ΔS0t confirms that ΔG0t is the simplest function and suggests that structural effects compensate in ΔH0t and ΔS0t.

Triple ion formation in solutions of alkaline sulfocyanides

Abstract: Through the infrared and Raman spectra of alkaline sulfocyanides in high-and low-polarity media, nitromethane (CH3NO2) and tetrahydrofuran (THF), triple cations and triple anions may be characterized. The formation of these species at the expense of ion pairs or more aggregated species such as dimers is discussed taking into account cation solvation, anion solvation, and the influence of cation-anion interaction.

Effect of solvation on the electron spin resonance spectrum of the superoxide ion

Abstract: The solvation of superoxide ions by protic solvents has been studied by e.s.r. spectroscopy. It is concluded that strong hydrogen bonds lift the degeneracy of the π-orbitals, with solvation being effectively confined to one of these orbitals. It is suggested that, when aqueous solutions are frozen, phase separation occurs so that the gz value (2.11) is not characteristic of the aquated ion. By extrapolation from data for aqueous methanolic glasses, we obtain a gz value of 2.065 for the aquated ion.Solutions in dry aprotic solvents generally give broad gz features at much lower fields. However, water in low concentrations is scavenged by the anions and this results in a shift of the gz feature into the 2.1 region. It is suggested that dry O–2 may add reversibly to dimethyl formamide to give a peroxy-radical adduct. Ion-pair formation with divalent cations has also been studied.