HOMO and LUMO energies from a 6-31G** full geometry optimization of a series of 7 cyclic hydroxamic acid derivatives were calculated. According to the LUMO energy, the molecules can be classified as hard electrophiles (E LUMO>3.0 eV) and soft electrophiles (E LUMO<2.5 eV). In general, hard electrophiles show a more important biological activity, suggesting that the active site of the biomolecule could be a hard nucleophile. This interpretation is in agreement with the electrophilic character of the hydroxamic function, as suggested by the C=O and O-H stretching frequency of vibration