Unshared Electron Pairs at Carbon

An unshared electron pair which occupies a compact orbital localized at a terminal carbon (e.g. an sp orbital with small r2p in eq. 3.4), causes a downfield shift of its 13C resonance. A comparison of carbon dioxide with carbon monoxide, and nitrile with cyanide anion and isocyanide [77] illustrates this  

On the other hand, odd-numbered carbons in conjugated dienyl anions are shielded [78]  

This has been explained by MO approximations in terms of a change of the charge density bond order term in eq. (3.4). 

---