Figure 2 . DFT adsorption structures. a) Top and side views of gas-MPBCP system. b) NBO donor and acceptor orbitals responsible for intermolecular interactions. The adsorption energy (Eads ) are expressed in kcal/mol calculated with the PBE0-D3/def2-TZVP method. Color code for spheres: dark gray (C); blue (N); yellow (H); red (O).