Excited state intramolecular proton transfer in 1-(trifluoroacetylamino)naphthaquinone: a CASPT2//CASSCF computational study

The excited state intramolecular proton transfer (ESIPT) in 1-( trifluoroacetylamino)-naphthaquinone (TFNQ) has been investigated using the CASSCF and CASPT2 computational approaches with the 6-31G( d) basis set. The structures and relative energies of critical points along the proton transfer reaction coordinate were optimized and the associated spectroscopic and electrostatic properties obtained. Combined quantum mechanical and molecular mechanical (QM/MM) Monte Carlo simulations were performed to elucidate solvent effects on the vertical excitation S-0 -> S-1. It was found that the ESIPT reaction is a barrierless process that takes place on a very flat potential energy surface ( PES) and the tautomeric structure of the reaction product is the only minimum on the excited state surface. The PES for both the ground and excited state from accurate electronic structure calcualtions will be used to parameterize empirical force fields in subsequent molecular dynamics simulations of the reaction in solution.