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 (3) pi-pi*, followed in order by (3)n-pi*, (1)n=pi*, and (1) pi-pi*. At the CIS level, the proton-transfer barriers in the excited states are all higher than the ground-state barrier; the exception is (1) pi-pi* 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 Moller-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.