Experimental and theoretical investigation of the molecular and electronic structure of 3-ethoxy-4-isopropylaminocyclobut-3-ene-1,2-dione

The title compound, 3-ethoxy-4-isopropylaminocyclobut-3-ene-1,2-dione (EIAC) has been synthesized and characterized by NMR, FT-IR, UV–vis spectroscopy and single-crystal X-ray diffraction. The 1H NMR spectra were recorded at 300 K and 315 K in CDCl3 to determine syn/anti conformers of the compound EIAC. Density functional theory (DFT) calculations, optimized geometrical parameters, vibrational frequencies and chemical shift values of syn/anti conformer in CDCl3 have been performed at B3LYP/6-311G(d) level, and compared with the experimental data. The values provided with the calculations support the experimental data of the compound EIAC. The presence of NH⋯O type intermolecular H bond can be perceived from the difference between experimental calculations and results of FT-IR and NMR calculations. In addition, B3LYP/6-311G(d) basis set has been used to calculate the molecular electrostatic potential, frontier molecular orbitals and electronic absorption spectra. HOMO–LUMO electronic transition of 5.12 eV is derived from the contribution of the bands n → σ* or π → π*. FT-IR, NMR and X-ray spectral results and additionally DFT calculations exhibit that the compound EIAC exists in keto-enamine tautomeric form. The experimental 1H NMR spectra recorded at 300 K and 315 K and theoretical 1H NMR data indicate that the compound EIAC is in syn conformer.

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► The compound 3-ethoxy-4-isopropylaminocyclobut-3-ene-1,2-dione (EIAC) was synthesized.
► The compound was characterized by NMR, FT-IR, UV–vis and X-ray crystallography.
► The 1H NMR was recorded at 300 K and 315 K to determine syn/anti forms of EIAC.
► The theoretical calculations were performed by DFT method.
► The compound exists in keto-enamine tautomeric form and syn conformer.