Lone pairs hyperconjugation

The interaction of the lone-pair electrons on an amine nitrogen with adjacent C—H bonds is another example of a hyperconjugative effect that can be described in MO language. The lone-pair electrons, when properly aligned with the C—H bond, lead to a... 

The three highest occupied orbitals of sulfoxides are the lone pairs ns and n0, as well as the 7iso bond210. The 1,3-dithietane 1-oxide adds a lone-pair ionization and destabilizes the n0 and nso radical-cation states compared with thietane oxide. According to a hyperconjugative MO model, the ns+ combination in 1,3-dithietane is destabilized by about leV relative to the basis orbital energy a(ns) due to the combination with the... 

The barriers for the rotation around the SS bond have been estimated from microwave data [40] and ab initio MO calculations [41, 42] as 34-36 kj mor (cis-barrier) and 24-25 kJ mol (trans-barrier), respectively. They are explained as the result of three effects First, the repulsion of the two 3p lone pairs at the sulfur atoms is lowest for r=90°. Second, the 3p lone pair electrons are partly delocalized into the antibonding a orbitals of the SH bonds originating from the neighboring sulfur atom. This hyperconjugation or 7i bond is at its maximum for r=90° but disappears for r=0° and 180°. Third,... 

Cieplak stated that the lone pair back-donation of C5 substituent also stabilizes the transition state during the syn approach since it improves hyperconjugation of the C5-H bond (extended hyperconjugation). He pointed out that for the cyclopen-tadienes having C5 substituents such as hydroxy, methoxy, amino, and chloro moiety, the hyperconjugative stabilization and the back-donation work in the same direction (Scheme 27). 

Cieplak apphed this effect to the explanation of the shift of selectivity in the reactions of the pentamethylcyclopentadienes 21,19, and 20 having substiments of formyl, vinyl, and (hydroxyimino)methyl moiety at the 5-positions [14]. He pointed out that the observed result is consistent with the notion that in the case of formyl moiety the hyperconjugative effect is enhanced by lone pair back-donation due to ... 

In this section we will describe some PES investigations on Group XIV element derivatives in which one or more sulphur or selenium atoms are bonded directly or indirectly to the metal120-132. The aspects investigated preferentially for these molecules were the electronic and conformational situation, the role played in the bonding by the chalcogen lone-pair orbitals and possible hyperconjugative interaction (n-cr or p -d ). 

The effectiveness of equatorial vs. axial bystanders at promoting the 1,2-H shift (MeO > Me > Ph) may be related to their ability to stabilize the partial positive charge that arises at the migration origin during the 1,2-H shift. In this scenario, the lone pairs of the MeO group are superior to the hyperconjugative and inductive properties of Me, whereas the conformationally dependent h-electron release of Ph is the least effective. 

Hyperconjugative interactions (Section 3.4.2) are another factor leading to bond bending. Such interactions are associated with the stereoelectronic influence of more remote bonds or lone pairs, particularly those anti (trans) to the bond of... 

From Table 3.22 one can see that typical hyperconjugative interactions of vinyl pi bonds with hydride bonds are rather weak (2 1 kcal mol-1), but those with lone pairs are considerably stronger (7-30 kcal mol-1). (Of course, a more polar pi bond... 

A particularly dramatic effect of hyperconjugation on the form of the ethylenic double bond NBOs is shown in Fig. 3.50. This figure depicts one of the two equivalent C=C bond and antibond NBOs of vinylamine, twisted slightly out of coplanarity with the nitrogen lone pair with dihedral

[Pg.219]

Figure 3.63 illustrates the gauche effect for vicinal lone pairs and polar C—F bonds with the examples of (a) hydrazine and (b) 1,2-difluoroethane, respectively. As seen in Fig. 3.63(a), the

lone pairs are anti to one another (thus squandering their powerful donor strength on vicinal moieties with no acceptor capacity) is disfavored by 3.2 kcal mol-1 relative to the preferred = 93.9° conformer in which each nN hyperconjugates effectively with... 

Stereoelectronic effects in chemical reactivity The bond-lengthening and -weakening influence of an antiperiplanar lone pair leads to strong stereoelectronic effects on chemical reactivity.97 In molecule 28a with lone-pair-bearing atom D adjacent to an A—B bond, a vicinal nD—s-cab hyperconjugative interaction can be associated (cf. Example 1.4 and Section 3.3.1) with a partial admixture of the alternative resonance structure 28b,... 

Finally, it is worth mentioning that 15N-NMR spectra of yohimbine (74), reserpine (109), and isoreserpine (514) also have been investigated (319). The 15N chemical-shift differences between C/D-cis- and trans-fused compounds could be explained by a hyperconjugative interaction between the antiperiplanar C—H bonds and the nitrogen lone pair characteristic for trans-fused compounds only. 

---