Tetra-, hexa-, and octanitrogen molecules: a quantum chemical design and thermodynamic properties
- PDF / 862,949 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 84 Downloads / 137 Views
2067
Tetra-, hexa-, and octanitrogen molecules: a quantum chemical design and thermodynamic properties D. V. Chachkova and O. V. Mikhailovb aKazan
Department of Joint Supercomputer Center of the Russian Academy of Sciences — Branch of the Federal State Institution "Scientific Research Institute for System Analysis of the Russian Academy of Sciences", 2/31 ul. Lobachevskogo, 420111 Kazan, Russian Federation. Fax: +7 (843) 292 1608. E-mail: [email protected] bKazan National Research Technological University, 68 ul. Karla Marksa, 420015 Kazan, Russian Federation. Fax: +7 (843) 236 5768. E-mail: [email protected]
The possibility for tetra-, hexa-, octa-, and decanitrogen to exist was analyzed using the QCISD(T)/TZVP and G4 quantum chemical calculations. The results obtained suggest the existence of only four allotropes of nitrogen whose molecules contain from four to eight atoms, viz., rectangular and tetrahedral N4, open-book N6, and cubic N8. The bond lengths and bond angles were calculated for all compounds, as well as selected thermodynamic parameters (standard enthalpy of formation fH, standard entropy of formation S, standard Gibbs energy of formation fG) of the compounds in the gas phase. The enthalpies and entropies of the oxidation reactions of each compound by molecular oxygen were calculated using the results of quantum chemical computations. All the oxidation reactions are highly exothermic and practically irreversible; therefore, the title allotropes of nitrogen may appear to be promising combustible materials. Key words: nitrogen, allotropes, molecular structure, quantum chemical design, QCISD(T)/ TZVP method, G4 method.
The nitrogen molecule contains two atoms held together by a triple bond.1 As a consequence, the molecule of dinitrogen, N2, is characterized by very high dissociation energy (941.64 kJ mol–1) and low reactivity. Solid dinitrogen is a crystalline substance which has three polymorphs (phases), viz., α-N2 (cubic lattice, space group P213, lattice constant a = 5.660 Å), which exists at T < 36.61 K; hexagonal close-packed-N2 (space group P63/mmc, lattice constants a = 3.93 Å and c = 6.50 Å), which exists at 36.61 < T < 63.29 К, and tetragonal γ-N2 existing at pressures above 350 MPa and T < 83.0 К.1 Among other allotropes of nitrogen, only tetranitrogen N4 was detected experimentally.2,3 Nevertheless, there are some indications of the possibility for other allotropes of nitrogen containing more than two atoms in their molecules to exist. They are based on the results of numerous quantum chemical calculations (see, in particular, Refs 2—20). According to those calculations, a rather large number of polynitrogens containing from four to sixty atoms in the molecule can exist at least in the gas phase.20 It is expected that such allotropes of nitrogen, would they be actually synthesized, could be used as materials with very high energy density and as high-energy rocket propellants or explosives.21,22
However, theoretical calculations reported in all publications mentioned above and in other stu
Data Loading...
