The highest occupied molecular orbital of a molecule isabbreviated as the HOMO. The lowest unoccupied molecularorbital in a molecule is called the LUMO. Experimentally,one can measure the difference in energy betweenthe HOMO and LUMO by taking the electronic absorption(UV-visible) spectrum of the molecule. Peaks in theelectronic absorption spectrum can be labeled as π 2p-π 2p*,σ2s-σ2s*, and so on, corresponding to electrons being promotedfrom one orbital to another. The HOMO-LUMO transitioncorresponds to molecules going from their groundstate to their first excited state. (a) Write out the molecularorbital valence electron configurations for the ground stateand first excited state for N2. (b) Is N2 paramagnetic or diamagneticin its first excited state? (c) The electronic absorptionspectrum of the N2 molecule has the lowest energypeak at 170 nm. To what orbital transition does this correspond?(d) Calculate the energy of the HOMO-LUMO transitionin part (a) in terms of kJ/mol. (e) Is the N—N bond inthe first excited state stronger or weaker compared to that inthe ground state?
The highest occupied molecular orbital of a molecule is
abbreviated as the HOMO. The lowest unoccupied molecular
orbital in a molecule is called the LUMO. Experimentally,
one can measure the difference in energy between
the HOMO and LUMO by taking the electronic absorption
(UV-visible) spectrum of the molecule. Peaks in the
electronic absorption spectrum can be labeled as π 2p-π 2p*,
σ2s-σ2s*, and so on, corresponding to electrons being promoted
from one orbital to another. The HOMO-LUMO transition
corresponds to molecules going from their ground
state to their first excited state. (a) Write out the molecular
orbital valence electron configurations for the ground state
and first excited state for N2. (b) Is N2 paramagnetic or diamagnetic
in its first excited state? (c) The electronic absorption
spectrum of the N2 molecule has the lowest energy
peak at 170 nm. To what orbital transition does this correspond?
(d) Calculate the energy of the HOMO-LUMO transition
in part (a) in terms of kJ/mol. (e) Is the N—N bond in
the first excited state stronger or weaker compared to that in
the ground state?
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