Solute structural effects

For the Flory-Huggins model of the activity, Equations (2C-4)-(2C-6), the data are reported in terms of a concentration dependent chi parameter. Data must be taken or extrapolated to the 6 = 0 limit in order to remove solution structure effects. 

In this article we shall focus on recent work involving dilute aqueous surfactant solutions. As a background the thermodynamics and statistics of these solutions will be discussed first (Section II). The distribution of substrate molecules in microheterogeneous solution is considered in Section m. It is decisive for the kinetics of elementary photochemical reactions (Section IV), which depend on the peculiar colloidal solution structure. Effects of the microscopic environments on photochemical reactions are treated in Section V. Finally, the use of known photochemical systems as probes for studying details of the structure of surfactant solutions will be considered in Section VI. 

Recent applications of the theory have been made for calculating the contribution the formation of a cavity gives to the free energy of transfer of a series of isomeric ketones(134), or various other solutes(132), from H2O to D2O, for the studies of solubility of many apolar gases in water and other polar solvents(135), for comparing the experimental thermodynamic data for the solution of rare gases(51), or some perfluorocarbon gases(136), in water at various temperatures with data calculated by means of the SPT. In a tentative made to extend the theory at aqueous solutions where solute-solute interactions have to be considered and hydrophobic interactions are operative, it has been shown that the dependence of the partial molar volumes and enthalpies of hard-sphere solutes in water on concentration and temperature are due to the anomalous trends the 63/6P and 63/6T (3 coefficient of isothermal compressibility) of pure water present, rather than to the solute structural effects(137). 

Structural effects on the rates of deprotonation of ketones have also been studied using veiy strong bases under conditions where complete conversion to the enolate occurs. In solvents such as THF or DME, bases such as lithium di-/-propylamide (LDA) and potassium hexamethyldisilylamide (KHMDS) give solutions of the enolates in relative proportions that reflect the relative rates of removal of the different protons in the carbonyl compound (kinetic control). The least hindered proton is removed most rapidly under these... 

Organic peroxide-aromatic tertiary amine system is a well-known organic redox system 1]. The typical examples are benzoyl peroxide(BPO)-N,N-dimethylani-line(DMA) and BPO-DMT(N,N-dimethyl-p-toluidine) systems. The binary initiation system has been used in vinyl polymerization in dental acrylic resins and composite resins [2] and in bone cement [3]. Many papers have reported the initiation reaction of these systems for several decades, but the initiation mechanism is still not unified and in controversy [4,5]. Another kind of organic redox system consists of organic hydroperoxide and an aromatic tertiary amine system such as cumene hydroperoxide(CHP)-DMT is used in anaerobic adhesives [6]. Much less attention has been paid to this redox system and its initiation mechanism. A water-soluble peroxide such as persulfate and amine systems have been used in industrial aqueous solution and emulsion polymerization [7-10], yet the initiation mechanism has not been proposed in detail until recently [5]. In order to clarify the structural effect of peroxides and amines including functional monomers containing an amino group, a polymerizable amine, on the redox-initiated polymerization of vinyl monomers and its initiation mechanism, a series of studies have been carried out in our laboratory. 

Samal et al. [25] reported that Ce(IV) ion coupled with an amide, such as thioacetamide, succinamide, acetamide, and formamide, could initiate acrylonitrile (AN) polymerization in aqueous solution. Feng et al. [3] for the first time thoroughly investigated the structural effect of amide on AAM polymerization using Ce(IV) ion, ceric ammonium nitrate (CAN) as an initiator. They found that only acetanilide (AA) and formanilide (FA) promote the polymerization and remarkably enhance Rp. The others such as formamide, N,N-dimethylformamide (DMF), N-butylacetamide, and N-cyclohexylacetamide only slightly affect the rate of polymerization. This can be shown by the relative rate (/ r), i.e., the rate of AAM polymerization initiated with ceric ion-amide divided by the rate of polymerization initiated with ceric ion alone. Rr for CAN-anilide system is approximately 2.5, and the others range from 1.04-1.11. 

Feng et al. [3] have studied the structural effect of acetanilide on the AAM polymerization either in water-for-mamide [3], water-acetonitrile [4], and water-DMF [26] mixed solution using Ce(IV) ion-acetanilide and its substituted derivatives as the initiator. The results showed that an electron donating substituent on the phenyl group would enhance the Rp, while an electron withdrawing group would decrease it, as shown in Table 1 [26]. 

Heterogeneities associated with a metal have been classified in Table 1.1 as atomic see Fig. 1.1), microscopic (visible under an optical microscope), and macroscopic, and their effects are considered in various sections of the present work. It is relevant to observe, however, that the detailed mechanism of all aspects of corrosion, e.g. the passage of a metallic cation from the lattice to the solution, specific effects of ions and species in solution in accelerating or inhibiting corrosion or causing stress-corrosion cracking, etc. must involve a consideration of the detailed atomic structure of the metal or alloy. 

Xia XH, Iwasita T, Ge E, Vielstich W. 1997. Structural effects and reactivity in methanol solution on polycrystalline and single-crystal platinum. Electrochim Acta 41 711-718. 

Markovic NM, Adzic RR, Cahan BD, Yeager EB. 1994. Structural effects in electrocatalysis— Oxygen reduction on platinum low-index single-crystal surfaces in perchloric-acid solutions. J Electroanal Chem 377 249-259. 

Sun SG, Cai WB, Wan LJ, Osawa M. 1999. Infrared absorption enhancement for CO adsorbed on Au films in perchloric acid solutions and effects of surface structure studied by cyclic voltammetry, scanning tunneling microscopy, and surface-enhanced IR spectroscopy. J Phys Chem B 103 2460-2466. 

The chelate ring size principle can have structural effects as well as effects on thermodynamic stability in aqueous solution. An example is coordination of metal ions by sugars (44). The cyclic polyol cts-inositol can coordinate metal ions in two distinct ways (Fig. 14) (45). In ax-ax-ax bonding (Fig. 14), the metal ion is part of three fused six-membered chelate rings. Alternatively, in ax-eq-ax coordination, the metal ion is part of two fused five-membered and one six-membered chelate rings. Angyal has noted that metal ions of radius more than 0.8 A adopt the ax-eq-ax structure (44), whereas with an ionic radius... 

Hybrid MPC-MD schemes may be constructed where the mesoscopic dynamics of the bath is coupled to the molecular dynamics of solute species without introducing explicit solute-bath intermolecular forces. In such a hybrid scheme, between multiparticle collision events at times x, solute particles propagate by Newton s equations of motion in the absence of solvent forces. In order to couple solute and bath particles, the solute particles are included in the multiparticle collision step [40]. The above equations describe the dynamics provided the interaction potential is replaced by Vj(rJVs) and interactions between solute and bath particles are neglected. This type of hybrid MD-MPC dynamics also satisfies the conservation laws and preserves phase space volumes. Since bath particles can penetrate solute particles, specific structural solute-bath effects cannot be treated by this rule. However, simulations may be more efficient since the solute-solvent forces do not have to be computed. 

At present, the problems in thiepin chemistry awaiting solution are (i) how to construct the thiepin skeleton under mild reaction conditions, (ii) what are the structural effects on thermal stability of thiepin, (iii) whether the thianorcaradiene is an intermediate of sulfur extrusion reaction or not, (iv) what is the molecular structure of the thiepin (planar or nonplanar), (v) what is the antiaromaticity of the thiepin ring. 

Through the recent developements in thiepin chemistry, many thiepin derivatives are now available for a detailed study of their properties. One of the most intriguing problems awaiting solution is the structural effects on the thermal stability of the thiepin system. 

In a sense each monolithic column is unique, or produced as a product of a separate batch, because the columns are prepared one by one by a process including monolith formation, column fabrication, and chemical modification. Reproducibility of Chro-molith columns has been examined, and found to be similar to particle-packed-silica-based columns of different batches (Kele and Guiochon, 2002). Surface coverage of a Chromolith reversed-phase (RP) column appears to be nearly maximum, but greater silanol effects were found for basic compounds and ionized amines in buffered and nonbuffered mobile phases than advanced particle-packed columns prepared from high purity silica (McCalley, 2002). Small differences were observed between monolithic silica columns derived from TMOS and those from silane mixtures for planarity in solute structure as well as polar interactions (Kobayashi et al., 2004). 

D.R. McMillin, Purdue University In addition to the charge effects discussed by Professor Sykes, I would like to add that structural effects may help determine electron transfer reactions between biological partners. A case in point is the reaction between cytochrome C551 and azurin where, in order to explain the observed kinetics, reactive and unreactive forms of azurin have been proposed to exist in solution (JL). The two forms differ with respect to the state of protonation of histidine-35 and, it is supposed, with respect to conformation as well. In fact, the lH nmr spectra shown in the Figure provide direct evidence that the nickel(II) derivative of azurin does exist in two different conformations, which interconvert slowly on the nmr time-scale, depending on the state of protonation of the His35 residue (.2) As pointed out by Silvestrini et al., such effects could play a role in coordinating the flow of electrons and protons to the terminal acceptor in vivo. 

The UV-VIS cutoffs of some model compounds clearly demonstrate a variety of structural effects discussed earlier. Although these spectra were obtained in solution (4xlO 4M in EtOH) the trends in transparency of these simple molecules are generally applicable to the crystalline state as well. Two of the more clear-cut comparisons involve the D-A substituted parent pyridine 73-74 and benzene 75-76 molecules (Figure 1). First, the cutoffs of the pyrrolidine substituted aromatics 74 and 76 have a bathochromic shift relative to their dimethylamino substituted analogs 73 and 74 due to the better donor... 

Numerous investigators have attempted to control the precursor structure and related solution chemistry effects with varying degrees of success, to influence subsequent processing behavior, such as crystallization tempera-ture.40-42,78,109 110 Particular attention has been given to precursor characteristics such as structural similarity to the desired product and the chemical homogeneity of the precursor species. For multicomponent films, this latter factor is believed to influence the interdiffusional distances associated with the formation of complex crystal structures, such as perovskite compounds. Synthetic approaches have been geared toward the preparation of multimetal species with cation stoichiometry identical to that of the desired crystalline phase.40 42 83 84... 

A fourth solvent structural effect refers to the average properties of solvent molecules near the solute. These solvent molecules may have different bond lengths, bond angles, dipole moments, and polarizabilities than do bulk solvent molecules. For example, Wahlqvist [132] found a decrease in the magnitude of the dipole moment of water molecules near a hydrophobic wall from 2.8 D (in their model) to 2.55 D, and van Belle et al. [29] found a drop from 2.8 D to 2.6 D for first-hydration-shell water molecules around a methane molecule. 

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