Final answer:
Pi electron energy in aromatic systems is generally lower than in nonaromatic rings or open chains due to resonance stabilization and electron delocalization. This results in a smaller HOMO-LUMO energy gap and can be observed in the UV absorption characteristics of substances like 1,3-butadiene. Nonbonding orbital transitions also indicate differences in energy levels, affecting observed wavelengths.
Step-by-step explanation:
When comparing the pi electron energy in aromatic rings to nonaromatic rings or open chains, there are several key concepts to consider. First, in molecules with conjugated systems, such as 1,3-butadiene, the delocalization of pi electrons across several adjacent p-orbitals results in the energy levels of the molecular orbitals (MOs) being different than in isolated pi bonds. This delocalization leads to resonance stabilization, which lowers the potential energy of the system, resulting in a smaller energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Comparatively, 1,3-butadiene absorbs UV light at a wavelength of 217 nm, indicating a smaller HOMO-LUMO energy gap than what would be observed with a simple pi-bond as in ethene.
Moreover, the energy of nonbonding molecular orbitals (n MOs) is typically higher than the energy of bonding pi orbitals (π). Consequently, transitions involving these nonbonding electrons, referred to as n - π* transitions, occur at longer wavelengths because the energy gap is smaller than that of pi - π* transitions. This is also reflected in the absorption spectra, where n - π* transitions are generally weaker than pi - π* transitions.
In a broader context, when comparing the energy levels in molecular orbitals, bonding molecular orbitals are lower in energy due to the increased stability from bond formation. Nonaromatic cyclic compounds and open chains lack extensive delocalization, leading to generally higher pi electron energy levels compared with delocalized, conjugated, or aromatic systems.