STRUCTURAL RELAXATIONS AND ENERGY EFFECTS OF INTRAMOLECULAR MOTIONS IN N,N-DIMETHYLNITRAMINE - A THEORETICAL-STUDY

The equilibrium geometry of the N,N-dimethylnitramine molecule and cha nges in the energy and structural parameters due to the internal rotat ion of the nitro group and the inversion of the N atom in the amino fr agment were calculated by the restricted Hartree-Fock (RHF) method and at the second-order Moller-Plesset (MP2) level of perturbation theory with inclusion of electron correlation using the 6-31G and 6-31G+* b asis sets. The one-dimensional potential functions of these motions ca lculated at the RHF/6-31G level were approximated by a truncated Four ier and power series, respectively. The frequencies of torsional and i nversion transitions were determined by solving direct vibrational pro blems for a non-rigid model, i.e., taking into account the molecular g eometry relaxation. The equilibrium conformation of the molecular skel eton of N,N-dimethylnitramine is nonplanar. Transition states of the i nternal rotation of the nitro group and inversion of the amine N atom are characterized by pronounced concerted changes in its bond angles a nd the length of the N-N bond. In the MP2/6-31G approximation, the he ight of the barrier to internal rotation calculated taking into accoun t the difference in the zero-point vibrational energies is equal to 9. 7 kcal mol(-1). Inversion in the amino fragment is accompanied by a re latively small energy change at the barrier height of similar to 1.0 k cal mol(-1) calculated in the same approximation.