Identifying the most energetic electrons in a molecule: The highest occupied molecular orbital and the average local ionization energy

The site of the greatest electronic density of the highest occupied molecular orbital (HOMO) is often taken to be the location of the most energetic electrons in a molecule, and thus the most likely site for reaction with an electrophile. However we show, by reference to the average local ionization energy on a molecular surface, that the HOMO alone often does not locate the most energetic electrons. This is because the HOMO, unlike the average local ionization energy, does not take explicit account of the fact that any molecular site has a significant probability of being occupied by electrons in lower-lying, less energetic molecular orbitals.

Automated summary

The site of the highest occupied molecular orbital (HOMO) may not always locate the most energetic electrons in a molecule due to the fact that it does not explicitly consider the probability of being occupied by electrons in lower-lying molecular orbitals.