Comprehensive spectroscopic (FT-IR, FT-Raman, 1H and 13C NMR) identification and computational studies on 1-acetyl-1H-indole-2,3-dione

Fourier transform infrared (FT-IR) and FT-Raman spectra of 1-acetyl-1H-indole-2,3-dione (N-acetylisatin) were recorded in the solid phase and analyzed. The molecular geometry, vibrational frequencies, infrared intensities, Raman activities and atomic charges were calculated using density functional theory (DFT/B3LYP) calculations with a standard 6-311++G(d,p) basis set. The fundamental vibrational modes of N-acetylisatin were analyzed and fully assigned with the aid of the recorded FT-IR and FT-Raman spectra. The simulated FT-IR and FT-Raman spectra showed good agreement with the experimental spectra. The stability of the molecule, arising from hyper-conjugative interactions and charge delocalization, was analyzed using natural bond orbital (NBO) analysis. The dipole moment (µ), polarization (α) and hyperpolarization (β) values of N-acetylisatin were also computed. The potential energy distribution (PED) was computed for the assignment of unambiguous vibrational fundamental modes. The HOMO and LUMO energy gap illustrated the chemical activity of N-acetylisatin. The energy and oscillator strength were calculated by DFT. Gauge–including atomic orbital NMR (1H and 13C) chemical shift calculations were performed and compared with the experimental values. Thermodynamic properties (heat capacity, entropy and enthalpy) of the compound at different temperatures were also calculated.