Excited-State Energetics and Proton-Transfer Barriers in Malonaldehyde

The ground and first four excited electronic states of malonaldehyde are examined by ab initio methods, using a variety of basis sets including 4-31G, 6-311G**, and 6-31+G**. The lowest-energy excited state is found to be ~T-T*, followed in order by 3n-~*, In**, and IT-*. At the CIS level, the proton-transfer barriers in the excited states are all higher than the ground-state barrier; the exception is IT-* for which the barrier is lower. An inverse correlation is noted between the height of each barrier and the strength of the intramolecular H bond. Correlation, when added via second-order Maller-Plesset perturbation theory, reduces the excitation energy of each state. As is typical of the ground state, electron correlation lowers the proton-transfer barrier in the excited states as well. Their correlated barriers are all smaller than that in the ground state. Indeed, in a number of excited states, the transfer potential is likely to contain only a single symmetric minimum.

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