A reinvestigation of the molecular structure of dimethyl-N-nitramine by gas electron diffraction, ab initio calculations of the molecular geometry and the force field and vibrational spectra

Abstract

The molecular structure of dimethyl-N-nitramine was reinvestigated with gas-phase electron diffraction (ED) and ab initio calculations. Ab initio calculations using different basis sets and HF, MP2 and DFT all predict a molecule with Cs symmetry and a pyramidal amine N bond configuration. The vibrational spectra were interpreted from the scaling of the harmonic force field, and vibrational amplitudes required for the ED analysis were calculated from this scaled force field. The following values (rg bond lengths in Å and ∠α angles in degrees with errors equal to three standard deviations) were found for the main parameters: r(N–O)=1.232(3), r(N–N)=1.387(3), r(N–C)=1.466(3), r(C–H)ave=1.114(9), ∠CCN=116.1(6), ∠CNC=122.4(27), ∠ONO=127.6(12), ∠NCHave=109.9(18). The sum of the bond angles around the amine N atom is 354.6(28)°. The geometrical parameters obtained from the ED analysis are in agreement with the ab initio calculations except that a more pyramidal amine N bond configuration is predicted by ab initio.