Structure and vibrational frequencies of 6,7-dimethoxy-1,4-dihydro-1,3-quinoxalinedione based on density functional theory calculations: The role of π-electron conjugation and back-donation

This work deals with the vibrational spectroscopy of 6,7-dimethoxy-1,4-dihydro-1,3-quinoxalinedione by means of quantum chemical calculations. The mid and far FT-IR and FT-Raman spectra are recorded in the condensed State. The fundamental vibrational frequencies and intensity of vibrational bands are evaluated using density functional theory (DFT) with the standard B3LYP/6-31G* method and basis set combination and is scaled using various scale factors which yields a good agreement between observed and calculated frequencies. The vibrational spectrum is interpreted with the aid of normal coordinate analysis based on scaled density functional force field. The results of the calculations are applied to simulate infrared and Raman spectra of the title compounds, which showed excellent agreement with the observed spectra. The infrared unscaled frequencies and intensities are used to disentangle the role played by back-donation in the title compound. For this purpose five other molecules are considered as references: ethane, dimethyl ether, anisole, p-nitro-anisole, and p-hydroxyanisole, in which back-donation has already been ascertained also experimentally. From the study of infrared intensities it is shown that no back-donation of electrons from the oxygen lone pairs takes place, independently of the conformation of the methoxy-group.