Results and discussion
The geometry and optimization of 1,3-phenylene-bridged cyclic pyrrole ( MPBCP)
The geometry of the meta-phenylene-bridged cyclic pyrrole (MPBCP) structure was optimized using PBE0-D3/def2TZVP with aC3v point group, adopting a non-planar geometry due to the presence of hydrogen atoms located in the cavity. The distance between the two N atoms in the middle of the pore is 4.73 Å and the pore diameter was measured as 3.84 Å. The C-N bond length of 1.37 Å is in agreement with similar bonds C-N in previous X-ray studies (1.38 Å). 37 The optimized structure of MPBCP is depicted in Figure 1. The adsorption spectrum of MPBCP structure showed a band at 369 nm, slightly high compared to the experimental adsorption of ortho-phenylene-bridged cyclic pyrrole (332 nm). 37
The high density of N atoms leads to the different interactions between gas molecules, due to, the adsorption of biogas molecules was studied (CO2, H2, N2 and CH4). Our results showed weak interactions between the gas and the MPBCP structure. For the case of the CO2 and N2 molecules, the most stable configurations adopted are a non-planar structure with Cs (CO2) and C3v point group for N2. Oxygen and nitrogen atoms, of both molecules, are coordinated to the hydrogens of sheet with 2.32 and 2.55 Å, respectively. These length values of coordination bonds were consistent with low Eads values -7.59 and -4.62 kcal/mol, see Figure 2. On the other hand, the adsorption of CH4and H2 molecules exhibits adsorption energies of H2, -1.97 and CH4, -5.61 kcal/mol. The NBO analysis shows the CO2 and N2, transfers charge to the nitrogen atoms with hyperconjugative interaction energies E (2) of nO→ σN-H = 2.01 kcal/mol, nN→ σN-H = 1.75 kcal/mol.
To increase the coordination with the biogas molecules, MPBCP was coordinated using two strategies: a) placing a lanthanum atom in the central cavity with a covalent interaction with the nitrogen atoms. b) maintaining the interaction of the lanthanum atom with the MPBCP structure, through coordination with a total charge of +3.