Structure and vibrational spectra of the salts of mononitroalkanes

The molecular geometries of the anions of nitromethane and 2-nitropropane were optimised and their harmonic force fields were calculated by the RHF/6-311G(d,p), MP2/6-311G(d,p) and B3LYP/6-311G(d,p) methods. The force fields obtained made it possible to reliably interpret the IR and Raman spectra of the Na⁺ salt of nitromethane, d₂-nitromethane and 2-nitropropane. The assignment proposed significantly improves the interpretation of vibrational spectra known so far. Some general conclusions on geometry and vibrational spectra of the salts of mononitroalkanes studied are made. The hybrid density functional method used (B3LYP) is shown to be in better agreement with experimental data available than the Hartree–Fock methods.