Limitations of Frontier Orbital and Charges Approaches in the Description of Electrophilic Aromatic Substitution

DFT calculations at B3LYP/aug-cc-pVDZ level of theory on some aromatic substrates showed that in the HOMO of nitrobenzene the atomic coefficients are not in agreement with the meta directing behavior of this compound. The atomic coefficients are the same in ortho and in meta position. The HOMO (or NHOMO in the case of benzaldehyde) is not in agreement with the experimental results when deactivating, meta orienting compounds are considered. Mulliken charges sometimes are not able to explain the observed reactivity. Hirshfeld charges allow to predict the orientation of the attack of an electrophile on the aromatic ring with the exception of nitrobenzene. Both HOMO atomic coefficients and charges are in agreement with the experimental results when deactivating, ortho-para orienting and activating compounds are tested.