Abinitio Calculations of Structural Features Not Easily Amenable to Experiment .74. Conformational-Analysis of Cyclotrimethylene-Trinitramine (Rdx)

by Coffin, James Michael; Newton, Susan Quinlyn; Ewbank, John David; Schäfer, Lothar; Van Alsenoy, Christian; Siam, Khamis S.

The structures of four forms of cyclotrimethylene-trinitramine (CH2NNO2)3 (RDX) were determined by SCF/4-21G ab initio gradient optimization. The forms considered are the chair forms AAA, AAE, AEE and EEE, in which the NO2 groups are either in the axial (A) position or in the equatorial (E) position, except for the EEE form which is nearly planar. The form AAA was identified as the most stable conformation, in agreement with gas phase and solution studies of the compound. Calculations of vibrational frequencies yielded imaginary values for all forms with equatorial NO2 groups. Thus, the latter are not energy minima in SCF/4-21 G space. Even though the 4-21G calculations are rather approximate for compounds of this type, they make it possible to describe characteristic structural trends involving axial and equatorial NO2 groups. Most importantly, axial N-N bonds are considerably longer (by about 0.05 angstrom) than equatorial ones. This finding is in agreement with the hypothesis by Stals that axial N-NO2 bonds in secondary nitramines are weaker than equatorial ones.

Journal of Molecular Structure-Theochem
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2210-2728; 0166-1280