Electronic spectra of nitroethylene

A systematic study of the electronic excited states of nitroethylene (C₂H₃NO₂) was carried out using the approximate coupled-cluster singles-and-doubles approach with the resolution of the identity (RI-CC2), the time dependent density functional theory with the CAMB3LYP functional (TDDFT/CAMB3LYP) and the DFT multireference configuration interaction (DFT/MRCI) method. Vertical transition energies and optical oscillator strengths were computed for a maximum of 20 singlet transitions. Semiclassical simulations of the ultraviolet (UV) spectra were performed at the RI-CC2 and DFT/MRCI levels. The main features in the UV spectrum were assigned to a weak n-π* transition, and two higher energy π_CC+O- π* bands. These characteristics are common to molecules containing NO₂ groups. Simulated spectra are in good agreement with the experimental spectrum. The energy of the bands in the DFT/MRCI simulation agrees quite well with the experiment, although it overestimates the band intensities. RI-CC2 produced intensities comparable to the experiment, but the bands were blue shifted. A strong π_CC+O-π* band, not previously measured, was found in the 8-9 eV range.