The electronic excited states of maleimide have been studied using multiconfigurational second-order perturbation theory in its multistate formulation (MS-CASPT2) and extended atomic natural orbital (ANO) basis sets. The calculation of the singlet-singlet and singlet-triplet transition energies comprises 10 valence singlet excited states, four valence triplet states,a nd the singlet 3s, 3p, and 3d members of the Rydberg series converging to the first four ionization limits. The main features of the absorption spectrum correspond to the 1~1A_1 →1~1A_2(nπ~*), 1~1A_1 → 1~1B_2(ππ~*), and 1~1A_1 → 2~1B_2(ππ~*) transitions computed at 3.29, 4.44, and 5.59 eV, respectively. The latter corresponds to the most intense band. The lowest Rydberg state is found to be at 5.98 eV in the high-energy side of the main band. A number of additional features are predicted in the higher-energy region of the spectrum, and our assignments follow the same trends as those offered earlier by Robin for N-methyl maleimide. The lowest triplet state is computed of nπ~* character in vacuo. However, arguments are given to rationalize the observed phosphorescence of maleimide in solution as being mainly of ππ~* nature.
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