IR spectra, crystal structure, dipole moment, ab initio and AM1 study of 1,2-dinitroethane

Abstract

IR spectra of 1,2-dinitroethane in the solid, solution and vapour phases, together with Raman measurements of the compound in the solid, liquid and solution states are reported and assignment of frequencies made. Vibrational frequencies of the gauche and trans rotamers of the molecule have also been calculated at the AM1, HF/3-21G*, HF/6-31G*, MP2/3-21G* and MP2/6-31G* levels and the results are compared with the experimental frequencies. Examination of the IR and Raman spectra suggests that the compound exists in the polar gauche conformation in the solid state and as a mixture of the gauche and trans rotamers in solution. The dipole moment of the compound has also been measured in benzene over a range of temperatures. Analysis of the relative permittivity data shows that at 25°C in solution, the compound exists approximately in the ratio 92% gauche to 8% trans. The experimentally derived values of the energy difference between the gauche and trans rotamers, the dihedral angle of the gauche rotamer and the gauche/trans population quotient are compared with values predicted by ab initio methods and the semiempirical MO program AMPAC using AM1 parametrization. The crystal and molecular structure of 1,2-dinitroethane has also been determined by single-crystal X-ray diffraction methods. The molecule crystallizes in the tetragonal space group /41/a, a = 13.305(1) Å, c = 11.121(3) Å, V = 1968.8(6) Å3, Z =16, Mo-Kα radiation, λ = 0.710 69 Å, μ = 0.148 mm-1. The structure was refined to a R factor of 0.033. The molecule adopts a gauche conformation with a N-C-C-N torsion angle of 73.5(2)°. The central C-C bond of the molecule is markedly shorter than a normal C-C bond. Each NO2 group is coplanar with the N-C-C plane containing the central carbon atoms.