Topic
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.
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76 citations
TL;DR: In this article, a conformational analysis of several substituted terthiophenes using ab initio calculations performed at the HF/3-21G* level is performed, and the most stable conformation of TT is found for dihedral angles θ = φ = 147.2°, whereas three maxima are located at 0°, 90°, and 180°.
Abstract: We report a conformational analysis of several substituted terthiophenes using ab initio calculations performed at the HF/3-21G* level. Geometries of terthiophenes having methoxy substituents in 3,3‘‘ positions (DMOTT), methyl groups in the same positions (DMTT), and ethyl substituents in 3‘,4‘ positions (DETT) are compared with that of the unsubstituted molecule (TT). For all these symmetrical molecules, it is observed that the two dihedral angles are independent of each other. The most stable conformation of TT is found for dihedral angles θ = φ = 147.2°, whereas three maxima are located at 0°, 90°, and 180°. The insertion of methoxy groups in 3,3‘‘ positions favors a more planar conformation with a higher rotational barrier at 90°. This behavior is explained by the electron donor properties of the methoxy groups. By contrast, the addition of two methyl groups at the same positions induces a twisting in the molecule which is caused by the steric hindrance between the methyl substituents and the sulfur a...
75 citations
TL;DR: In this article, the barrier to rotation of a novel ligand is derived from inelastic neutron scattering measurements of its rotational transitions and interpreted in terms of the various types of interactions that give rise to it.
Abstract: The fact that certain metal complexes will bind hydrogen in molecular form has provided an opportunity to examine in the solid state the simplest hindered molecular rotation problem, namely, that of a dumbbell molecule in a double-minimum potential with one rotational degree of freedom. The barrier to rotation of this novel ligand is derived from inelastic neutron scattering measurements of its rotational transitions and interpreted in terms of the various types of interactions that give rise to it. These include the direct electronic interaction between dihydrogen and the metal, electronic effects (via the metal) of the other ligands on the metal, and steric effects
75 citations
TL;DR: Comparison of the effects of fluorinated alcohols as promoting solvents in three reactions is reported, and the positive correlation between rate constants and H-bonding donation ability for sulfide oxidation and imino Diels-Alder reaction brings to light the role of this property.
Abstract: The influence of substituents on the properties of tri- and hexafluorinated alcohols derived from 2,2,2-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) was examined. Measurements of specific solvent−solute interactions revealed that H-bond donation (HBD) of fluorinated alcohols is sensitive to the steric hindrance of the OH group, whereas their Bronsted acidity is dependent only on the number of fluorine atoms. For hexafluorinated alcohols (HFAs), their association with amines characterized by X-ray diffraction showed that the balance between HBD and acidity is influenced by their structure. Moreover, the ability of HFAs to donate H-bonds is exerted in synclinal (sc), synperiplanar (sp), and also antiperiplanar (ap) conformations along the C−O bond. Comparison of the effects of fluorinated alcohols as promoting solvents in three reactions is reported. The positive correlation between rate constants and H-bonding donation ability for sulfide oxidation and imino Diels−Alder reaction brin...
75 citations
TL;DR: In this article, a spiro-bicyclic system was developed to mimic the type II β-turn, a secondary structural feature found in many bioactive peptides, and the asymmetric synthesis and conformational analysis of derivative 6 starting from (R)-2-allylproline was reported.
Abstract: A novel highly constrained spiro-bicyclic system (5) has been developed to mimic the type II β-turn, a secondary structural feature found in many bioactive peptides. This system simultaneously restricts three (Φ 2 , Ψ 2 , and Φ 3 ) of the four torsion angles that characterize the type II β-turn. As a test of the design, the asymmetric synthesis and conformational analysis of derivative 6 starting from (R)-2-allylproline is reported. Temperature dependent NMR chemical shift studies in CDCl 3 suggest that the amide proton of 6 is involved in an intramolecular hydrogen bond.Aso,NOE measurements place this hydrogen under the plane of the bicyclic ring system in proper proximity for this hydrogen bond to form with the acetyl carbonyl oxygen.Modeling studies of 6 produced eight minimum-energy conformations with torsion angles close to those of the classical type II β-turn.A comparison of the minimum-energy conformer of this molecule with the classical type II β-turn gave an RMS fit=0.161 A °
75 citations