Steric effects

About: Steric effects is a research topic. Over the lifetime, 16112 publications have been published within this topic receiving 319615 citations. The topic is also known as: steric hindrance.

Computational Evidence of the Importance of Substituent Bulk on Agostic Interactions in Ir(H)2(PtBu2Ph)2

Abstract: While Ir(H)2(PtBu2Ph)2+ has been shown experimentally to have two agostic tBu groups, ab initio B3LYP calculations on IrH2[P(Et)H2]2+ show that the CH3 group of the phosphine ligand does not form any agostic bond with the strongly electron-deficient (14-valence electron) metal. In contrast, integrated molecular mechanics/molecular orbital (IMOMM) calculations on the full complex Ir(H)2(PtBu2Ph)2+ duplicate the experimentally observed agostic interaction. Thus, at least in this case, the agostic interaction is due in part to the trapping of a C−H bond in the vicinity of the metal by the steric effects of the other groups of the bulky phosphine. This necessity of steric “constraint” identifies an additional influential factor for the agostic interaction.

Steric stabilization of nanoparticles with grafted low molecular weight ligands in highly concentrated brines including divalent ions

Abstract: Whereas numerous studies of stabilization of nanoparticles (NPs) in electrolytes have examined biological fluids, the interest has grown recently in media with much higher ionic strengths including seawater and brines relevant to environmental science and subsurface oil and gas reservoirs. Given that electrostatic repulsion is limited at extremely high ionic strengths due to charge screening, we have identified ligands that are well solvated in concentrated brine containing divalent cations and thus provide steric stabilization of silica nanoparticles. Specifically, the hydrodynamic diameter of silica nanoparticles with grafted low molecular weight ligands, a diol ether, [3-(2,3-dihydroxypropoxy)propyl]-trimethoxysilane, and a zwitterionic sulfobetaine, 3-([dimethyl(3-trimethoxysilyl)propyl]ammonio)propane-1-sulfonate, is shown with dynamic light scattering to remain essentially constant, indicating lack of aggregation, at room temperature and up to 80 °C for over 30 days. An extended DLVO model signifies that steric stabilization is strongly dominant against van der Waals attraction for ∼10 nm particles given that these ligands are well solvated even in highly concentrated brine. In contrast, polyethylene glycol oligomers do not provide steric stabilization at elevated temperatures, even at conditions where the ligands are soluble, indicating complicating factors including bridging of the ether oxygens by divalent cations.

Structure, Reactivity, Spectra, and Redox Properties of Cobalt(III) Hexaamines

Abstract: Publisher Summary This chapter discusses the effect of structural properties of cobalt (III) hexa-amines on their other properties. The chapter discusses complexes ranging from cobalt hexa-ammine, to complexes composed of three secondary and three tertiary amine donors. The chapter discusses only those coordination compounds, having a complete CoN 6 core, thus excluding the enormous number of CoN 6 - x L x complexes. Ammonia and organic amines with an aliphatic skeleton are described in the chapter. The chapter discusses complexes, containing just one kind of ligand—that is, homoleptic complexes. Structural properties and their relationship to kinetic stability (toward hydrolysis and racemization), electronic structure, reduction potentials, and electron transfer processes are explained in this chapter. The standard preparative procedure for the Co(III)–amine complexes consist of the reaction of an aqueous or alcoholic solution of a Co(II) salt with the free base of the amine. Oxidation in the solution is achieved by air, oxygen, H 2 0 2 , or other oxidants. In this chapter, the term molecular mechanics describes a method used frequently to predict structures and relative energies of related complexes (usually isomers). The method involves summing the potential energy of all the interatomic interactions in terms of bond deformation, valence angle deformation, torsion angle deformation, and nonbonded interactions. The chapter discusses the effects of structure on the other properties of the amine complexes predominantly in terms of the steric effects.

Studies on the Biosynthesis of Cholesterol. XIX. Steric Course of Hydrogen Eliminations and of C-C bond Formations in Squalene Biosynthesis

Abstract: 1. Three molecules of isopentenyl pyrophosphate are used in the biosynthesis of one molecule of farnesyl pyrophosphate. In this process, one hydrogen atom is added and three are eliminated: these three were all originally attached to C-4 of an earlier precursor, mevalonate. It is demonstrated that these eliminations are stereospecific and that all three occur in the same stereochemical sense. The absolute configuration of the eliminated hydrogen is defined. 2. In the association of three isopentenyl pyrophosphate molecules to form farnesyl pyrophosphate, two new C-C bonds are formed with expulsion of two pyrophosphate ions. It is demonstrated that both these processes occur with inversion of configuration at the pyrophosphate-bearing carbon atom. 3. In the biosynthesis of squalene from two molecules of farnesyl pyrophosphate, one farnesyl residue suffers exchange of one hydrogen atom and one new C-C bond is formed. It is demonstrated that this bond is formed with inversion of configuration at that farnesyl residue which does not exchange hydrogen.