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.

Manganese(III) meso-tetrakis(ortho-N-alkylpyridyl)porphyrins. Synthesis, characterization, and catalysis of O2˙− dismutation

Abstract: A series of ortho isomers of meso-tetrakis(N-alkylpyridyl)porphyrins (alkyl being methyl, ethyl, n-propyl, n-butyl, n-hexyl, and n-octyl) and their Mn(III) complexes were synthesized and characterized by elemental analysis, uv/vis spectroscopy, electrospray ionization mass spectrometry and electrochemistry. An increase in the number of carbon atoms in the alkyl chains from 1 to 8 is accompanied by an increase in: (a) lipophilicity, as measured by the chromatographic retention factor, Rf; (b) metal-centered redox potential, E1/2 from +220 to +367 mV vs. NHE, and (c) proton dissociation constant, pKa2 from 10.9 to 13.2. A linear correlation was found between E1/2 and Rf of the Mn(III) porphyrins and between the pKa2 and Rf of the metal-free compounds. As the porphyrins become increasingly more lipophilic, the decrease in hydration disfavors the creation of charge, while enhancing the electron-withdrawing effect of the positively charged pyridyl nitrogen atoms. Consequently, E1/2 increases linearly with the increase in pKa2, a trend in porphyrin basicity opposite from the one we previously reported for other water-soluble Mn(III) porphyrins. All of these Mn(III) porphyrins are potent catalysts for superoxide dismutation (disproportionation). Despite the favorable increase of E1/2 with the increase in chain length, the catalytic rate constant decreases from methyl (log kcat = 7.79) to n-butyl, and then increases such that the n-octyl is as potent a SOD mimic as are the methyl and ethyl compounds. The observed behavior originates from an interplay of hydration and steric effects that modulate electronic effects.

Lattice Resistance to Hydrolysis of Si−O−Si Bonds of Silicate Minerals: Ab Initio Calculations of a Single Water Attack onto the (001) and (111) β-Cristobalite Surfaces

Abstract: Hydrolysis of Si−O−Si linkages of β-cristobalite by a single H2O molecule is studied within the cluster approach at the DFT (B3LYP) and MP2 levels of theory. The 6-31G(d) and 6-311G(d) basis sets are used. Cluster models, including from 6 up to 14 Si atoms, of the (001) and (111) surface planes are considered. These models are specially designed to take into account the steric constraints imposed by the solid matrix on the Si−O−Si linkages and their nearest surroundings. For comparison, the hydrolysis of the Si−O−Si bridge of the free (HO)3Si−O−Si(OH)3 molecule is also calculated. The computed activation energy of the reaction (ΔEa) for the (001) and (111) planes of β-cristobalite is larger by 5 and 16 kcal/mol, respectively, than for (HO)3Si−O−Si(OH)3 (17 kcal/mol). The higher energy barrier for the surface is due to the resistance of the lattice to the relaxation of the activated complex of the reaction. The difference in ΔEa between the (001) and (111) planes suggests that the larger the number of Si−O...

Copper(I)/Dioxygen Reactivity of Mononuclear Complexes with Pyridyl and Quinolyl Tripodal Tetradentate Ligands: Reversible Formation of Cu:O2 = 1:1 and 2:1 Adducts

Abstract: Copper(I) complexer possessing a series of related tripodal tetradentate ligands with pyridyl- and/or quinolylcontaining groups have been investigated in reactions with dioxygen (O 2 ). The ligand variations allow for the terting of effects of ligand donor ability and steric factors. Copper-dioxygen complex stabilitier, preference for formation of 1:1 Cu-O 2 and/or 2:1 Cu 2 -O 2 adducts, Cu n -O 2 (n=1, 2), spectroscopic propesties, and reactivity characteristics have been invertigated