Electronic Spectroscopy of Maleimide and Its Isoelectronic Molecules. I. Maleimide and N-Alkylmaleimides

The electronic absorption spectra of maleimide and two N‐alkylmaleimides are presented. Molecular‐orbital classifications of four electronic transitions with energies $\text{〈 6.5\hspace{0.17em}}\mathrm{eV}$ are made for maleimide. Vibrational structure appearing in two of the transitions in the N‐alkylmaleimides in the vapor phase is interpreted and assignments of the active vibrational modes discussed. A series of hot bands derived from the 2000‐Å band of N‐methylmaleimide is analyzed and employed in the association of the corresponding active ground‐ and excited‐state vibrational modes. The vacuum‐ultraviolet vapor absorption spectrum of N‐methylmaleimide is presented. Phosphorescence emissions from maleimides in glassy media at 77°K suggest that the T₁ states of these molecules are ${}^3{\Gamma }_{{\mathrm{n\; \pi }}^{*}}$ .