5864
J. Phys. Chem. 1991, 95, 5864-5869
the inclusion sites formed by the cavities are not fully occupied by H20. Vacancies allow for other molecules such as NO2 to form clathrates with the HNIW molecules. The EPR results implicate the N 12-N 12a bond (Table IV) and symmetrically equivalent positions (Table 1) as the major position for radical formation. This is one of the nitramine bonds that forms the "cage" inclusion center. For NO2 radical formation at this position, the NO2 would effectively be in the cage immediately when formed. This provides an opportunity for trapping NO2 at the inclusion site, especially if the cavity does not contain a guest H 2 0 molecule. Annealing the crystal at room temperature provides kinetic energy to the NO2 radicals that might be sufficient to induce diffusion into the cage sites from other locations. From section IIIA, it is shown that more NO2 is produced per quantum of UV from HNIW than from RDX or HMX. There are weak signals (not shown) in I and I1 spectra that indicate that other NO2 conformations exist. No attempt was made to study these additional conformations because the signals were too weak and masked by the more intense signals from I or 11. Another factor that might contribute to NO2 formation and stability is the presence of other nitrate impurities. Studies by Box and Rustgi show that HNOo facilitates the formation of NO2 in X-irradiated nitrate salt crystals of gly~ylglycine.~~ The reaction as shown in eq 4 proceeds by electron scavenging by the R N H 3 + N 0 3 - z RNH3+ + N0302-
-
2NOje2-
2n+
2N02
+ 20H-
(4)
(5)
NO; anion to form which is paramagnetic. There are two factors that were shown to contribute to stability: (i) ~
(34) Rustgi,
~~~
S.N.; Box, H.C . J . Chem. Phys. 1973, 59, 4763.
the N0302-is stable as long as the temperature is low enough to prohibit proton transfer from a neighboring molecule or (ii) deuteration of neighboring molecules prohibits NOjo2-decay. Warming the crystal to room temperature facilitates proton transfer from adjacent molecules and results in reduction of NO
