Anti-Electrostatic Pi-Hole Bonding: How Covalency Conquers Coulombics

Intermolecular bonding attraction at π-bonded centers is often described as “electrostatically driven” and given quasi-classical rationalization in terms of a “pi hole” depletion region in the electrostatic potential. However, we demonstrate here that such bonding attraction also occurs between closed-shell ions of like charge, thereby yielding locally stable complexes that sharply violate classical electrostatic expectations. Standard DFT and MP2 computational methods are employed to investigate complexation of simple pi-bonded diatomic anions (BO−, CN−) with simple atomic anions (H−, F−) or with one another. Such “anti-electrostatic” anion–anion attractions are shown to lead to robust metastable binding wells (ranging up to 20–30 kcal/mol at DFT level, or still deeper at dynamically correlated MP2 level) that are shielded by broad predissociation barriers (ranging up to 1.5 Å width) from long-range ionic dissociation. Like-charge attraction at pi-centers thereby provides additional evidence for the dominance of 3-center/4-electron (3c/4e) nD-π*AX interactions that are fully analogous to the nD-σ*AH interactions of H-bonding. Using standard keyword options of natural bond orbital (NBO) analysis, we demonstrate that both n-σ* (sigma hole) and n-π* (pi hole) interactions represent simple variants of the essential resonance-type donor-acceptor (Bürgi–Dunitz-type) attraction that apparently underlies all intermolecular association phenomena of chemical interest. We further demonstrate that “deletion” of such π*-based donor-acceptor interaction obliterates the characteristic Bürgi–Dunitz signatures of pi-hole interactions, thereby establishing the unique cause/effect relationship to short-range covalency (“charge transfer”) rather than envisioned Coulombic properties of unperturbed monomers.

The Synthesis of Associative Copolymers with Both Amphoteric and Hydrophobic Groups and the Effect of the Degree of Association on the Instability of Emulsions

The acrylamide (AM)/methacryloyl ethyl sulfobetaine (SPE)/behenyl polyoxyethylene ether methacrylate (BEM) terpolymer (PASB) was synthesized by soap-free emulsion polymerization. Four types of PASBs were synthesized by adjusting the moles of AM and BEM with constant total moles of monomers. The synthesized copolymers were characterized by Fourier-transform infrared spectroscopy, thermogravimetry, molecular weight, and viscosity. By measuring the microscopic morphology and backscattered light intensity of the emulsions, the instability process of the emulsions prepared by PASBs was investigated in detail. The main instability processes of the emulsions prepared from PASBs within 45 min were flocculation and coalescence. The intermolecular association of copolymer PASBs was dominated by the behenyl functional groups on the molecular chains. The stability of the emulsions, which were prepared from isoviscosity aqueous solutions controlled by the concentration of the associative copolymers, was increased with the degree of association of copolymers. The hydrophobic association between the copolymer molecules can further slow down the flocculation and coalescence of the emulsion droplets on the basis of the same aqueous solution viscosity, which is one of the reasons for improving the stability of the emulsion.

Crystal and molecular structures of some phosphane-substituted cymantrenes [(C5H4 X)Mn(CO)LL′] (X = H or Cl, L = CO, L′ = PPh3 or PCy3, and LL' = Ph2PCH2CH2PPh2)

UV irradiation of tetrahydrofuran solutions of [CpMn(CO)3] (Cp = π-C5H5 or π-C5H4Cl) in the presence of the phosphanes PPh3 or PCy3 (Cy = cyclohexyl) and Ph2PCH2CH2PPh2 yields the substitution products [CpMn(CO)2PR 3] (R = Ph or Cy) and [CpMn(CO)(Ph2PCH2CH2PPh2)], namely, dicarbonyl(η5-cyclopentadienyl)(triphenylphosphane-κP)manganese(I), [Mn(C5H5)(C18H15P)(CO)2], 1a, dicarbonyl(η5-1-chlorocyclopentadienyl)(triphenylphosphane-κP)manganese(I), [Mn(C5H4Cl)(C18H15P)(CO)2], 1b, dicarbonyl(η5-cyclopentadienyl)(tricyclohexylphosphane-κP)manganese(I), [Mn(C5H5)(C18H33P)(CO)2], 2a, dicarbonyl(η5-1-chlorocyclopentadienyl)(tricyclohexylphosphane-κP)manganese(I), [Mn(C5H4Cl)(C18H33P)(CO)2], 2b, carbonyl(η5-cyclopentadienyl)[1,2-bis(diphenylphosphanyl)ethane-κ2 P,P′]manganese(I), [Mn(C5H5)(C26H24P2)(CO)], 3a, and carbonyl(η5-1-chlorocyclopentadienyl)[1,2-bis(diphenylphosphanyl)ethane-κ2 P,P′]manganese(I), [Mn(C5H4Cl)(C26H24P2)(CO)], 3b, The crystal structure determinations show a very small influence of the chlorine substitution and a moderate influence of the phosphane substitution on the bond lengths. The PR 3 groups avoid being eclipsed with the C—Cl bonds. All the compounds employ weak C—H...O interactions for intermolecular association, which are enhanced by C—H...Cl contacts in the chlorinated products.

Quinacridone pigments

This chapter surveys the structural and synthetic chemistry and the industrial applications of quinacridones, a small but extremely important group of high-performance carbonyl (or polycyclic) organic pigments. They are based on one of the most important new chromophoric systems developed specifically for pigment applications after the introduction of the phthalocyanines, and currently occupy a prominent position in the red to violet shade areas. A historical perspective on the discovery and commercial development of the quinacridones is presented initially. There then follows an illustrated discussion of the structural chemistry of the pigments, encompassing both molecular and crystal structures. Throughout the chapter, specific features of their molecular structures and the nature of the intermolecular association within the crystals are related to their influence on the color and technical performance in application, in which they exhibit some of the highest standards of heat stability, solvent resistance, and fastness to light and weather encountered in organic pigments. Finally, a survey of the principal current applications of the specific individual commercial quinacridone pigments is presented.

Effect of Multivalent Cations on Intermolecular Association of Isotactic and Atactic Poly(Methacrylic Acid) Chains in Aqueous Solutions

The formation of nanoparticles of two poly(methacrylic acid) (PMA) isomers, atactic (aPMA) and isotactic (iPMA), was investigated in aqueous solutions in the presence of mono- (Na+) and multivalent cations (Mg2+ and La3+). Using dynamic (DLS) and static light scattering (SLS), we show that PMA nanoparticles have characteristics of microgel-like particles with a denser core and a swollen corona. iPMA aggregates are stable at a much higher degree of neutralization (αN) than the aPMA ones, indicating a much stronger association between iPMA chains. This is explained by proposing segregation of ionized and unionized carboxyl groups within the iPMA aggregates and subsequent cooperative hydrogen-bonding between COOH groups. The calculated shape parameter (ρ) suggests different behavior of both isomers in the presence of Mg2+ ions on one hand and Na+ and La3+ on the other. The microgel-like particles formed in the presence of Mg2+ ions have a more even mass distribution (possibly a no core-shell structure) in comparison with those in the presence of Na+ and La3+ ions. Differences between the aggregate structures in the presence of different ions are reflected also in calorimetric experiments and supported by pH and fluorimetric measurements. Reasons for different behavior in the presence of Mg2+ ions lie in specific properties of this cation, in particular in its strong hydration and preference towards monodentate binding to carboxylate groups.