Theoretical Studies on Ag-Based Homolytic Bond Dissociation Energies

The breaking and formation of the Ag-X (X = C, H, O, S) bonds are vital to many Ag-catalyzed/mediated reactions, such as CO₂ fixation and C-H activation. Adequate description of the bond dissociation energies (BDEs(Ag-X)) would not only help us to understand the nature of these Ag-X bonds, but also provide important instructions on rational design of Ag-based catalysts/additives. To achieve this goal, 41 diverse DFT functionals (e.g. B3LYP, B2PLYP, M06-2X, PBE, TPSSH) and two ab initio methods (MP2 and CCSD(T)) with different basis sets were assessed to benchmark against 6 homolytic BDEs (i.e. BDE(Ag-H), BDE(Ag-F), BDE(Ag-O), BDE(Ag-S), BDE(Ag-CH₃) and BDE(Ag-OH)) derived from experiments and/or CBS limit standards. Considering both the accuracy and computational expense, the theoretical protocol BLYP with the basis set cc-pVTZ-PP for Ag and may-cc-pVTZ for other atoms was found superior, with a precision of 1.2 kcal/mol. According to the benchmarks, the common methods B3LYP and M06-2X were found to underestimate the Ag-X BDEs while PBE overvalues them; higher-level basis sets could increase the accuracy. Further, with the aid of the calibrated theoretical methods, the ligand effect (L_nAg-X, L=H₂O, CO, CO₂ and NH₃ (n=1-5)) and the substituent effect (Ag-CH₂R, R=H, OH, SH, CH₃, Cl, etc) were systematically investigated. Note that after ligation with simple groups the Ag-X bonds became stronger compared to bare Ag-X bonds. Interestingly, for the ligands H₂O and CO, the Ag-X BDEs were lowest when n=3.