Position of peripheral substituents and influence on ECD spectra. It is known that the ECD spectra are sensitive to the position of peripheral aromatic substituents. However, in the present case formation of H-bonds between the guest molecules and porphyrin prevent rotation of the aryl groups. As a result, the aryl positions change insignificantly (Figure ), and do not affect the ECD spectra. In contrast, the orientation of Et groups differs from that found in the P[SO4 2-][HSO4 -]4 crystal structure. In the complex studied the Et groups are placed in such a way as to reduce corresponding steric interaction with the guests (Figure ). The calculated rotational barrier is found to be 6.10 kcal . mol -1 (Figure ), hence indicating the possibility of free rotation at room temperature. The flexibility of β-alkyl conformations in nonplanar porphyrins has been noted frequently in structural studies. Figure . Orientation of the ethyl groups relative to the porphyrin plane in the P[SO4 2-][HSO4 -]4 crystal structure and in the lowest energy conformer C porphyrin complex with two (R)-CSA guests.