[r4mbini]

I am working my way through Ian Fleming's 'Molecular Orbtials and Organic Chemical Reactions' and I am having trouble with one of the exercise questions. (Ch2 Q1)

It reads: Given that two pi bonds conjugated together have a lower energy than two separate pi bonds and a C-H bond conjugated with a pi bond also lowers the energy, explain why two C-H bonds conjugated to each other are not stabilised.

http://i.imgur.com/5dUZG.jpg is my attempt at some form of MO diagram.

Any help is appreciated, thanks.

[souro10]

Why does conjugation of alternate pi-single-pi bonds lead to stability?
Due to sidewise overlap of the pi orbitals which result in a new kind of more stable MO to be formed.

Is such an overlap possible with sigma bonds? Sigma bonds are formed by head-on overlap of orbitals. How can any two sigma bonds on different parts of a molecule interact with each other? Try visualizing the situation with some software/ molecular modelling kit.

[r4mbini]

so the interaction between a C-H sigma bond and an empty p orbital in hyperconjugation is on the limit of how far away the orbitals will have to be to interact in any meaningful way?