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Lecture 14 - Nitrenes and Carbenes

If a small energy dierence between the and p orbitals

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Unformatted text preview: the species is called singlet, if the two electrons are unpaired the species is called triplet carbene. Triplet carbenes can be observed by ESR (electron spin resonance). F. Sladojevich Definition and Structure RelaKve Stability of Singlet and Triplet Carbenes: Due to electron repulsion, there is an energy cost in pairing both electrons in the orbital. If a small energy difference between the σ and p orbitals exists, the electrons will remain unpaired (triplet). If a large gap exists between the σ and p orbitals the electrons will pair in the σ orbital (singlet). Chem 106 SubsKtuents effect on the Singlet- Triplet state: let’s compare the singlet/triplet energies for methylene and difluorocarbene: For most triplet carbenes the singlet spin state that would arise by pairing up the two electrons lies only about 9.5 kcal/mol above the ground (triplet) state. QuesKon: how do you explain the difference in bond angles? F. Sladojevich Definition and Structure When the H’s in methylene are replaced with different ligands, the difference between σ and pπ orbital change: •  σ- EWG subs5tuents favor the singlet state •  σ- EDG subs5tuents favor the triplet state •  π- EDG subs5tuents favor the singlet state (- F, - Cl, - Br, - I, - NR2, - OR, - SR etc. •  π- EWG subs5tuents favor the singlet state (COR, CN, BR2, etc. See Bertrand, Chem. Rev. 2000, 100, 39- 91 Let’s consider the case π- EDG subs5tuents (- X). The orbital diagrams is reported below: Chem 106 In this case the interac5on with the lone pairs of the subs5tuent increases the energy of the pπ orbital, while the σ energy is not significantly affected. As a result the energy gap σ- pπ increases and the singlet state is favored. This delocaliza5on can be described in terms of resonanc...
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