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

Abstract

Harmonic force fields for the molecule of N,N-dimethylnitramine were calculated in the RH !=/6-31 G* and M P2/6-31G** 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 ~SN-amino-and tSN-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 NOar/NOst t and NOstr/NN~r interactions remain even after scaling the force fields.Key words: N,N-dimethylnitramine, vibrational spectra, quantum-chemical calculation, force field.Interpretation of vibrational spectra of N,N-dimethylnitramine and its Dr-, tSN-nitro, and tSN-amino isotopomers was changed several times as new experimental information became available; t-9 however, it has not been completed as yet. In particular, the assignment of the frequencies of NN and NC stretching vibrations 2-6 is rather contradictory. The bands in the spectral region below 450 cm -I, characteristic of deformation vibrations of a methyl-substituted amino group and torsional vibrations, 5-8 have no reliable assignment. The lack of reliable experimental data for the lowfrequency spectral region can significantly affect the interpretation of the spectra as a whole. The results of numerous studies on normal coordinate analysis 5"s.s,9 indicate the complex character of the vibrations. Contradictory results of numerous attempts of analyzing vibrational spectra of N,N-dimethylnitramine and a number of other simplest nitramines suggest that a complete interpretation of these spectral data in the framework of conventional methods is impossibld, t A correct choice and assignment of the fundamental frequencies are impossible without preliminary knowledge on the force field. Ab initio quantum-chemical calculations are a powerful source of such an inlormation.t~ Typical calculation errors can be effectively compensated for by scaling the force fields in the course of the solution of the inverse spectral problem with refinement of a small set of scaling factors by fitting theoretical vibrational frequencies to the experimental ones.Our experience on using the force fields calculated by the restricted Hartree--Fock (RHF) method with the 6-31G* basis set and their scaling in the course of interpretation of vibrational spectra of a number of nitro derivatives shows that this approach is highly efficient, tl-t3 For three simplest nitramines, (CH3)2NNO2, CH3NHNO2, and H2NNO2, and their isotopomers we obtained a c...

Chemical physics of decomposition of energetic materials. Problems and prospects

“…Quantum-chemical calculations of the force field, analysis of vibrational spectra of several nitro compounds and the force constants of stretching vibrations, internuclear distances and spectral characteristics of the simplest nitramines were reported. 179 Ab initio (including multiconfigurational) quantum-chemical methods were employed to calculate the potential energy surfaces of nitroso compounds in the ground state and in the lowest excited electronic state and to determine the equilibrium geometric parameters, vibrational frequencies and the potential functions of internal rotation of molecules. 180 The structures of nitroso compounds were calculated and the Y7NO (Y = C, N, O, S) bond dissociation energies determined.…”

The monomolecular mechanism of the gas‐phase thermal decomposition of primary N‐nitramines

“…At present, there are few data about the geometrical and electronic structures of secondary nitramines. In most cases, the results were given by using the electron diffraction investigation and quantum‐chemical methods 1, 2. There is not enough precise experimental data on thermal decomposition.…”