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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: The steric modulation of the donor/acceptor separation (rDA) leads to distinctive electron-transfer rates between sterically hindered donors/acceptors and their more open (unsubstituted) parents.
Abstract: Transient 1:1 precursor complexes for intermolecular self-exchange between various organic electron donors (D) and their paramagnetic cation radicals (D+*), as well as between different electron acceptors (A) paired with their anion radicals (A-*), are spectrally (UV-NIR) observed and structurally (X-ray) identified as the cofacial (pi-stacked) associates [D, D+*] and [A-*, A], respectively. Mulliken-Hush (two-state) analysis of their diagnostic intervalence bands affords the electronic coupling elements (HDA), which together with the Marcus reorganization energies (lambda) from the NIR spectral data are confirmed by molecular-orbital computations. The HDA values are found to be a sensitive function of the bulky substituents surrounding the redox centers. As a result, the steric modulation of the donor/acceptor separation (rDA) leads to distinctive electron-transfer rates between sterically hindered donors/acceptors and their more open (unsubstituted) parents. The latter is discussed in the context of a continuous series of outer- and inner-sphere mechanisms for organic electron-transfer processes in a manner originally formulated by Taube and co-workers for inorganic (coordination) donor/acceptor dyads-with conciliatory attention paid to traditional organic versus inorganic concepts.

106 citations

Journal ArticleDOI
TL;DR: It is demonstrated that steric interactions alone are insufficient to account for the spatial characteristics of polynucleotide chains.
Abstract: A simplified scheme for treating the spatial configurations of polynucleotide chains is developed using the rotational isomeric state approximation and statistical methods applicable to linear systems of interacting subunits. As a consequence of geometric constraints imposed by the skeletal structure and of the severity of certain steric interactions, it is possible to represent the repeat unit comprising six skeletal bonds by two virtual bonds of fixed length. The configuration of the polynucleotide chain as a whole may be conveniently described by an alternating succession of these two virtual bonds. Moreover, analysis of steric interactions suggests that bond rotations governing the mutual orientation of a given pair of successive virtual bonds should be sensibly independent of the rotations affecting the mutual orientation of other pairs. The statistical mechanical treatment of configuration-dependent properties is much simplified in consequence of this mutual independence. Mean-square dimensions calculated by giving equal weights to all sterically allowed conformations are much smaller than values determined by Felsenfeld and co-workers. The calculated dimensions are markedly increased, however, by placing certain arbitrary restrictions on the rotations about selected pairs of skeletal bonds. It is thus demonstrated that steric interactions alone are insufficient to account for the spatial characteristics of polynucleotide chains. The dimensions are also found to be sensitive to the conformation of the ribose ring of each nucleotide unit, but, insofar as the influences of steric interactions are concerned, the dimensions do not depend on the heterocyclic base attached to the ribose ring.

106 citations

Journal ArticleDOI
TL;DR: In this article, temperature-dependent optical absorption measurements have revealed important thermochromic effects in poly(3-dodecylthiophene) and poly( 3-octyloxythiophene), which are related to a planar to non-planar transition of the main polymer chain.
Abstract: Temperature-dependent optical absorption measurements have revealed important thermochromic effects in poly(3-dodecylthiophene) and poly(3-octyloxy-4-methylthiophene) between 25 and 150°C which are related to a planar to non-planar transition of the main polymer chain. This conformational transition is caused by an increase of the repulsive intrachain steric interactions upon heating, which then force the thiophene backbone to adopt a non-planar conformation. On the other hand, as observed with poly(3-octyloxythiophene), if the side chains do not lead to large steric interactions, the polymer can maintain a highly conjugated structure even at high temperatures while, if the steric interactions are too strong (e.g. poly(3,3′-dihexyl-2,2′-bithiophene)), no co-planar conformation can be adopted.

106 citations

Journal ArticleDOI
TL;DR: A new family of low-coordinate nickel imides supported by 1,2-bis(di-tert-butylphosphino)ethane was synthesized and it was indicated that the unpaired electron is mostly localized on the imide nitrogen for the Ni(III) complexes.
Abstract: A new family of low-coordinate nickel imides supported by 1,2-bis(di-tert-butylphosphino)ethane was synthesized. Oxidation of nickel(II) complexes led to the formation of both aryl- and alkyl-substituted nickel(III)-imides, and examples of both types have been isolated and fully characterized. The aryl substituent that proved most useful in stabilizing the Ni(III)-imide moiety was the bulky 2,6-dimesitylphenyl. The two Ni(III)-imide compounds showed different variable-temperature magnetic properties but analogous EPR spectra at low temperatures. To account for this discrepancy, a low-spin/high-spin equilibrium was proposed to take place for the alkyl-substituted Ni(III)-imide complex. This proposal was supported by DFT calculations. DFT calculations also indicated that the unpaired electron is mostly localized on the imide nitrogen for the Ni(III) complexes. The results of reactions carried out in the presence of hydrogen donors supported the findings from DFT calculations that the adamantyl substituent was a significantly more reactive hydrogen-atom abstractor. Interestingly, the steric properties of the 2,6-dimesitylphenyl substituent are important not only in protecting the Ni═N core but also in favoring one rotamer of the resulting Ni(III)-imide, by locking the phenyl ring in a perpendicular orientation with respect to the NiPP plane.

105 citations

Journal ArticleDOI
TL;DR: Experimental behaviour and stability of these carbene-borane adducts are confirmed and the adduct is stable at room temperature in the absence of suitable substrates.
Abstract: Tris[3,5-bis(trifluoromethyl)phenyl]borane reacts with the sterically demanding Arduengo carbenes 1,3-di-tert-butylimidazolin-2-ylidene and 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene to form isolable normal adducts. In the case of 1,3-di-tert-butylimidazolin-2-ylidene, the adduct exhibits dynamic behaviour in solution and frustrated-Lewis-pair (FLP) reactivity. Fast cleavage of dihydrogen and THF, the C-H activation of phenylacetylene, and carbon dioxide fixation were achieved by using solutions of this adduct in benzene. This adduct is stable at room temperature in the absence of suitable substrates; however, thermal rearrangement into an abnormal carbene-borane adduct can be observed. In contrast, the 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene adduct exhibits no evidence of FLP reactivity or of dissociation in solution. DFT calculations confirmed the experimental behaviour and stability of these carbene-borane adducts.

105 citations


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