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

The molecular structure of dimethyl-N-nitramine was reinvestigated with gas -phase electron diffraction (ED) and ab initio calculations. Ab initio calc ulations using different basis sets and HF, MP2 and DFT all predict a molec ule with C-s symmetry and a pyramidal amine N bond configuration. The vibra tional spectra were interpreted from the scaling of the harmonic force fiel d, and vibrational amplitudes required for the ED analysis were calculated from this scaled force field. The following values (r(g) bond lengths in An gstrom and angle(alpha) angles in degrees with errors equal to three standa rd 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), angle CCN = 116.1( 6), angle CNC = 122.4(27), angle ONO = 127.6(12), angle NCHave = 109.9(18). The sum of the bond angles around the amine N atom is 354.6(28)degrees. Th e geometrical parameters obtained from the ED analysis are in agreement wit h the ab initio calculations except that a more pyramidal amine N bond conf iguration is predicted by ab initio. (C) 1999 Elsevier Science Limited. All rights reserved.