Steric stabilization II

Summary A theory of steric stabilization is developed that is essentially an extension ofFischer's solvency theory. It treats both interpenetration and compression using a latice approach. This permits the introduction of segment density distribution functions. One virtue of the new theory is that it is in good qualitative agreement with the results of experiment. Some limitations of the theory are also noted.     

Steric stabilization I

Summary A comparison is presented between the qualitative predictions of the various theories of steric stabilization with the results of experiments. It is shown that with the notable exception ofFischer's solvency theory, all of the theories advanced to-date are at odds with results of experiments.Possible reasons for these discrepancies are discussed.     

Steric stabilization in worse thanθ-solvents

Summary The stabilization of colloidal particles by nonionic polymers that exhibit loopy adsorption is considered in terms of an exponentially decaying segment density distribution function. It is shown that the steric interaction, whether repulsive or attractive, in the interpenetrational domain is drastically reduced for an exponential function, compared with that for a constant or a Gaussian distribution function. This reduction originates in the small segment densities that occur in the outer regions of steric layers exhibiting loopy adsorption. The net effect is that incipient instability in such systems can only be generated by significant penetration into the worse than 8-solvent domain. Any correlation between the critical flocculation point and the -point is thus weakened and stable dispersions can be prepared in worse than -solvents for the stabilizing chains in free solution. Both phenomena have been observed experimentally. It is predicted that, in general, the penetration into the worse than B-solvent domain will be greater for nonaqueous than for aqueous dispersion media. The conformational effects are reinforced by any changes in the thermodynamic parameters due to loopy adsorption.With 5 figures and 1 table     

Steric stabilization of polystyrene colloids using thiol-ended polyethylene oxide

Thiol-ended polyethylene oxide (I) has been prepared from the esterification of thioglycolic acid with monomethylether of polyoxyethylene glycol. Emulsion polymerization of styrene (and, in a few cases, methylmethacrylate as comonomer) was carried out in the presence of I using either water-soluble azo initiator or t-butylhydroperoxide. In the former case, bimodal particle size distribution was obtained while monodisperse latexes could be prepared in the latter case. Then a redox system was formed from I and t-BuOOH so that I was both an initiator and a transfer agent. Good steric stabilization of the latexes was observed. The polyethylene oxide sequence of I was partly incorporated at the surface of the latex particles, but the incorporation yield remained limited (between 7 and 18%). Most of the resdue of I remained in the serum.     

Selective transition state stabilization via hyperconjugative and conjugative assistance: stereoelectronic concept for copper-free click chemistry

Dissection of stereoelectronic effects in the transition states (TSs) for noncatalyzed azide-alkyne cycloadditions suggests two approaches to selective transition state stabilization in this reaction. First, the formation of both 1,4- and 1,5-isomers is facilitated via hyperconjugative assistance to alkyne bending and C···N bond formation provided by antiperiplanar σ-acceptors at the propargylic carbons. In addition, the 1,5-TS can be stabilized via attractive C-H···F interactions. Although the two effects cannot stabilize the same transition state for the cycloaddition to α,α-difluorocyclooctyne (DIFO), they can act in a complementary, rather than competing, fashion in acyclic alkynes where B3LYP calculations predict up to ～1 million-fold rate increase relative to 2-butyne. This analysis of stereoelectronic effects is complemented by the distortion analysis, which provides another clear evidence of selective TS stabilization. Changes in electrostatic potential along the reaction path revealed that azide polarization may create unfavorable electrostatic interactions (i.e., for the 1,5-regioisomer formation from 1-fluoro-2-butyne and methyl azide). This observation suggests that more reactive azides can be designed via manipulation of charge distribution in the azide moiety. Combination of these effects with the other activation strategies should lead to the rational design of robust acyclic and cyclic alkyne reagents for fast and tunable "click chemistry". Further computational and experimental studies confirmed the generality of the above accelerating effects and compared them with the conjugative TS stabilization by π-acceptors.     

Steric stabilization of a monomeric proalumatrane: experimental and theoretical studies

Treatment of tripodal tris(3-tert-butyl-2-hydroxy-5-methylbenzyl)amine (L) with 1 equiv of trimethylaluminum in toluene gave the stable proalumatrane (AlL) (1) [wherein L = tris(3-tert-butyl-5-methyl-2-oxidobenzyl)amine] featuring a distorted trigonal monopyramidal four-coordinate aluminum geometry. An analogous reaction uses the less sterically congested isomer of L, namely, tris(5-tert-butyl-2-hydroxy-3-methylbenzyl)amine provided dimeric (AlL')2 (2) [wherein L' = tris(5-tert-butyl-3-methyl-2-oxidobenzyl)amine], which contains two bridging alumatrane moieties possessing five-coordinate TBP aluminum geometries. Reaction of AlL with water provided the adduct H2O.AlL (3), a species that is representative of a coordinatively stabilized intermediate in the hydrolysis of an aluminum alkoxide. Theoretical calculations revealed that considerable stabilization energy is obtained by the coordination of a water molecule to the tetracoordinate aluminum in AlL and that this result is consistent with the postulate that the Lewis acidity of AlL exceeds that of boron trifluoride, despite the presence of the transannular N-->Al bond in AlL.     

Steric stabilization of spherical colloidal particles: Implicit and explicit solvent

We present the results of Monte Carlo simulations and density functional theory treatment of interactions between spherical colloidal brushes both in implicit (good) solvent and in an explicit polymeric solution. Overall, theory is seen to be in good agreement with simulations. We find that interactions between hard-sphere particles grafted with hard-sphere chains are always repulsive in implicit solvent. The range and steepness of the repulsive interaction is sensitive to the grafting density and the length of the grafted chains. When the brushes are immersed in an explicit solvent of hard-sphere chains, a weak mid-range attraction arises, provided the length of the free chains exceeds that of the grafted chains.     

Steric strain release-directed regioselective functionalization of meso-methyl Bodipy dyes

Starting from some meso-methyl Bodipy dyes, the corresponding meso-styryl derivatives were synthesized by regioselective Knoevenagel-type condensation with different aromatic aldehydes. The reactions were driven by the alleviation of the structural strain of the alkyl substituted Bodipys that could override the differential acidities of the methyl protons at the pyrrole ring of the Bodipy moiety.     

Steric stabilization of core-shell nanoparticles in liquid carbon dioxide at the vapor pressure

Nondilute nanoparticle dispersions were stabilized in liquid CO2 at 25 degrees C at pressures as low as the vapor pressure for greater than 30 min. By modifying hydrophilic silica with a trifunctional silylating agent, (heptadecafluoro-1,1,2,2-tetrahydrodecyl)triethoxy silane, a cross-linked polymer shell was formed around the silica core. The presence of the shell led to weaker Hamaker interactions between approaching fluoro-silica composite particles and enabled dispersibility at weaker solvent conditions (low pressures) than for metals with larger Hamaker constants. Steric stabilization of the nanoparticles was provided by low-molecular-weight perfluorodecane side chains at the surface of the fluoro-silica composite shell. Compared to polymeric chains, the perfluorodecane side chains are more easily solvated and thus stabilize nanoparticle dispersions in CO2 at much lower pressures, even down to the vapor pressure.     

Steric stabilization of iron (III) hydroxide sols by various amino acids

The dispersion and coagulation phenomena of iron(III) hydroxide sols were investigated as a function of pH in the absence and presence of amino acids. The amino acids used were glycine,L--alanine,DL--amino-n-butyric acid,L-valine,L-leucine,L- isoleucine,L-glutamic acid andL-arginine. The turbidity measurements of the iron-(III) hydroxide sols, which were prepared by pouring an aqueous iron(III) chloride solution into boiling distilled water, were carried out using a spectrophotometer with an addermixer device and an automatic recording system. The zeta potentials of sol particles were obtained by ultra-microelectrophoresis. The change in turbidity of the sol, as a measure in stability of the sol, increased with increasing pH in the region of pH 2–8, and reached a maximum at the isoelectric point of the particles. The coagulation at the isoelectric point was prevented by adding amino acids, and the stabilization had an optimum point at concentrations which depended upon the kinds of amino acids. The remarkable dispersing effect of amino acids which occurred near the isoelectric point of the particles at the suitable concentration of the ammo acids may be due to the steric protection by amino acid adsorbed. The protective action was explained according to a modified DLVO theory, the modification for London-van der Waals force being applied in order to take the effect of the adsorption layer into account.     

Steric stabilization of thermally responsive N-isopropylacrylamide particles by poly(vinyl alcohol)

Poly(vinyl alcohol) (PVA) was used as a steric stabilizer for the dispersion polymerization of cross-linked poly(N-isopropylacrylamide) (PNIPAM) in water. A series of reactions were carried out using PVA of varying molecular weight and degree of hydrolysis. Under appropriate conditions, PNIPAM particles of uniform and controllable size were produced using PVA as the stabilizer. The colloidal stability was investigated by measuring changes in particle size with temperature in aqueous suspensions of varying ionic strength. For comparison, parallel colloidal stability measurements were conducted on PNIPAM particles synthesized with low-molecular-weight ionic surfactants. PVA provides colloidal stability over a wide range of temperature and ionic strength, whereas particles produced with ionic surfactants flocculate in moderate ionic strength solutions upon collapse of the hydrogel as the temperature is increased. Experimental results and theoretical consideration indicate that sterically stabilized PNIPAM particles resulted from the grafting of PVA to the PNIPAM particle surface. The enhanced colloidal stability afforded by PVA allows the temperature-responsive PNIPAM particles to be used under physiological conditions where electrostatic stability is ineffective.     

New epothilone congeners from Sorangium cellulosum strain So0157-2

As a continuous work to find more epothilone congeners produced by the epothilones A and B producing Sorangium cellulosum strain So0157-2 in the large-scale fermentation (5000?L), we reinvestigated the chemical compositions of the fermentation broth. Consequently, two new epothilone variants (1-2) and one new natural epothilone derivative (3) were isolated from the fermentation broth. Their structures were established as 16-ethyl epothilone B (1), 6-desmethyl-16-hydroxymethyl epothilone C (2) and 20-ethyl epothilone A (3), respectively, by an extensive NMR analysis.     

Steric stabilization and functionalization of magnetite particles and preparation of colloid magnetite dispersions in oligomeric media

Method for producing oligomer-based dispersions of magnetite, stabilized with an oligomeric surfactant having a hydroxy group at the free end of the chain, was studied in order to obtain polymeric nanocomposites chemically bound to the polymeric matrix from these dispersions.     

Homolytic versus Heterolytic Hydrogen Evolution Reaction Steered by a Steric Effect

Several H?H bond forming pathways have been proposed for the hydrogen evolution reaction (HER). Revealing these HER mechanisms is of fundamental importance for the rational design of catalysts and is also extremely challenging. Now, an unparalleled example of switching between homolytic and heterolytic HER mechanisms is reported. Three nickel(II) porphyrins were designed and synthesized with distinct steric effects by introducing bulky amido moieties to ortho‐ or para‐positions of the meso‐phenyl groups. These porphyrins exhibited different catalytic HER behaviors. For these Ni porphyrins, although their 1e‐reduced forms are active to reduce trifluoroacetic acid, the resulting Ni hydrides (depending on the steric effects of porphyrin rings) have different pathways to make H₂. Understanding HER processes, especially controllable switching between homolytic and heterolytic H?H bond formation pathways through molecular engineering, is unprecedented in electrocatalysis.     

Steric stabilization of polyvinyl alcohol adsorbed on silica/water and water/oil interfaces

The steric hindrance of various polyvinyl alcohol samples was investigated by measuring the force-distance curves between crossed quartz filaments with adsorbed layers of PVA. Results were obtained by changing the concentration of macromolecules, the average macromolecular weight, the electrolyte concentration, the kind of electrolytes, the temperature and by adding organic solvents. The results were compared with calculations using the Hesselink-Vrij-Overbeek Theory.It could be shown that only a small amount of “longer” tails is responsible for the stabilization but the segment distribution functions are different for different samples.     

Steric effects on nascent vibrational distributions of the HCN product produced in CN radical reactions with ethane, propane and chloroform

Time-resolved IR emission spectroscopy (TRIES) has been used to study infrared emission in the 3400–3100 cm⁻¹ region from HCN molecules produced when CN radicals abstract a hydrogen atom from ethane, propane, and chloroform. From these observations the nascent vibrational distributions of the HCN produced were derived. The nascent vibrational population distributions of the product HCN in all of the reactions are non-statistical and inverted in both the pure CH stretch (00p) and CH stretch—bend (0np) series.     

Homoallyl-cyclopropylcarbinyl cation manifold. Trimethylsilyl versus aryl stabilization

A series of E- and Z-1-aryl-5-trimethylsilyl-3-buten-1-yl trifluoroacetates were solvolyzed in CD3CO2D, and rates of reaction as well as products derived from these reactions were determined. Hammett plots showed a break, which was indicative of a mechanistic change from a kC process when the most electron-donating substituents were attached to the aryl group to a kDelta process involving formation of cyclized beta-silyl carbocation intermediates for electron-withdrawing groups. In the case of p-CH3O substitution (a kC extreme), the cationic intermediate captures solvent (95%) or loses a proton (5%). In the case of m-CF3 substitution (a kDelta extreme), the beta-silyl cation intermediate desilylates to give vinylcyclopropane products. Substituents with intermediate electronic properties give more complex product mixtures. Solvolysis of pure Z-trifluoroacetate (p-CH3) gives small amounts of E-trifluoroacetate (p-CH3) along with the E-substitution product. This isomerization suggests that the cyclized beta-silyl cation can isomerize and then reopen to a classical aryl-stabilized cation. By way of contrast, B3LYP/6-31G* computational studies show only cyclized beta-silyl cations as energy minima. Open kC cations are higher-energy nonminimum energy structures.     

Bond energy, aromatic stabilization energy and strain in IPR fullerenes

Various models applied to DFT structures and energies of 2-D and 3-D aromatic molecules shed new light on the effects of strain and aromaticity in these systems. The cyclic pi electron delocalisation does not stabilize the fullerene C60 formation; and 5-6 and 6-6 CC bonds have near-identical bond stretch potentials.