Hyperconjugation, lone pair orbital effects

Hyperconjugation, lone pair orbital effects, 101 Hyperthyroidism, endogenous peroxide determination, 667... 

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

The small stabilization is attributable to hyperconjugation with the o- and o -orbitals formed between the phosphorus and its substituent, and is therefore of hyper-conjugative nature. This was recognized from early photoelectron spectroscopic investigations [167], but since the extent of the effect is small, the literature mainly contains cautious statements about its nature. The interaction between the n-system, the sPH-orbitals, and the phosphorus lone pair can clearly be seen in Fig. 3. 

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