V. A. Shlyapochnikov

The structure of nitrobenzene and the interpretation of the vibrational frequencies of the CNO2 moiety on the basis of ab initio calculations

Abstract The vibrational spectra of nitrobenzene and its para - d 1 , d 5 , 16 O 18 O, 18 O 2 and 15 N isotopic modifications are evaluated using the RHF/6-31G* ab initio harmonic force field. A rigorous interpretation of the experimental CNO 2 moiety bands is carried out. Systematic deficiencies of the SCF method are effectively removed by applying scale factors optimized previously for a number of aliphatic nitro compounds. Fully optimized geometries are also reported for planar and orthogonal nitrobenzene conformations at the RHF and MP2 computational levels using the standard 6-31G* and 6-31G** basis sets. Theoretical geometries and barriers to internal rotation are compared with available experimental data. The calculations suggest that steric factors affect appreciably the structural parameters of the CNO 2 fragment in the equilibrium planar conformation and consequently the potential function for internal rotation in nitrobenzene.

On the structure and spectra of dinitramide salts

Abstract The preparation for the isotopically substituted dinitramide salts has been proposed. The 14 N and 15 N NMR spectra of the 15 N-enriched dinitramide anion in solution are measured and interpreted for the first time. New experimental IR and Raman spectra are obtained for the NH 4 + and K + salts of dinitramide in the solid state and in solution. The assignments are made with the help of quantum-mechanical force field calculations and normal-coordinate analysis. The harmonic vibrational spectrum of the dinitramide anion calculated at the B3LYP/6-31+G(d) level in the presence of solvent correlates best with the experimental data. The structure and spectra of the M + CH(NO 2 ) 2 − and M + N(NO 2 ) 2 − (M denotes a counterion) are compared and discussed. Topology of the electron charge density in the dinitramide anion is analyzed within the framework of Baders theory of atoms in molecules.

STRUCTURE AND VIBRATIONAL SPECTRA OF DINITROMETHANE AND TRINITROMETHANE

Abstract The molecular geometries of dinitromethane and trinitromethane were optimized and their harmonic force fields were calculated by the DFT/B3LYP method. The force fields obtained made it possible to interpret reliably the vibrational spectra of dinitromethane, trinitromethane and a number of isotopomers of trinitromethane. Some general conclusions on geometry and vibrational spectra of the molecules under study are made. The hybrid density functional method used is shown to predict the reliable structural parameters and vibrational frequencies for polynitromethanes.

DINITRAMIDE AND ITS SALTS. VII: SPECTRA AND STRUCTURE OF DINITRAMIDE SALTS

The vibrational and electronic spectra of dinitramide salts MN(NO2)2 (M=K, Na, Li, NH4, Fe, Ag, Mn, Mg, Rb, C(NH2)3) have been studied. The IR and Raman spectra of solutions and melts of the salts have been satisfactorily interpreted on the basis of the anion model withC2v symmetry. The complication of the spectra in the crystalline phase has been explained by restructurization of the anion that reduces its symmetry and makes the nitro groups nonequivalent.

A quantum-chemical study of 1,2,3,4,5,6,7,8-octaazanaphthalene and itsN-oxides

Calculations of hypothetical molecules of octaazanaphthalene and itsN-oxides were performed by the MNDO method with full geometry optimization. Probable decomposition reactions of these compounds were considered. Compounds with more pronounced alternation of charges on adjacent atoms were shown to be more thermodynamically favorable and thermally stable.

QUALITATIVE ESTIMATION OF THE DEPENDENCE OF THE NUMBER OF ENERGETIC COMPOUNDS ON THEIR OXYGEN COEFFICIENT

The ratio between the numbers of structural formulas of C,H,N,O-containing energetic compounds belonging to the classes of fuels (low values of the oxygen coefficientA), explosives (mediumA), and oxidants (highA values) was studied by a computer generation procedure. The number of the theoretically possible structural formulas was found to decrease rapidly on going from fuels to explosives and then to oxidants; this observation agrees with the data on the numbers of various energetic compounds currently used and proposed. The strategy of the search for new compounds with the specified properties is described in brief, and its applicability to the search for explosives and oxidants with a small (up to 12) number of atoms in a molecule is evaluated.

Molecular structures of trinitromethane derivatives

The molecular structures of trinitromethane derivatives XC(NO2)3 (X = F, Cl, Br, NC, NF2, N3) were studied using the density functional approach. The rules for changing the configurations of substituents in these compounds were revealed. Acceptability of the method employed for the calculations of trinitromethane derivatives is discussed.

Interpretation of the vibrational spectra of nitramines on the basis ofab initio calculations

It is shown that, unlike conventional methods of vibrational spectroscopy, the use ofab initia harmonic force fields, IR and Raman intensities, and depolarizations makes possible a rigorous interpretation of the experimental spectra of the simplest aliphatic nitramines (CH3)2NNO2, CH3NHNO2, H2NNO2, and their isotopomers. The scale factors, which were introduced for each compound to remove the systematic errors of the SCF MO LCAO calculation by fitting the parameters to the observed frequencies, were mutually adjusted during the solution of the inverse vibrational problem. The set of transferable scale factors established in this work can be used directly to analyze spectra of larger molecules. Some common patterns of the force fields and vibrational spectra of nitramines are discussed.

Mechanism of formation and monomolecular decomposition of aci-nitromethanes: a quantum-chemical study

A quantum-chemical study of the reactions of formation of aci-nitromethane (aci-NM) and aci-dinitromethane (aci-DNM) and their decomposition with elimination of water was carried out. The methods employed were the ab initio RHF method with inclusion of electron correlation at the MP2 level of theory and the Dunning—Hay double zeta basis set augmented with polarization d-functions on heavy-element atoms, the DFT approach at the B3LYP level, and the semiempirical PM3 method. The formation of aci-NM and aci-DNM was found to be the limiting stage of the mechanism under study. For DNM, the barrier to reaction is substantially lower than for NM. The estimates of the heights of the barriers to formation found from density functional calculations at the B3LYP/6-311++G(df,p) level (258 kJ mol–1 for aci-NM and 218.5 kJ mol–1 for aci-DNM) are thought to be the most reliable.

Some peculiarities of the influence of molecular structure on the activation energy of radical gas-phase decomposition of aliphatic nitro compounds

Geometric parameters of nitro and fluoronitro derivatives of nitromethane, nitroethane, I-nitropropane, and I-nitrobutane (for which experimental data on kinetics of radical gas-phase decomposition are available) have been determined by the MINDO/3 method. Correlations between changes in logarithm of the activation energy and those in the lengths of C-N and C-H bonds as well as in the ionization potentials have been found. The major changes in the C-N- and C-H- bonds occur when bulky atoms and groups (N02, Cl, Br, and 1) are introduced into the molecules. Nonempirical calculations of energies of dissociation of the C-N bonds in the molecules of nitromethane and its halogen derivatives have been carried out. An equation was proposed which allows one to perform a high-accuracy determination of the activation energy for radical gas-phase decomposition of nitroalkanes using the coefficients of steric interactions in the molecules calculated by the methods of molecular mechanics.