50 Synthesis And 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)
|51 NO2 and HNO3 free dinitrogen pentoxide, N2O5 was synthesized by the oxidation of NO2 with ozone. Two principal nitration systems have been investigated: (i) N2O5 in pure nitric and sulfuric acid and (ii) N2O5 in anhydrous organic solvents (CH2Cl2, CH3NO2, CH3CN, CHCl3, CCl4). The Raman spectra of pure dinitrogen pentoxide in the solid state and in solution at different temperatures (-15°C, 0°C, 20°C) were recorded.|
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]
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.
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.
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.
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.
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.
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.
|#||Part of this work was carried out in collaboration with and interest of the WIWEB.|
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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)