A theoretical study of the vibrational spectrum of maleimide

B3LYP and MP2 theoretical calculations with 6-31++G(d,p) basis set have been performed to interpret the infrared spectrum of maleimide. The anharmonicity effect has been included to correct the calculated harmonic frequency values. As expected, this has allowed a better agreement with the experimental ones, especially B3LYP values. The more pronounced changes have been verified to the XH (X = C or N) stretching modes.The calculated IR intensities compare favorably well with their corresponding relative experimental values in vapor phase. The large discrepancy is found to the symmetric CC stretching mode of the heterocyclic ring. It appears to be a strong band at 897 cm−1, in contrast to B3LYP and MP2 calculated values. The calculated intensities due to the out-of-plane and in-plane bending and stretching modes for the XH oscillators (X = C or N) have been adequately interpreted in terms of hydrogen atomic charges and XH charge-fluxes extracted from the modified charge–charge flux-overlap model (CCFOM) for infrared intensities.