Computational comparison of the structures of the benzyl...

Computational comparison of the structures of the benzyl cation (A) and singlet phenyl carbene (B) indicate a much greater degree of double-bond character for the exocyclic bond in $\mathbf{A}$ than in $\mathbf{B}$. Provide a rationale for this difference, both in $\mathrm{VB}$ and $\mathrm{MO}$ terminology.

Key Concepts

Resonance Stabilization

Resonance stabilization explains the delocalization of charge through multiple contributing structures. In systems where conjugation allows for the dispersal of positive charge or unpaired electrons over several atoms, a structure can acquire partial double-bond character that strengthens and shortens specific bonds.

Valence Bond Theory

Valence Bond theory describes bonding in terms of localized electron pairs and resonance structures. It provides a way to rationalize differences in double-bond character by assessing the extent to which different resonance forms contribute to the overall electronic structure, indicating how delocalization of positive charge can enhance bond order.

Molecular Orbital Theory

Molecular Orbital theory explains bonding via the combination of atomic orbitals into molecular orbitals that extend over the entire molecule. It accounts for double-bond character by examining the mixing and delocalization of electrons in bonding and antibonding orbitals, and how effective orbital overlap increases the bond order through extensive conjugation.

Orbital Hybridization and Overlap

Orbital hybridization and the quality of orbital overlap play a crucial role in determining bond strength and character. A greater overlap between adjacent atomic orbitals facilitates the formation of stronger, more delocalized ? bonds, thereby imparting significant double-bond character as electrons can be distributed over several centers.

Carbene Electronic Structure

Carbene centers typically feature a divalent carbon with either singlet or triplet configurations, which influences their bonding interactions with adjacent atoms. The electronic configuration of carbenes, including the availability of empty or half-filled orbitals, affects the degree to which conjugation and double-bond characteristics can be established in neighboring bonds.

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