The equilibrium geometry of the N,N-dimethyLnitramixle molecule and changes in the energy and structural parameters due to the internal rotation of the rfitro group and the inversion of the N atom in the amino fragment were calculated by the restricted Hartree--Fock (RHF) method and at the second-order Mr (MP2) level of perturbation theory with inclusion of electron correlation using the 6-31G* and 6-31G** basks sets. The one-dimensional potential functions of these motions calculated at the RHF/6-31G* level were approximated by a truncated Fourier a.ad power series, respectively. The frequencies of torsional and inversion transitions were determined by soNing direct vibrational problems for a non-rigid model, i.e., taking into account the molecular geometry relaxation.The equilibrium conformation of the molecular skeleton of N,N-dimethylnitramine is nonplanar. Transition states of the internal rotation of the nitro group and inversion of the amine N atom are characterized by pronounced concerted changes in its bond angles and the length of the N--N bond. in the MP2/6-31G* approximation, the height of the barrier to internal rotation calculated taking into account the difference in the zero-point v~rational ener~es is equal to 9.7 kcal mo[ -~-. Inversion in the amino fragment is accompanied by a relatively small energy change at the barrier height of-1.0 kcal tool -1 calculated in the same approximation.Key words: N,N-dimethylnitramine, quazltum-chemical calculation, moleculax structure, internal rotation, inversion of the amine nitrogen atom.The specific properties of a large number of complex energetic compounds containing the CoN--NO 2 nitramino fragment are determined by structural flexibility and conformational mobility of this fragment, l,z However, experimental and theoretical investigations of the simplest nitramines (which can be considered as model systems) carried out up to the present did not lead to a clear understanding of specific features of their structure and intramotecular dynamics.The structure of the N,N-dimethylnitramine molecule, Me2NNO 2, was studied experimentaRy in the gas phase 3 and in the crystal, 4-8 as well as by ab initio quantum-chemical calculations. 9-It The authors of the electron diffraction study 3 believe that the molecule has a planar skeleton of C2v symmetry. At the same time, they could not totally reject nonplanar models of C 2 and Cs symmetry, although they reason that the nonplanar structure is an artifact caused by vibrational effects.

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...

The possible combination of two methods for the calculation of multicenter two-electron integrals using STO and B function basis sets is discussed. The first method (Method I), which is of approximate nature, is based on a simplified version of the so-called X-factorization method [A.W. Niukkanen and L. A. Gribov, Theor. Chim. Acta 62, 443 (1983)], where the radial part of the two-center one-electron density pa&) is represented as a sum of two radial functions f&) and fb(r), placed on two different centers a and b. After such a transformation, the calculation of the two-electron integrals boils down to the calculation of some type of Coulomb integrals. The second method (Method 11), which calculates each integral separately to a given accuracy, is based on Mobius-type quadrature used for a three-dimensional integral representation for the two-electron integral of B functions [E. 0. Steinborn and H. H. H. Homeier, Int. J. Quantum Chem. Symp. 24, 349 (1990)l. In Method I, the choice of the radial functions placed on the different centers has an essential influence on the final value of the multicenter integrals. In the present study of Method I, a rather simple approximation of the radial part was made that reproduced the qualitative behavior of the molecular integral curves as function of the geometry rather well. At the present state of development, Method I produces fast order-of-magnitude estimates that are useful for screening purposes, i.e., to decide which integrals have to be evaluated more accurately by other methods like Method 11. Method I1 reproduces data given in the literature [R. M. Pitzer and D. P. Merrifield, J. Chem. Phys. 52, 4782 (1970)

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