The ortho-carbons in the porphyrin cations 4+ and 6+ have similar partial charges (differing up to 0.006) (Figure and Table ). However, in the [α2β2-P[H2SO4]2] 4+ complex, due to the small size of the guest molecule only one H-bond (instead of two) can be formed between the NH3 + groups of porphyrin and oxygens of each H2SO4 guest molecule. This leads to differences between the NBO partial charges of ortho-carbons of up to 0.021 for phenyls with the H-bonded and non-bonded NH3 + groups.