Figure 1. Optimized geometry of the truncated Zn-MOF (BP86/def2-TZVPP).
The first step consisted in optimizing the Zn-MOF as well as the interacting system Zn-MOF-analyte with the ORCA package.40 All structures were optimized via the gradient-corrected Becke–Perdew (BP86)41exchange-correlation functional using the triple-zeta valence with two sets of polarization functions (def2-TZVPP) 42 basis set. In the second stage of this work, we performed a study of the optical properties of the Zn-MOF and Zn-MOF-analyte systems by means of the Time-dependent Density Functional Theory (TD-DFT) approach. To simulate the vertical transitions, 50 excitations were computed at the hybrid exchange-correlation functional (B3LYP) 43 as well as the Coulomb Attenuated Method (functional, CAM-B3LYP)44 and def2-TZVPP basis set. Finally, we analyzed the nature of the interaction Zn-MOF-analyte based on the energy decomposition analysis (EDA) proposed by Morokuma–Ziegler.45 The interaction energy (ΔEint) between two defined fragments according to the EDA scheme can be divided into four components:
ΔEint = ΔEElec + ΔEPauli+ ΔEOrb + ΔEDisp (1)
Where ΔEElec accounts for the classical electrostatic interaction between the fragments as they are brought to their positions in the final structure. The second term ΔEPauli is related to the repulsive interaction of Pauli, between occupied orbitals of both molecular fragments. The third term ΔEOrbexpresses the possible interactions between molecular orbitals related to the charge transfer, polarization, etc. This term can be analyzed by the natural orbital of chemical valence method proposed by Mitoraj.46-47 The term ΔEDispdescribes the dispersion forces acting between the fragments. We consider the long-range interactions, using Grimme’s D3 dispersion correction for EDA calculations.48