Theoretical characterization of intramolecular proton transfer in the ground and the lowest-lying triplet excited states of 1-amino-3-propenal: a methodological comparison

Several theoretical methods are employed to characterize the intramolecular proton transfer in the ground state and in the lowest-lying 3nπ* and 3ππ* excited states of 1-amino-3-propenal. The geometrical parameters, the relative energy of the two tautomeric forms, the energy barrier for the proton transfer, and the energy difference between the ground and the excited states predicted by the different methods are compared. It was found that: (1) the CASPT2 results are in good agreement with those obtained using the CCSD(T) method; (2) the CIS method and the CASSCF method with a medium-sized active space yield poor geometries and overestimate the adiabatic energy excitations and the energy barriers for the proton transfer; and (3) the B3LYP method provides good adiabatic excitation energies, although B3LYP energy barriers are systematically underestimated. For qualitative studies of ESIPT processes in more extended molecular systems, in which computational cost prevents the use of advanced post-Hartree–Fock methods, the CIS-MP2//CIS methodology is recommended. However, we have not found a low-cost method among the methods studied that is capable of providing a quantitative description of the proton-transfer processes. If such a description is required, we recommend the use of single-point CASPT2 calculations with a medium-sized active space performed using CIS-optimized geometries.