Proton transfer in the ground and lowest excited states of malonaldehyde: A comparative density functional and post-Hartree–Fock study

Abstract

Intramolecular proton transfer in the ground and the lowest two excited electronic states of malonaldehyde has been investigated by using density functional and post-Hartree–Fock methods. Our best estimates of the energy barriers governing proton transfer in the ground and lowest triplet state are quite low (4.3 and 6.6 kcal/mol, respectively), whereas a significantly higher barrier (12.0 kcal/mol) is obtained for the second triplet state. The coupled cluster approach provides reliable results already with relatively compact basis sets, its only drawback being the very unfavorable scaling with the number of active electrons. Among the cheaper methods, those based on the many-body perturbative approach provide good results for the ground electronic state, but their performances strongly deteriorate for excited states. The overestimation of correlation energy by conventional density functional methods produces an excessive degree of conjugation in the backbone of malonaldehyde with the consequent underestimation of energy barriers governing proton transfer. A more coherent picture is offered by a hybrid density functional/Hartree–Fock approach, which couples good structural predictions with a reduced, although still not negligible, underestimation of energy barriers. Furthermore, different electronic states are described with comparable accuracy.