Ground states, eclipsed

Hyperconjugation between sp3 hybridized atoms can have important implications for the ground-state conformation of organic compounds. It has, for example, been suggested that the energy difference between the staggered and the eclipsed conformations of ethane is due to both hyperconjugation and repulsion [2-5]. The fact that... 

The same authors, in a different paper (93JA3494), have shown that the N-R substituent prefers to be equatorial and that the conformation about the exocyclic N-R bond is due to a rapid equilibrium between two nearly eclipsed conformations (cf. Scheme 37b). The second ground state of the dynamic process, the own-conformation (after 180° of rotation), could not be detected experimentally but was calculated by molecular dynamics to be less stable by 5.5 kcal/mol than the preferred, vy -conformers, e.g., 99h (cf. Scheme 37). The transition state in 99b-e and 99g-l occurs when the exocyclic substituent has its maximum interaction with an equatorial methyl group about 120° on either side of the ground state. The dynamic behavior of 99h,i and 99o (R = Bu) was also studied by the MM3 force field (99CEJ449) the authors concluded that the experimental barriers of 99h,i and 99o belong to a complex combination of ring inversion, N-inversion and rotation about the exocyclic bond. 

Thioacetone exhibits a planar eclipsed-eclipsed structure in the singlet ground state, whereas it is expected to present a pyramidal gauche-gauche structure in its first triplet excited state. The deformation involves now both methyl rotations and the inversion of the molecule. So, it is a three-dimensional problem [37]. 

According to ab initio studies on 3-buten-2-ol32, it is expected that in the absence of a particular steric bias, the ground state conformations of electron-rich allylic alcohols favor a conformation in which the allylic hydrogen eclipses the double bond (model A) whereas electron-poor alkenes prefer a conformation with the carbon-oxygen bond in the plane of the double bond (model B)33. X-ray crystallographic structures further support this trend16d 101. 

The Lewis acid-mediated addition of electrophiles to allylsilanes has been extensively studied [10 c], In most cases the addition of an electrophile to an allylsilane proceeds via an anti Se process. In the ground state structure, simple allylsilanes are known to prefer the conformation wherein the allylic hydrogen eclipses the double bond. The electrophile can then approach the double bond from the same side as the allylmetal (syn Se) or from the side opposite the allyl-... 

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