Electronic Spectroscopy of Maleimide and Its Isoelectronic Molecules. III. Quantum Chemical Calculations

The electronic transitions of energy <6.5 eV observed in maleic anhydride, maleimide, and 4‐cyclopentene‐3, 5‐dione are assigned using all‐valence‐electron computational results and experimental data. In the one case where an experimental ground state dipole moment is available (i.e., maleic anhydride: 3.94 D), the calculated moment of 3.81 D provides excellent agreement. The solvent‐polarity sensitivity and chemical substitution effects observed for several of the lower‐energy transitions are discussed in terms of calculated ground‐state and excited‐state electron densities. The T₁ states of all three isoelectronic molecules are tentatively assigned as ${\text{1\hspace{0.17em}}}^3{B}_2{\mathrm{(n\pi }}^{*})$ states.