Molecular mechanics‐based measures of steric effects: Customized code to compute Ligand repulsive energies

Ligand repulsive energies, E_R, have been demonstrated to provide reliable steric parameters for ligands in organometallic systems. To date, ligand repulsive energies have been computed manually using commercially available molecular mechanics code. We report a customized code, ERCODE, that calculates ligand repulsive energies. Some reported E_R values differ from those in the literature due to a modified conformational search strategy presented. Updated ligand repulsive energies for 100 phosphines, 12 phosphites, 26 amines, and 54 alcohols, ethers, and sulfides are presented. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 239–246, 2000     

Conformations and rotational barriers of 2,2′‐bithiazole and 4,4′‐dimethyl‐2,2′‐bithiazole semiemperical,ab initio,and density functional theory calculations

Conformational properties of 2,2′‐bithiazole and 4,4′‐dimethyl‐2,2′‐ bithiazole have been studied by using AM1 and PM3 semiemperical methods and ab initio HF/6‐311+G* and B3LYP/6‐311+G* calculations. All methods agree that the planar s‐trans conformation is the global minimum and the perpendicular conformation is the transition state. Additional local minima were found using the Hartree–Fock (HF) and B3LYP levels for 2,2′‐bithiazole while for 4,4′‐dimethyl derivative the minima was located only at the MP2//B3LYP level. The barrier heights for rotation are 1.72, 7.69, and 7.88 kcal/mol at the PM3, HF, and B3LYP levels, respectively, and methyl substitution did not affect appreciably this value. Fourier expansion terms and bond orders were used to explain the origins of the rotational barrier in terms of π conjugation, electrostatic interaction, and steric effects, which represent the main factors in the shape of the rotational barrier. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 79: 367–377, 2000     

Modifying the Steric Properties in the Second Coordination Sphere of Designed Peptides Leads to Enhancement of Nitrite Reductase Activity

Protein design is a useful strategy to interrogate the protein structure‐function relationship. We demonstrate using a highly modular 3‐stranded coiled coil (TRI‐peptide system) that a functional type 2 copper center exhibiting copper nitrite reductase (NiR) activity exhibits the highest homogeneous catalytic efficiency under aqueous conditions for the reduction of nitrite to NO and H₂O. Modification of the amino acids in the second coordination sphere of the copper center increases the nitrite reductase activity up to 75‐fold compared to previously reported systems. We find also that steric bulk can be used to enforce a three‐coordinate Cu^I in a site, which tends toward two‐coordination with decreased steric bulk. This study demonstrates the importance of the second coordination sphere environment both for controlling metal‐center ligation and enhancing the catalytic efficiency of metalloenzymes and their analogues.     

Silicon Phthalocyanines with Axial Dendritic Substituents

Nonscattering glasses that are indefinitely stable with respect to crystallization can be prepared from the title compounds. In these solid solutions, the dendritic substituents effectively suppress interactions between the phthalocyanine units (see the structure depicted on the right).     

Reactive surfactants in heterophase polymerization. Part XXII—incorporation of macromonomers used as stabilizers in styrene dispersion polymerization

The effect of several parameters on the incorporation yield of poly(ethylene oxide) macromonomers at the surface of the particles, for the dispersion polymerization of styrene in ethanol–water mixtures, has been studied. The reactivity of the macromonomer is a key parameter in the mechanism of stabilization of the micrometer-size polymer particles, because it partly determines the amount and the composition of the copolymer stabilizer available at any moment during the process. The polarity of the reaction medium also strongly influences the polymerization process: higher incorporation yield and grafting density were obtained in medium of lower polarity. Besides, a chain length of around 50 ethylene oxide units for the macromonomer were needed to produce stable monodisperse particles with a significant incorporation yield. Thus, an incorporation yield as high as 53% and a grafting density corresponding to a surface area of 232 Ų/molecule have been obtained in a one-step process by using a methacrylate macromonomer. In an optimized two-step process resulting in monodisperse polymer particles, 80% incorporation yield with a very high grafting density (175 Ų/molecule) were reached. The particles with high grafting density (surface area lower than 600 Ų/molecule) could be transferred in water and exposed to a freeze–thaw cycle without massive flocculation, illustrating the efficiency of the steric stabilization. © 1997 John Wiley & Sons, Ltd.     

Quinoline formation via a modified Combes reaction: examination of kinetics,substituent effects,and mechanistic pathways

This is the first reported investigation of the Combes condensation employing 19F NMR spectroscopy to monitor intermediate consumption and product formation rates. The reaction was found to be first order in both the diketone and aniline. Product regioselectivity and reaction rates were found to be influenced by substituents on the diketones and anilines with rates varying as much as five fold. The consumption rate of key imine and enamine intermediates mirrored quinoline formation rates, in accord with rate determining annulation. A ρ of ?0.32 was determined for this cyclization. While the sign of the reaction constant is consistent with rate limiting electrophilic aromatic substitution (EAS), the magnitude is likely a composite value, resulting from opposing substituent effects in the nucleophilic addition and EAS steps. Mechanistic details and reaction pathways supporting these findings are proposed. Copyright © 2008 John Wiley & Sons, Ltd.     

Catenation of 1,3‐Diphosphacyclobutane‐2,4‐diyl Units Having 2,4,6‐Tri‐tert‐butylphenyl Protecting Groups and a P‐sec‐Butyl Group in the Ring

Two and three stable 1‐sec‐butyl‐2,4‐bis(2,4,6‐tri‐tert‐butylphenyl)‐1,3‐diphosphacyclobutane‐2,4‐diyl units were catenated to construct multi‐biradical derivatives by utilizing 1,3‐di‐, 1,4‐di‐, and 1,3,5‐trimethylenebenzenes as bridging groups, respectively. UV/Vis spectroscopic and cyclovoltammetric (CV) properties of the multi‐biradicals indicate a non‐conjugative interaction between the concatenated biradical units.