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 Igor F. Shishkov a , Lyudmila V. Khristenko a , Victor A. Sipachev a , Lev V. Vilkov a , Svein Samdal b, * , Snefrid Gundersen b , Mauricio A. Palafox c a Department of Chemistry, Moscow State University, Moscow 119899, Russia b Department of Chemistry, University of Oslo, PO Box 1033 Blindern, N-0315 Oslo, Norway c Departamento de Quimica-Fisica I (Espectroscopia), Facultad de Ciencias Quimicas, Universidad Complutense, Madrid 28040-ES, Spain Received 14 September 1998; received in revised form 5 December 1998; accepted 5 December 1998 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 C s 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 (r g bond lengths in A ˚ and  a 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), NCH ave  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.  1999 Elsevier Science Limited. All rights reserved. Keywords: Dimethyl-N-nitramine; Electron diffraction; Vibrational spectra 1. Introduction Dimethyl-N-nitramine, (CH 3 ) 2 NNO 2 , (DMNA) and nitramide, H 2 NNO 2 , (NA) are the two basic molecules for the nitramines. These two molecules have previous been studied both experimentally and theo- retically. The results from microwave spectroscopy [1] and ab initio calculations [2] for NA agree on that the amine N atom has a pyramidal bond config- uration. However, a previous electron diffraction (ED) investigation on DMNA [3] concluded that there is no significant deviation from C 2v with a planar skeleton. This is not in agreement with the results of the ab initio calculations at the HF/6-31** and MP2/6- 31** level [4], where a pyramidal bond configuration is predicted. Therefore, we wanted to reinvestigate the ED data for DMNA and include results from the ab initio calculations in order to determine the amine N Journal of Molecular Structure 485–486 (1999) 153–161 0022-2860/99/$ - see front matter  1999 Elsevier Science Limited. All rights reserved. PII: S0022-2860(98)00847-3 * Corresponding author. Tel.: +47-22-855-458; fax: +47-22- 855-402. E-mail address: svein.samdal@kjemi.uio.no (S. Samdal)