The geometric, electronic, and thermodynamic parameters of six known isomers of the molecular structure of dinitrogen tetraoxide N 2 O 4 were calculated by the quantum-chemical DFT/B3LYP density functional method with the 6-311++G(3df) basis set. The structure of a new isomer of dinitrogen tetraoxide NONO 3 , which is characterized by a local potential energy minimum and corresponds to a stationary state of the N 2 O 4 isomer, was calculated. The DFT calculations showed that this structure NONO 3 is characterized by a significant negative charge on the NO 3 fragment and a positive charge on the NO fragment. The calculated dipole moment of nitrosonium nitrate NO NO + − 3 in the gas phase is 4.13 D.Key words: density functional method, quantum-chemical calculations, dinitrogen tetraoxide, nitrosonium nitrate.Dinitrogen tetraoxide N 2 O 4 is a highly toxic and highly corrosive substance. It is a gas that readily condenses to a liquid with boiling point T = 294 K. When cooled below T = 262 K the liquid solidifies to a colorless crystalline mass [1]. The most stable form of dinitrogen tetraoxide N 2 O 4 is the planar symmetrical structure O 2 N-NO 2 (D 2h symmetry), in which two molecules of nitrogen dioxide NO 2 are weakly linked by a central bond through the nitrogen atoms (r N-N = 1.78 Å) [2].Isomers of N 2 O 4 with the unsymmetrical O 2 NONO structure are also formed when gaseous NO 2 is deposited on a cooled surface (9-77 K) [3,4]. Analysis of the IR spectra of solid sprayed films of N 2 O 4 suggests the existence of two isomers with unsymmetrical structure O 2 NONO, denoted as the D and D´isomers [3]. The isomerization processes in N 2 O 4 in the sprayed solid films can also be stimulated by UV irradiation [5,6].When N 2 O 4 deposited on a cooled surface is heated to 180 K, spontaneous autoionization of the dinitrogen tetraoxide molecules with the formation of nitrosonium nitrate NO NO + − 3 can occur [3,4]. However, autoionization is not observed when deposited films containing only the symmetrical O 2 N-NO 2 structure are heated [7]. It can thus be assumed that the ionic form in the solid phase is formed during heating of the metastable molecular isomer with a substantial dipole moment, the presence of which and also the presence of charge polarization gives rise to the energy favorability for crystallization of the ionic structure in view of the crystal field energy [8]. showed that the molecular structure of the unsymmetrical O 2 NONO isomer with the trans configuration has the largest dipole moment (m » 2.6 D) (see Fig. 1b). The IR spectrum of this isomer, calculated in [9], corresponds the experimentally observed IR spectrum of the D isomer [3,5]. Thus, the observed thermally induced transformation of crystalline N 2 O 4 from the molecular phase to the ionic NO NO + − 3 phase can be attributed to the D isomer O 2 NONO with the trans configuration (Fig. 1b), as was done in [6]. 260040-5760/08/4401-0026

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By functional density quantum-chemical method (DFT/B3LYP using the 6-311++G(3df)) it has been shown that the molecular structures of N 2 O 5 with C s and C 2 symmetries are energetically equivalent. It follows from calculations of the vibrational frequencies that both structures are characterized by potential energy minima and correspond to stationary states of the N 2 O 5 molecule. It is proposed, on the basis of a comparison of the calculated and experimental vibrational spectra of N 2 O 5 , that dinitrogen pentaoxide exists in the gas phase as an equimolecular mixture of N 2 O 5 molecules with C s and C 2 symmetry, while in the solid phase it is characterized by the C 2 molecular structure.Key words: the functional density method, quantum chemical calculations, molecular structure, N 2 O 5 . N 2 O 5 was first obtained in 1840 as a result of the reaction of AgNO 3 with Cl 2 . Currently the characteristic reaction for the preparation of N 2 O 5 is the reaction of NO 2 with ozone. The finding of N 2 O 5 in the stratosphere of the Earth indicates an important role of gaseous N 2 O 5 in the nitrogen cycle for the destruction of the ozone layer, so that studies of the reactivity and electronic structure of N 2 O 5 has been carried out intensively [1,2].Nitric anhydride (dinitrogen pentaoxide, N 2 O 5 ) exists as colorless, transparent, easily decomposed (at 32°C) crystals. In the solid state N 2 O 5 has a crystalline structure formed of NO 2 + and NO 3 − ions, but after sublimation it consists of molecules with the covalent structure O 2 N-O-NO 2 in the gaseous phase. The molecular structure is retained for several hours in the solid state after rapid cooling of the gas on a surface at -180°C. Over time or on warming to -80°C the covalent structure O 2 N-O-NO 2 reverts to the ionic structure (NO 2 + : NO 3 − ) [3]. The first detailed investigation of the vibrational structure and the molecular structure of N 2 O 5 was carried out more than 40 years ago [4] and was based on the assumption that the N 2 O 5 molecule has a planar structure with C 2v symmetry.In agreement with the structural data [5] the double bonds between the four oxygen atoms and the two nitrogen atoms in N 2 O 5 were equal to 0.1188 nm, and the bonds between the oxygen atom at the center of the molecule and the nitrogen atoms were equal to 0.1498 nm. The angle between the central oxygen atom and the nitrogen atoms was equal to 111.8°and the angle between the nitrogen atoms and the outer oxygen atoms was equal to 133.2°. The authors of [5] concluded that the energy of the molecular structure of N 2 O 5 was minimal when the two torsional angles of rotation t 1 = t 2 » 30°(structure with symmetry C 2 , see Fig. 1c). Subsequent quantum-chemical study [6] showed that the planar configuration of the molecule was unstable and the structure with symmetry C 2 had the greatest energetic stability. 660040-5760/07/4301-0066