A combined DFT, MP2 and CASSCF study on the properties of Tropolone-(H2O)n (n = 0-2) complexes

The S0 and S1 structures, excitation energies, intramolecular proton transfer reactions of Tropolone (TRN) and its complexes with water were investigated by DFT, TD-DFT, CASSCF and MP2 methods. Structure analysis indicates that the S1 structure of TRN monomer is nonplanar, which is consistent with the experimental results. The role of water acting as a bridge between tautomers was also discussed. Orbital analysis concludes that the S1 states of TRN-(H2O)n (n = 0-2) complexes come from π → π* transitions. The adiabatic excitation energies of TRN, TRN-H2O, TRN-(H2O)2 are 74.71, 79.26 and 77.43 kcal/mol at CASSCF(10, 8)/cc-pVDZ level, which agree with the available experimental values, and at TD-DFT/6-31g(d, p) level the vertical excitation energies are 88.61, 82.59 and 88.37 kcal/mol, respectively. The proton transfer energy profile indicates that the water complexing to TRN leads to the proton transfer more difficult.