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.

Flexing analysis of ethane internal rotation energetics

Abstract: A flexing analysis of the ethane barrier energy in terms of structural (ΔEstruct), steric exchange (ΔEsteric), and hyperconjugative charge-transfer (ΔEdeloc) energy contributions has been carried out using natural bond orbitals. No evidence is found for the view that the ethane staggered equilibrium geometry or the C–C bond expansion that accompanies rotation results from steric exchange repulsion interactions. The analysis shows that ΔEstruct and ΔEdeloc have very different stereoelectronic dependencies, but that the ΔEsteric and ΔEdeloc dependencies are antagonistic. All of their contributions are strongly affected by the C–C bond expansion, with the result that the barrier mechanism cannot be understood without taking into account their different relaxation dependencies. Neglect of C–C expansion leaves the charge-transfer interactions paramount by subduing the steric and structural contributions. These interactions are found to be an important determinant for the expansion. The strong expansion depende...

Controlling the Frequency of Macrocyclic Ring Rotation in Benzylic Amide [2]Catenanes

Abstract: A combination of variable-temperature 1H NMR spectroscopy and molecular mechanics calculations have been used to probe the factors that determine the rate of macrocyclic ring rotation in benzylic amide [2]catenanes. The results show that the interlocked macrocycle dynamics are governed by a delicate combination of steric effects, intricate inter-macrocyclic arrays of hydrogen bonds, π−π stacking, and T herringbone-type interactions. A cascade of hydrogen-bond ruptures and formations is the principal event during circumvolution (complete rotation of one macrocyclic ring about the other) but is accompanied by a series of cooperative conformational and co-conformational rearrangements that help to stabilize the energy of the molecule. The experimental picture is consistent both when activation energies are measured from the coalescence of NMR signals and when rate constants are directly measured by spin polarization transfer by selective inversion recovery (SPT-SIR) methods. The nature of the circumrotationa...

Adducts of Nitrous Oxide and N-Heterocyclic Carbenes: Syntheses, Structures, and Reactivity

Abstract: N-Heterocyclic carbenes (NHCs) react at ambient conditions with nitrous oxide to give covalent adducts. In the crystal, all compounds show a bent N2O group connected via the N-atom to the former carbene carbon atom. Most adducts are stable at room temperature, but heating induces decomposition into the corresponding ureas. Kinetic experiments show that the thermal stability of the NHC-N2O adducts depends on steric as well as electronic effects. The coordination of N2O to NHCs weakens the N-N bond substantially, and facile N-N bond rupture was observed in reactions with acid or acetyl chloride. On the other hand, reaction with tritylium tetrafluoroborate resulted in a covalent modification of the terminal O-atom, and cleavage of the C-N2O bond was observed in a reaction with thionyl chloride. The coordination chemistry of IMes-N2O (IMes = 1,3-dimesitylimidazol-2-ylidene) was explored in reactions with the complexes CuOTf, Fe(OTf)2, PhSnCl3, CuCl2, and Zn(C6F5)2. Structural analyses show that IMes-N2O is able to act as a N-donor, as an O-donor, or as a chelating N,O-donor. The different coordination modes go along with pronounced electronic changes as evidenced by a bond length analysis.