A CASSCF-CASPT2 study of the excited-state intramolecular proton transfer reaction in 1-amino-3-propenal using different active spaces

In this work we analyze how the choice of the active space in the CASSCF (t he complete-active-space multiconfiguration self-consistent-field method) a nd CASPT2 (the second-order perturbation theory based on the CASSCF referen ce wave function) calculations affects the computed potential energy curves (PECs) for the intramolecular proton transfer reaction in the ground state and the two lowest lying singlet excited states of 1-amino-3-propenal. As anticipated, the results revealed that, qualitatively, the proton transfer in the different states can be correctly described even by minimal active s paces, which include the orbitals involved in the electronic excitation of the considered state and the antibonding sigma orbital corresponding to the bond formed by the molecule with the migrating hydrogen atom. However, qua ntitatively, the relative energies of the two tautomers and the energy barr iers computed at the CASSCF level change when the active space is increased , indicating importance of the dynamic electron correlation. Introducing th e dynamic correlation effects via CASPT2 makes the calculated energy parame ters more uniform among the different active spaces. The analysis suggested certain optimal active spaces for studying proton transfer reactions in sy stems similar to 1-amino-3-propenal. The PEC calculations for excited state s showed that the results are sensitive to the molecular geometries used in the calculations, particularly near the transition point. (C) 1999 John Wi ley & Sons, Inc.