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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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Journal ArticleDOI
TL;DR: Structural modifications such as the replacement of dodecyl groups in 2a with hydrogen atoms resulted in a stable dimer structure in 2b due to diminished steric hindrance, as supported by quantum chemical calculations.
Abstract: Disc-shaped hexa-peri-hexabenzocoronenes (HBCs) peripherally substituted by flexible dodecyl chains (molecule 1) or rigid polyphenylene dendrons (molecules 2a,b and 3) were efficiently synthesized. Steric hindrance arising from the substituents, from less hindered dodecyl to bulky dendrons, was utilized to program the self-assembly of the HBC cores in solution. The high tendency of the hexadodecyl-substituted HBC 1 to aggregate was determined by concentration and temperature-dependent 1H NMR spectroscopic measurements and nonlinear least-squares analysis of the experimental data. The rigid dendrons in molecule 2a suppress the π−π interactions of the HBC cores to a certain extent, and a slow (with respect to the NMR time scale) monomer−dimer equilibrium is observed. This unique equilibrium was further controlled by temperature, concentration, and solvent to afford discrete monomeric or dimeric species. Further structural modifications such as the replacement of dodecyl groups in 2a with hydrogen atoms resu...

144 citations

Journal ArticleDOI
TL;DR: In this paper, a vibrational spectroscopic method was used to study silanol groups which have been modified after reaction with a variety of very reactive hydrogen sequestering agents (D2O, ZnMe2, BCl3, TiCl4, AlMe3, GaMe3 and BEt3).
Abstract: Chemical modification of the silica surface can provide a powerful method for probing the nature of the surface hydroxyl groups and how these might be modified via thermal activation. In the present paper, it is shown how vibrational spectroscopic methods (infrared and Raman) can be used to study silanol groups which have been modified after reaction with a variety of very reactive hydrogen sequestering agents (D2O, ZnMe2, BCl3, TiCl4, AlMe3, GaMe3, BEt3 and (SiMe3)2NH), having differing steric dimensions and it is demonstrated that a nearly complete vibrational spectrum for some of the corresponding new surface species can be obtained. With the use of fast scanning FTIR spectroscopy, spectra were able to be recorded in less than a second and the application of these time-resolved methods has been used to probe differences in the reactivity of isolated and H-bonded silanol groups. The spectroscopic evidence suggests that the ability of a reactant to react bifunctionally, i.e. to react with more than one SiOH group, is important in determining the initial reactivity of the H-bonded silanols relative to those that are isolated or non-H-bonded. Further, the number of inaccessible and H-bonded silanols that do not react increases as the apparent size of the reactant molecule increases, regardless of whether the reactant can react bifunctionally. Finally, for the larger probe molecules used ((SiMe3)2NH and BEt3), the new chemisorbed surface species block other H-bonded silanols and prevent them from reacting with either of these probes.

144 citations

Journal ArticleDOI
TL;DR: The change in pressure needed to bring egg phosphatidylcholine bilayers into contact from their equilibrium separation in excess water has been determined as a function of both distance between the bilayers and water content as discussed by the authors.
Abstract: The change in pressure needed to bring egg phosphatidylcholine bilayers into contact from their equilibrium separation in excess water has been determined as a function of both distance between the bilayers and water content. A distinct upward break in the pressure-distance relation appears at an interbilayer separation of about 5 A, whereas no such deviation is present in the pressure-water content relation. Thus, this break is not a property of the dehydration process per se, but instead is attributed to steric repulsion between the mobile lipid head groups that extend 2-3 A into the fluid space between bilayers. That is, electron density profiles of these bilayers indicate that the observed break in the pressure-spacing relation occurs at a bilayer separation where extended head groups from apposing bilayers come into steric hindrance. The pressure-spacing data are used to separate steric pressure from the repulsive hydration pressure, as well as to quantitate the range and magnitude of the steric interaction. An appreciable fraction of the measured steric energy can be ascribed to a decrease in configurational entropy due to restricted head-group motion as adjacent bilayers come together.

144 citations

Journal ArticleDOI
TL;DR: A series of alkyl (1-3), aryl (6), and benzo-annulated (4, 5) heteroaromatic triphenylene analogues with B(2)N( 2)C(2), found to result in a dimeric, head-to-tail herringbone packing motif, held together by close intermolecular B...N interactions of 3.
Abstract: A series of alkyl (1−3), aryl (6), and benzo-annulated (4, 5) heteroaromatic triphenylene analogues with B2N2C2 cores have been synthesized via chelation of pyridazine derivatives using difunctional Lewis acidic diborabiphenyl precursors. In contrast to triphenylene, NICS(1) calculations on 1 suggested high aromaticities for the central (−11.3 ppm) and outer borabenzene rings (−7.7 ppm), along with nonaromatic behavior for the pyridazine ring (−0.7 ppm). Crystal structure analyses supported this analysis. When the a- and c-faces of the pyridazine moiety were free of substitution (1, 3), planar structures resulted, but upon substitution, a twisted B2N2C2 core was observed due to steric repulsion of neighboring hydrogen atoms (e.g., 5). The increase of steric bulk from H (1) to iPr (3) in the planar species was found to result in a dimeric, head-to-tail herringbone packing motif, held together by close intermolecular B···N interactions of 3.39 A. One-electron reduction by Cp*2Co was found to afford the radi...

144 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
2023942
20221,917
2021346
2020292
2019296
2018307