Efficiency of post-Hartree-Fock force field for the interpretation of vibrational spectra of N,N-dimethylnitramine

Harmonic force fields for the molecule of N,N-dimethylnitramine were calculated in the RHF/6-31G* and MP2/6-31G** approximations. Scaling of the force fields obtained made it possible to reliably interpret the vibrational spectra of light and perdeuterated compounds reported in the literature. The assignment is confirmed by good reproducibility of experimental isotope shifts upon N-15-amino- and N-15-nitrosubstitution. The frequencies of intramolecular vibrations in far IR and Raman spectra as well as in neutron inelastic scattering spectra for the light and perdeuterated samples of solid N,N-dimethylnitramine were identified using the force field calculated with the inclusion of electron correlation (MP2). Although general structures of the force fields calculated in the RHF and MP2 approximations are similar, considerable differences in the force constants of the NO and NN stretching vibrations and especially in the constants of the NOstr/NOstr and NOstr/NNstr interactions remain even after scaling the force fields.