Abstract:
New energetic salts NF 4  XeF 7  and (NF 4 ) 2  XeF 8  are prepared by reacting NF 4  HF 2  with XeF 6  and exposing NF 4  XeF 7  to blue 4880Å laser light.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to energetic inorganic salts and more particularly to salts containing the NF 4   +   cation. 
     NF 4   +   salts are key ingredients for solid propellant NF 3  --F 2  gas generators, as shown by D. Pilipovich in U.S. Pat. No. 3,963,542, and for high detonation pressure explosives, as shown by K. O. Christe in U.S. Pat. No. 4,207,124. The synthesis of NF 4   +   salts is unusually difficult because the parent molecule NF 5  does not exist and the salts must be prepared from NF 3  which amounts formally to a transfer of F +   to NF 3  according to: 
     
         NF.sub.3 +F.sup.+ →NF.sub.4.sup.+ 
    
     Since fluorine is the most electronegative of all elements, F +   cannot be generated by chemical means. This difficult synthetic problem was overcome by K. O. Christe, et al as shown in U.S. Pat. No. 3,503,719. By the use of an activation energy source and a strong volatile Lewis acid, such as AsF 5 , the conversion of NF 3  and F 2  to an NF 4   +   salt became possible: ##STR1## However, only few Lewis acids are known which possess sufficient strength and acidity to be effective in this reaction. Therefore, other indirect methods were needed which allowed conversion of the readily accessible NF 4   +   salts into other new salts. Two such methods are presently known. The first one involves the displacement of a weaker Lewis acid by a stronger Lewis acid, as shown by K. O. Christe and C. J. Schack in U.S. Pat. No. 4,172,881 for the system: 
     
         NF.sub.4 BF.sub.4 +PF.sub.5 →NF.sub.4 PF.sub.6 +BF.sub.3 
    
     but is again limited to strong Lewis acids. The second method is based on metathesis, i.e., taking advantage of the different solubilities of NF 4   +   salts in solvents such as HF or BrF 5 . For example, NF 4  SbF 6  can be converted to NF 4  BF 4  according to: ##STR2## This method has successfully been applied by K. O. Christe, et al as shown in U.S. Pat. Nos. 4,108,965; 4,152,406; and 4,172,884 to the synthesis of several new salts. However, this method is limited to salts which have the necessary solubilities and are stable in the required solvent. The limitations of the above two methods are quite obvious and preempted the synthesis of NF 4   +   salts of anions which are either insoluble in these solvents or are derived from Lewis acids weaker than the solvent itself and therefore are displaced from their salts by the solvent. 
     SUMMARY OF THE INVENTION 
     The limitations of the previously known reactions for the synthesis of NF 4   +   salts are overcome by the present invention. It was found that NF 4   +   salts derived from very weak and volatile Lewis acids, such as XeF 6 , which are weaker than the solvent itself, can be prepared by the following method. A solid having the composition NF 4  HF 2 .nHF, where n ranges from about 0.5 to 10, was obtained as described by K. O. Christe et al in Inorganic Chemistry, 19, 1494 (1980). Repeated treatments of NF 4  HF 2 .nHF ,with a large excess of XeF 6  followed by removal of the volatile products at ambient temperature, permitted to shift the following equilibrium: ##STR3## quantitatively to the right. 
     For applications, such as solid propellant NF 3  --F 2  gas generators for chemical HF-DF lasers, the NF 3  --F 2  yields must be as high as possible and no volatile products which would deactivate the excited species can be tolerated. The highest usable fluorine contents theoretically available from the thermal decomposition of a previously known sufficiently stable NF 4   +   salt were 64.6 and 59.9 weight percent for (NF 4 ) 2  NiF 6  and (NF 4 ) 2  MnF 6 , respectively. Although these fluorine yields are high, the solid NiF 2  and MnF 3  byproducts render their formulations difficult to burn and require higher fuel levels thus reducing the practically obtainable fluorine yields. Consequently, NF 4   +   salts decomposing exclusively to NF 3 , F 2  and inert diluents, such as noble gases or nitrogen, were highly desirable. The new NF 4  XeF 7  salt, described in this invention, fulfills all of these requirements and provides a theoretical usable fluorine yield of 62.9 weight percent. On decomposition, it produces only NF 3 , F 2   and inert Xe. A further increase in the usable fluorine yield to 71.7 weight percent, the highest presently known value, was achieved by converting NF 4  XeF 7  into (NF 4 ) 2  XeF 8  according to: 
     
         2NF.sub.4 XeF.sub.7 →(NF.sub.4).sub.2 XeF.sub.8 +XeF.sub.6 
    
     This conversion was achieved by irradiating the yellow NF 4  XeF 7  salt with blue 4880 Å light from an Ar ion laser. The yellow NF 4  XeF 7  strongly absorbs the blue light and is photolytically decomposed to (NF 4 ) 2  XeF 8  and gaseous XeF 6 . Since (NF 4 ) 2  XeF 8  is white it does not absorb the blue light and is not further decomposed. Therefore, this invention also provides a new, selective, laser-induced, photolytic method for converting NF 4  XeF 7  into (NF 4 ) 2  XeF 8 . The latter compound not only provides the highest NF 3  --F 2  yield of any presently known compound, but also gives the highest theoretical detonation pressures in explosive formulations (about 50 kbar higher than corresponding formulation containing (NF 4 ) 2  NiF 6 ). 
     Accordingly, an object of this invention is to provide a new compound. 
     Another object of this invention is to provide new energetic NF 4   +   compositions which are useful in explosives, and solid propellants. 
     A further object of this invention is to provide NF 4   +  compositions for solid propellant NF 3  --F 2  gas generators for chemical HF-DF lasers which deliver a maximum of NF 3  and F 2  while not producing any gases which deactivate the chemical laser. 
     Still another object of this invention is to provide a novel method of preparing new energetic compounds. 
     These and other objects of this invention will become apparent from the following detailed description. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     EXAMPLE 1 
     Dry CsF (15.54 mmol) and NF 4  SbF 6  (15.65 mmol) were loaded in the drybox into one half of a prepassivated Teflon U metathesis apparatus. Dry HF (9 ml liquid) was added on the vacuum line and the mixture was stirred with a Teflon-coated magnetic stirring bar for 45 minutes at 25° C. After cooling the apparatus to -78° C., it was inverted and the NF 4  HF 2  solution was filtered into the other half of the apparatus. Most of the HF solvent was pumped off during warm up from -78° C. towards room temperature until the first signs of NF 4  HF 2  decomposition became noticeable. At this point the solution was cooled to -196° C. and XeF 6   (17.87 mmol) was added. The mixture was warmed to 25° C. and stirred for 12 hours. Although most of the XeF 6  dissolved in the liquid phase, there was some evidence for undissolved XeF 6 . Material volatile at 25° C. was removed in a static vacuum and separated by fractional condensation through traps kept at -64° and -196° C. Immediately, a white copious precipitate formed in the reactor, but disappeared after about 10 minutes resulting in a clear colorless solution. As soon as the first signs of NF 4  HF 2  decomposition were noted, removal of volatiles was stopped and the reactor cooled to -196° C. The HF collected in the -196° C. trap was discarded, but the XeF 6  collected in the -64° C. trap was recycled into the reactor resulting in a yellow solution at room temperature. This mixture was stirred at 25° C. for several hours, followed by removal of the material volatile at 25°  C. in a dynamic vacuum. The volatiles were separated by fractional condensation through traps kept at -210°, -126° and -64° C. and consisted of NF 3  (˜0.3 mmol), HF (˜11 mmol), and XeF 6 , respectively. The reactor was taken to the drybox and the solid products were weighed. The yellow filtrate residue (5.149 g, weight calcd for 15.54 mmol NF 4  XeF 7  =5.506 g, corresponding to a yield of 93.5 percent) consisted of NF 4  XeF 7 , and the white filter cake (5.78 g, weight calcd for 15.54 mmol of CsSbF 6  =5.72 g) consisted of CsSbF 6 . The composition of these solids was confirmed by vibrational and  19  F NMR spectroscopy, pyrolysis and analysis of the pyrolysis residue for NF 4   + , Cs +   and SbF 6   - . Based on these results, the reaction product had the following composition (weight %): NF 4  XeF 7  (98.01), NF 4  SbF 6  (0.88 ) and CsSbF 6  (1.11). 
     The NF 4  XeF 7  salt was identified by its Raman spectrum which exhibited the bands characteristics for NF 4   +   (1159, 1149, (ν 3 ), 841, (ν 1 ), 603 (ν 4 ), 440 (ν 2 ) and XeF 7   -   (558, 495, 464, 233, 212 cm -1 ). 
     EXAMPLE 2 
     A sample of NF 4  XeF 7  was exposed at room temperature for prolonged time to blue 4880 Å laser light. Photolytic decomposition of NF 4  XeF 7  occurred resulting in (NF 4 ) 2  XeF 8  formation (time of exposure depends upon the intensity and power of the light source) ##STR4## The (NF 4 ) 2  XeF 8  salt was identified by its Raman spectrum which exhibited the bands characteristic for NF 4   +   (1158, 1145, 841, 602, 440 cm -1 ) and XeF 8   2-   (500, 433, 416, 380, 374 cm -1 ). 
     Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.