Syntheses & Raman spectra of dinitrogen pentoxide, N2O5

Janna Trofimova, Gunnar Spieß and Thomas M. Klapötke*
Institute of Inorganic Chemistry,
University of Munich (LMU),
Meiserstrasse 1, D-80333 Munich (Germany)
fax: +49-89-5902-382,
e-mail: tmk [at] anorg.chemie.uni-muenchen.de

Abstract

52 Introduction

The majority of energetic materials are organic compounds, which derive their energy from the nitro group -NO₂. Dinitrogen pentoxide, N₂O₅, has proven to be very useful in the nitration of a variety of substrates including aromatic compounds, alkanes, alkenes, alcohols, sugars, oximes, amines and amides in solution and in the gas phase.[ref. 1]

The synthesis of dinitrogen pentoxide, N₂O₅, was first described in preliminary studies as early as in 1849. N₂O₅ can be obtained by dehydration of white fuming HNO₃ with phosphorus pentoxide (the yield is 50%).² Anodic oxidation of nitrogen(IV)oxide in 100% HNO₃ using controlled potential techniques yields solutions of N₂O₅ / HNO₃.³ Direct ozonolysis of nitrogen(IV)oxide is a more satisfactory method from the standpoint of ease of preparation and purity (NO₂ and acid free N₂O₅).⁴ X-Ray diffraction studies show that solid N₂O₅ consists of an ionic array of linear NO₂⁺ (N-O 1.15 Å) and planar NO₃^– (N-O 1.24 Å) ( Fig. 1). In the gas phase [ref. 5,6] (Fig. 2) and in solution (CCl₄, CHCl₃ etc.) the compound is molecular while the ionic form will predominate in polar solvents.[ref. 5-7]

Fig. 1 Ionic structure of dinitrogen pentoxide.[ref. 8]

Fig. 2 Covalent structure of dinitrogen pentoxide in the vapor phase.[ref. 8-10]

53 Decomposition

N₂O₅ decomposes unimoleculary to nitrogen(IV)oxide and oxygen in the gas phase [ref. 11,12]:

The decomposition of N₂O₅ in HNO₃ is very slow and again unimolecular. [ref. 13-16] There are, however, a rather large number of compounds that are not compatible with acid solutions. The rate of decomposition of N₂O₅ in chemically inert solvents was found to occur in the following order. [ref. 13]

(fast) CHCl₃ > CCl₄ > C₂H₂Cl₄ > CH₃NO₂ (slow)

54 Experimental Section

Synthesis of dinitrogen pentoxide

The synthesis of the dinitrogen pentoxide has been described previously.[ref. 3-5] N₂O₅ in the solid state was prepared by oxidation of NO₂/N₂O₄ with ozone. Ozone was produced from dried oxygen (Messer-Griesheim, 5.0) using a Fischer Scientific type OZ – 502 ozone generator (120 L/h). NO₂ (Linde) was commercially available and was dried prior to use (P₄O₁₀).

The flow rate of nitrogen(IV)oxide was set with the aid of a flow meter so that the color of the NO₂ was discharged just as the mixed NO₂/O₃ gas stream entered the glass-bead area of the reactions tube. Three N₂O₅ collectors were cooled to -78°C with a CH₂Cl₂/dry ice slush. After about 3 hours of operation under the conditions described above, approximately 15 g of NO₂ free nitrogen(V)oxide, N₂O₅, condensed. N₂O₅ is a white solid, which is sometimes slightly colored from condensed unreacted nitrogen(IV)oxide. If necessary, the nitrogen(V)oxide can be stored in the flasks at -80°C for a few weeks with relatively little decomposition.

55 Raman Spectra and Discussion

Figure 3 shows the solid state Raman spectra of neat N₂O₅ under various conditions.

An excess of N₂O₅ was dissolved in the dry acids (Figure 6) and in anhydrous solutions of CH₂Cl₂, CH₃NO₂, CH₃CN, CHCl₃, CCl₄ and CD2Cl₂ at about -20°C (Figure 4 and Figure 5). Pyrex 4 mm Raman tubes were used as sample containers for both the solids and the solutions.

To investigate the stability of dinitrogen pentoxide, N₂O₅, in various solutions, Raman spectroscopic measurements were carried out at -15°C, 0°C and 21°C (Figure 4 and Figure 5).

The Raman spectra were all recorded directly after the solutions were prepared (within 5 minutes) between 4000 and 200 cm^-1 on a Perkin-Elmer SPECTRUM 2000 spectrophotometer (50 scans). For solid-phase spectra of the dinitrogen pentoxide, typical settings were: 200 mW, 50 scans, 180° geometry.

Strong peaks: 1397 cm^-1(NO₂⁺) and 1047 cm^-1 (NO₃-); weak peaks: 722 cm^-1(NO₃^–) and 533 cm^-1 (NO₂⁺), (Figure 3; cf. ref. 17).

Fig. 3 Raman spectra of solid dinitrogen pentoxide, N₂O₅.

The Raman spectra obtained from a solution of N₂O₅ in CH₂Cl₂ are shown in Figure 4.

Fig. 4 Raman spectra of CH₂Cl₂ (top), neat dinitrogen pentoxide and dinitrogen pentoxide in CH₂Cl₂ solution at -15°C, 0°C and 21°C.

Similar spectra were obtained for CH₃NO₂, CH₃CN, CHCl₃ and CCl₄, solutions. The decomposition of N₂O₅ in chemically inert solvents is the fastest in CHCl₃ (nonpolar) and the slowest in CH₃NO₂:

CHCl₃ > CCl₄ > CH₂Cl₂ > CH₃CN > CH₃NO₂

The observed order of stability seems to reflect the fact that N₂O₅ is more stable when it dissolves as ions (i.e. as NO₂⁺ and NO₃^–) rather than as the weekly bound covalent species.

a.

b.

Fig. 5 Raman spectra of the dinitrogen pentoxide in CH₂Cl₂, CH₃NO₂, CH₃CN and CHCl₃ solution: a) at -15°C and b) at 21°C.

In very polar solvents such as HNO₃, N₂O₅ is completely dissociated into NO₃^– and NO₂⁺ ions (Fig 6 ).

Fig. 6 Raman spectra of the dinitrogen pentoxide in HNO₃ solution.

To demonstrate how unstable N₂O₅ is in solution we recorded Raman spectra of saturated solutions of N₂O₅ at room temperature in CHCl₃ and CCl₄ after 2 hours and after one day. In all these cases complete decomposition had taken place and only spectra of the pure solvents were obtained (Fig. 7) because the decomposition products of N₂O₅ are volatile gases (NO₂, O₂).

Fig. 7 Raman Spectra of neat N₂O₅ (top), CHCl₃ (bottom) and of solutions of N₂O₅ in CHCl₃ at 21°C recorded after 2 hours and after one day.

56 Conclusions

The Raman spectra of the dinitrogen pentoxide were recorded for seven different samples under various conditions. Dinitrogen pentoxide is reasonably stable in nitric acid and in polar and non-polar organic solutions. However, as discussed, when N₂O₅ is heated in solution, decomposition or organic reactions may occur.

Acknowledgment

The authors wish to thank the University of Munich for financial support. We are indebted to and thank Professor P. Hendra for many valuable technical discussions.

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Received 25th February 1998, received in revised format 10th March 1998, accepted 11th March 1998.

REF: Int. J. Vib. Spect., [www.irdg.org/ijvs] 2, 1, 50-56 (1998)