ECD spectra are sensitive to the molecular geometry making the determination of the lowest energy conformers and the averaging of the final spectra essential. However, not all the geometry changes are significant. For example, in porphyrin complexes, the position and conformation of a guest, as well as the degree of macrocycle nonplanarity play a major role in the ECD spectra simulations. The position of aromatic or peripheral chromophoric substituents can also affect the simulated spectra, but their influence strongly depends on their relative position. The position of non-chromophoric peripheral substituents, such as alkyl groups, is considered unimportant, since these substituents usually do not have electron density in their frontier molecular orbitals resulting in a minor influence on the electronic structure. For example, ethyl groups are often ignored, or are replaced with methyl groups or hydrogen atoms for simplicity. However, it was also reported that in some cases Et groups may have some influence on ECD spectra. Position of guests and influence on ECD spectra. Based on the crystallographic data we concluded that one porphyrin cation binds two CSA molecules in each of its cavities. Using this ratio and the P[SO4 2-][HSO4 -]4 crystal data the corresponding host-guest complexes were built with a subsequent conformational search. First, a conformational search was performed "manually", as we decided that the limited size of the cavity and H-bond formation between the host and guest molecules would significantly simplify the conformational landscape and limit the number of low-energy conformations. By manual building potentially low-energy conformations (containing the maximum number of H-bonds) we hoped to circumvent more time consuming and expensive conformational analysis. The conformational search was performed for each of two solvents (chloroform and dichloromethane) using BP86-D3BJ/def2-SVP and the SMD solvent model implemented in Gaussian 16. Although the two porphyrin cavities are comparable in size, they differ in the position of NH3 + groups. In one cavity the ammonia groups are placed closer to each other with a distance of ~4.9 Å [between NH3 + (N3) and NH3 + (N4)], and in another cavity the distance between the NH3 + groups is ~8.7 Å [between NH3 + (N1) and NH3 + (N2)] (see Figure ). This determines the position of the (R)-CSA molecules and the bonding mode with the host. In all lowest-energy conformers, despite the different positions of the guests, both (R)-CSA molecules form four Hbonds with the porphyrin cation: one or two with core protons and up to three with the NH3 + groups of the porphyrin cation (Figures 4 and Table ). It should be noted that the H-bond lengths formed between the host and guests are very similar in dichloromethane and chloroform, as well as in acetonitrile with differences of only up to 0.05 Å (Table ).