Contribution of steric and electrostatic repulsion forces to the stability of styrene latices copolymerized with acrylic acid

Acrylic acid (AA) is used in many emulsion polymerization formulations to improve the colloidal stability of the latex product. The improved stability originates from electrostatic repulsion complemented with steric repulsion. The strength of the electrostatic and steric repulsion forces in a styrene (S)/AA copolymer latex was investigated at different pH values, electrolyte concentrations, and temperatures. A comparison was made with an S homopolymer latex. Transmission electron microscopic pictures, combined with visual inspections, provided understanding of the mechanisms leading to coagulation in polystyrene (PS)/AA copolymer latices. Colloidal stability of the unswollen sodium dodecyl sulfate stabilized PS latex is based on electrostatic repulsion. Destabilization by sodium chloride resulted in aggregation. The acidic PS/AA latex remained stable against aggregation at high electrolyte concentrations because of steric repulsion. The acidic PS/AA latex showed a strong tendency to flocculate at increasing electrolyte concentrations. Flocculation was not observed for high‐pH PS/AA latices at high electrolyte concentrations. Steric repulsion of the acid PS/AA latex was lost at temperatures higher than the critical coagulation temperature (35 °C), and flocculation was followed by aggregation and coalescence. The high‐pH PS/AA latex was stable even at high electrolyte concentrations and temperatures up to 80 °C because of strong electrosteric stabilization. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2985–2995, 2003     

DSC of etched aluminum foils for use in electrolytic capacitors

A DSC method for evaluating the surface area of etched Al foils for use in high performance electrolytic capacitors is presented. A linear relationship between the etching degree (effective surface area) and the thermal resistance of the sample is obtained by means of DSC, based on the transient phenomenon. This method using the transient state in DSC measurement is not only novel, but also rapid and simple in evaluating the surface area of an etched aluminum foil. The method is effective even when the Al foil has a naturally oxidized surface.The authors wish to thank to Mr. Adachi and Mr. Koike of ELNA Co., Ltd. for supplying us with etched aluminum foils. This work is partially supported by the Research Fund of North Shore College of SONY Institute. R. O. whishes to express her gratitude to the support.     

Bond strengths: the importance of hyperconjugation

[Structure: see text] Gronert (J. Org. Chem. 2006, 71, 1209) has challenged the importance of hyperconjugation in determining C-H bond dissociation enthalpies (BDEs) in alkanes. Electron paramaganetic resonance spectra of H3CCH2*, (H3C)2CH*, and (H3C)3C* show significant positive spin on their beta-H3C groups' hydrogens. A 55%/45% partitioning of these spins between hyperconjugation and spin polarization mechanisms linearly correlates with the C-H BDEs in methane, ethane, propane, isobutane and propene. Hyperconjugation is an important factor determining alkane C-H BDEs.     

Chemical origin of blue- and redshifted hydrogen bonds: intramolecular hyperconjugation and its coupling with intermolecular hyperconjugation

Upon formation of a H bond Y...H-XZ, intramolecular hyperconjugation n(Z)-->sigma*(X-H) of the proton donor plays a key role in red- and blueshift characters of H bonds and must be introduced in the concepts of hyperconjugation and rehybridization. Intermolecular hyperconjugation transfers electron density from Y to sigma*(X-H) and causes elongation and stretch frequency redshift of the X-H bond; intramolecular hyperconjugation couples with intermolecular hyperconjugation and can adjust electron density in sigma*(X-H); rehybridization causes contraction and stretch frequency blueshift of the X-H bond on complexation. The three factors--intra- and intermolecular hyperconjugations and rehybridization--determine commonly red- or blueshift of the formed H bond. A proton donor that has strong intramolecular hyperconjugation often forms blueshifted H bonds.     

Heteroatoms and substituent effects: The importance of heteroatom hyperconjugation

We have found that the specific rate of α‐sulfonyl carbanion formation in a β‐substituted sulfone shows a sizable dependence on the H C_α C_β X torsion angle. Defining k_N = (k_exch)_X/(k_exch)_model (where the model has X = H or an alkyl group) we observed for a collection of β‐alkoxy sulfones (X = OR) acceptable agreement with the expression log k_N = a + b cos² θ (where a = 1.70 and b = 2.62). Extension to other β‐substituents (X = RS, R₂N, and R₃N⁺) yields the same pattern, with the last showing very large dependence of k_N on the torsion angle (b = 6.3). These observations are ascribed to the presence (in addition to the inductive and field effects) of negative hyperconjugation responsible for accelerations of 1000‐fold and more, deriving from donation of the incipient negative charge on carbon into the σ*_{C X} orbital in the transition state. These observations reflect, and at the same time underline, the importance of the low‐lying antibonding orbitals present in heteroatomic molecules. © 2002 Wiley Periodicals, Inc. Heteroatom Chem 13:397–405, 2002; Published online in Wiley Interscience (www.interscience.wiley.com). DOI 10.1002/hc.10067     

Quantitative evaluation of hyperconjugation in the cyclopropylcarbinyl cation and in cyclopropylborane

An orbital deletion procedure (ODP) at HF/6-311G** have been used to evaluate the hyperconjugation effects in the cyclopropylcarbinyl cation (1) and in cyclopropylborane (2), as well as the conjugation effects in the allyl cation (3) and in vinylborane (4). The hyperconjugation (or conjugation) energies have been quantified by ODP in which the critical “vacant” carbocation (or boron) p orbital is “deactivated”. Comparisons between the bisected conformations of 1 with 3, and 2 with 4 demonstrate that cyclopropane can be just as effective as a π-electron donor as a C=C double bond.     

The dichotomy of dimetallocenes: coaxial versus perpendicular dimetal units in sandwich compounds

Theoretical studies of the dimetallocene (eta5-C5H5)2Zn2 lead to optimized D5h or D5d structures in which the Zn-Zn bond is coaxial with the C5 axes of the two Cp rings, with a Zn-Zn distance of 2.33 A, corresponding to a Zn-Zn single bond. (eta5-C5H5)2Ni2 (singlet state) and (eta5-C5H5)2Cu2 (triplet) have similar structures with a NiNi triple bond (2.06 A) and a Cu=Cu double bond (2.22 A). However, DFT computations on (C5H5)2Ni2 and (C5H5)2Cu2 (both singlet states) lead to a totally different type of optimized structure (Ci symmetry) lying at significantly lower energies, with the metal-metal bonds perpendicular to the C5 axes of the Cp rings.     

Metal coordination architectures of N-acyl-salicylhydrazides: The effect of metal ions and steric repulsion of ligands to their structures of polynuclear metal complexes

Three polynuclear transition metal complexes [Mn₈(DMF)₈(L1)₈] · 4DMF (1), [Mn₆(DMF)₆(L2)₆] · [Mn₆(DMF)₄(H₂O)₂(L2)₆] · 2DMF (2), [Cu₃(L3)₂(py)₂] (3) of the pentadentate ligands N-acyl-salicylhydrazides were synthesized and characterized, their crystal structures were investigated. The oxidation state and properties of the central metal ions are important in crystal structure formation, trivalent Mn(III) ion which easily form stable octahedral coordination metallamacrocycle complexes, metallacrowns 1 and 2 were obtained; while bivalent Cu(II) ion is easier to form square planar, trinuclear complexes 3 was obtained. The steric effect of the N-acyl side chains also plays an important role in the structures of these polynuclear complexes. The magnetic property of 1 was also investigated.     

The role of hyperconjugation in the conformational analysis of methylcyclohexane and methylheterocyclohexanes

Full geometry optimizations were carried out at the HF/6-31G** and B3LYP/6-31G** levels for methylcyclohexane, 2-, 3-, and 4-methyltetrahydropyran, 2-, 3-, and 4-methylpiperidine, 2-, 3-, and 4-methylthiane, 2-, 4-, and 5-methyl-1,3-dioxane, and 2-, 4-, and 5-methyl-1,3-dithiane and also for S-methyl thianium. Constrained geometry optimizations were carried out for methylcyclohexane, 2-methyl-1,3-dioxane, and the axial conformers of 2- and 3-methyltetrahydropyran and 2- and 3-methylpiperidine. The steric repulsion model, which is believed to account for the conformational energies of the cited compounds, was tested by stretching bonds and bending angles so that the axial methyl group is either forced to approach the ring gamma methylenes or get farther away from them. The calculated energies show that the energy costs of these perturbations are not dependent on the distances between the axial methyl group and the ring gamma methylenes and are not dependent on whether the methyl is axial or equatorial. It is shown that, besides the steric repulsion model, the conformational energies of the compounds studied are dictated by hyperconjugative interactions involving mainly the methine hydrogen. The C[bond]C lengths of the axial and equatorial conformers of methylcyclohexane are shown to be related to hyperconjugation.     

Hyperfine coupling to β-muonium and the theory of hyperconjugation

The theory of hyperconjugation, orσ-π delocalization, which has been very successful in explaining the properties of carbocations, and the ESR parameters for a range of radicals, is invoked to explain two aspects of the results for muonated radicals. One is the commonly observed fact that the reduced muon hyperfine coupling constants in a range of radicals are greater by a factor of ca. 1.2 than the corresponding proton coupling constants. The other is the preference of C-Mu bonds in positionsβ to radical centers to occupy an eclipsed site which maximizesσ-π overlap. This theory, which has been largely dismissed by others, still seems to be an attractive and simple explanation of both phenomena. It is suggested that one alternative theory, which is termed the “zeropoint energy theory,” is closely linked to the hyper-conjugation theory proposed herein.     

Substituent effects and the role of negative hyperconjugation in siloxycarbene rearrangements

Substituent effects and the role of negative hyperconjugation in 1,2-silyl migration and decarbonylation of methoxy(substituted-siloxy)carbenes have been investigated using quantum chemical calculations and natural bond orbital analysis. It has been found that sigma-electron-withdrawing substituents generally lower the barriers for 1,2-silyl migration and decarbonylation, consistent with symmetry-forbidden concerted rearrangements involving intramolecular front-side nucleophilic attack by the carbene lone pair at silicon and by the methoxy oxygen at silicon, respectively. However, while good linear Hammett correlations are obtained for 1,2-silyl migration, those obtained for decarbonylation are poor. In addition, there appears to be a relationship between the extent of pertinent hyperconjugative interactions in the siloxycarbene conformers and the ease of intramolecular reactivity. As a matter of fact, the finding that 1,2-silyl migration is more favorable than decarbonylation seems to be primarily related to stronger negative hyperconjugation between the carbene lone pair and the O-Si antibonding orbital, compared to that between the methoxy oxygen n(sigma) lone pair and the O-Si antibonding orbital. Moreover, the activation enthalpies for 1,2-silyl migration decrease linearly with stronger negative hyperconjugation, although no such correlation could be established for decarbonylation.     

An ESR study of the hyperconjugation in the phosphorescent state of acenaphthene

ESR absorption has been observed in a single-crystal solid solution of acenaphthene in durene under irradiation at 120K with light from a high-pressure mercury lamp. The observed hyperfine structure gives evidence for hyperconjugation of the ethylene group protons. The guest and host molecular planes are nearly parallel to each other, and the guest in-plane axes deviate from the host ones by ca 40°. For comparison, 1,1,2,2-tetradeuteroacenaphthene was also prepared and studied.     

Valence-shell charge concentrations and electron delocalization in alkyllithium complexes: negative hyperconjugation and agostic bonding

In this paper we present the results of density functional theory (DFT) calculations on the ethyl ligand and some related organic moieties; we then proceed to consider a range of alkyllithium complexes studied by DFT calculations and high-resolution X-ray and neutron diffraction. Topological analysis of the charge density is used to follow changes in the electronic structure of the organic fragment. The charge concentrations (CCs) in the valence shell at the alpha and beta atoms reveal faithfully the delocalization of the lone pair at the Calpha atom or of the Li-C bonding electrons. Negative hyperconjugation is thus shown to arise from delocalization of the lone pair or the Li-C bonding electrons over the alkyl fragment, with depletion of the metal-directed charge concentration at Calpha, and characteristic ellipticity profiles for the bonds involved in hyperconjugative delocalization. In the case of so-called lithium agostic complexes, we show that close Li.H contacts are a consequence of this delocalization and further secondary interactions, with Li.H-C agostic interactions, playing only a minor role. The ellipticity profiles and the magnitude of the CCs at Calpha provide a quantitative measure of the extent of delocalization, and show excellent agreement between experiment and theory.     

Intrinsic acidity of dimethylhalonium ions: evidence for hyperconjugation in dimethylhalonium ylides in the gas phase

The intrinsic acidity of dimethylhalonium ions has been determined, both by theoretical methods and by gas-phase reactions of the isolated ions with pyridine bases. The calculated geometry of the dimethylhalonium ions shows a bent structure with the C-X-C angle decreasing in the order Cl > Br > I. Thermochemical calculations for the reaction of the dimethylhalonium ions with pyridine, 2,6-dimethylpyridine, and 2,6-di-tert-butylpyridine indicate that proton transfer, with the formation of the dimethylhalonium ylide is endothermic, whereas methyl transfer, with formation of methylhalide, is exothermic. The endothermicities for proton transfer are, nevertheless, dependent on the steric hindrance of the base. The bulkier the bases, the less endothermic the proton-transfer reaction is. Experimental gas-phase reactions support the calculations, showing that methyl transfer is the major reaction of dimethylchloronium and dimethyliodonium with pyridine, whereas proton transfer, as well as single electron transfer, is observed for the bulkier bases. The calculations also indicate that acidity increases in the order chloronium > bromonium > iodonium. NBO calculations predict that hyperconjugation with the sigma carbon-halogen orbital plays a role in stabilizing the halonium ylide species in the gas phase.     

Theory and application of photoelectron spectroscopy : XXX. HgC hyperconjugation

Evidence for a hyperconjugative splitting due to the interacting HgC σ_u and ethylene π(b_1u) MOs in allylmercuric chloride is presented.     

Role of negative hyperconjugation and anomeric effects in the stabilization of the intermediate in SNV reactions

The role of negative hyperconjugation and anomeric and polar effects in stabilizing the XZHCbetaCalphaYY'- intermediates in SNV reactions was studied computationally by DFT methods. Destabilizing steric effects are also discussed. The following ions were studied: X = CH3O, CH3S, CF3CH2O and Y = Y' = Z = H (7b-7d), Y = Y' = H, Z = CH3O, CH3S, CF3CH2O (7e-7i), YY' = Meldrum's acid-like moiety (Mu), Z = H, (8b-8d), and YY' = Mu, Z = CH3O, CH3S, CF3CH2O (8e-8i). The electron-withdrawing Mu substituent at Calpha stabilizes considerably the intermediates and allows their accumulation. The hyperconjugation ability (HCA) (i.e., the stabilization due to 2p(Calpha) --> sigma*(Cbeta-X) interaction) in 8b-8d follows the order (for X, kcal/mol) CH3S (8.5) > CF3CH2O (7.6) approximately CH3O (7.5). The HCA in 8b-8d is significantly smaller than that in 7b-7d due to charge delocalization in Mu in the former. The calculated solvent (1:1 DMSO/H2O) effect is small. The stability of disubstituted ions (7e-7i and 8e-8i) is larger than that of monosubstituted ions due to additional stabilization by negative hyperconjugation and an anomeric effect. However, steric repulsion between the geminal Cbeta substituents destabilizes these ions. The steric effects are larger when one or both substituents are CH3S. The anomeric stabilization (the energy difference between the anti,anti and gauche,gauche conformers) in the disubstituted anions contributes only a small fraction to their total stabilization. Its order (for the following X/Z pairs, kcal/mol) is CF3CH2O/CH3S (8i, 4.9) > CF3CH2O/CH3O (8h, 3.9) > CH3O/CH3S (8g, 3.3) > CH3S/CH3S (8f, 2.9) > CH3O/CH3O (8e, 2.4). Significantly larger anomeric effects of ca. 8-9 kcal/mol are calculated for the corresponding conjugate acids.     

Electronic basis of improper hydrogen bonding: a subtle balance of hyperconjugation and rehybridization

The X[bond]H bond length in X[bond]H...Y hydrogen bonded complexes is controlled by a balance of two main factors acting in opposite directions. "X[bond]H bond lengthening" due to n(Y)-->sigma(H[bond]X) hyperconjugative interaction is balanced by "X[bond]H bond shortening" due to increase in the s-character and polarization of the X[bond]H. When hyperconjugation dominates, X[bond]H bond elongation is reflected in a concomitant red shift of the corresponding IR stretching frequency. When the hyperconjugative interaction is weak and the X-hybrid orbital in the X[bond]H is able to undergo a sufficient change in hybridization and polarization, rehybridization dominates leading to a shortening of the X[bond]H and a blue shift in the X[bond]H stretching frequency.