Assuming 2s-2p mixing to be non-operative, which of the following specie(s) can undergo transition from gerade M.O. to ungerade M.O. on absorbing radiation.

Question

TestHub · Accepted Answer

Without 2s-2p mixing, the molecular orbital energy ordering for second-period homonuclear species is:

$σ_{1 s} < σ_{1 s}^{*} < σ_{2 s} < σ_{2 s}^{*} < σ_{2 p_{z}} < π_{2 p_{x}} = π_{2 p_{y}} < π_{2 p_{x}}^{*} = π_{2 p_{y}}^{*} < σ_{2 p_{z}}^{*}$

Orbital Symmetries: Bonding $σ_{2 p_{z}}$ is gerade ( $g$ ), while bonding $π_{2 p}$ is ungerade ( $u$ ).

Configuration of C₂²⁺ (10 electrons): $σ_{1 s}^{2} σ_{1 s}^{* 2} σ_{2 s}^{2} σ_{2 s}^{* 2} σ_{2 p_{z}}^{2}$ .

Transition: The Highest Occupied Molecular Orbital (HOMO) is $σ_{2 p_{z}}$ ( $g$ ) and the Lowest Unoccupied Molecular Orbital (LUMO) is $π_{2 p}$ ( $u$ ).

Thus, C₂²⁺ undergoes a gerade to ungerade ( $g \to u$ ) electronic transition upon absorbing radiation.

Correct Option: (B)

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