In most acid-base reactions, a HOMO-LUMO combination form new HOMO and LUMO of the product. Orbitals whose shapes allow significant overlap and whose energies are similar form useful bonding and antibonding orbitals. On the other hand, if the orbital combinations have no useful overlap, no net bonding is possible and they can not form acid-base product. Even when the orbital shapes match, several reactions may be possible, depending on the relative energies. A single species can act as an oxidant, an acid, a base or a reductant, depending on the other reactant