Larger polynitrogens

Strout and co-workers calculated possible dissociation pathways of open-chain N9 and Nn. The lowest barrier was estimated to be 16.6 and 14.1 kcal/mol for N9(N9- N7 + N2) and Nn (Nn- N9+ N2), respectively, at the CCSD(T)//MP2 level [67], Calculations of N7, N9, and Nn species suggest that the open-chain structures for these odd-numbered molecules will not be candidates for energetic materials since fragmentation barriers are too low. 

Manaa has considered the fullerene-like N60 molecule, and proposed that it could be formed from six bispentazoles (N10), or [N5+N5 ] salts under extreme conditions [73]. AMI and SCF levels of theory confirmed that N60 is a local minimum, and the reaction energy of N60 - 6N,0 is estimated to exothermic by 2430 kcal/mol at the SCF level, indicating N60 would be a super-high-energetic molecule if synthesized. 

Calculations of polynitrogens larger than N,2 have determined relative energetics and vibrational frequencies of possible isomers, but not activation barriers. Future work should explore the kinetic stabilities of these larger polynitrogens on their potential energy surfaces to validate their possible application as energetic materials. 

Lee et al. [77] considered the E atom in ENn as a catalyst, and pointed out that EN can dissociate into E + N with a high activation barrier followed by low activation barriers for decomposition of N into (n/2)N2. They also suggested that p-block elements such as Al and Ga would be expected to have strong covalency in the bonding with polynitrogens. In contrast, elements such as transition metals (Ti, Zr, Hf, Th), alkali (Na, K), and alkaline earth (Mg, Ca) metals are expected to form rather ionic complexes with polynitrogen species. 

Azide compounds of hypervalent main group elements such as Si and Ge are also highly explosive [83,84], but their structures and energetics have not been widely explored. Quite recently, the crystal structure of the Si(N3)62 anion has been reported, which has 90% nitrogen content, enough to be a possible high-energetic material candidate[83,84]. It is noted that Si(N3)62 (E = Ge, Pb) have been synthesized and are structurally known [83], 

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Neutral molecules are the polynitrogen species of main interest because they represent pure polymerized nitrogen without the esthetic distraction of a counterion, because they have been the most difficult to realize experimentally, and because being pure nitrogen they are in principle the best candidates for high-energy-density materials. Closed-shell species would seem to be of particular interest, because most stable molecules fall into this class, but radicals and even-electron open-shell molecules will also be mentioned where appropriate. We will consider N3, N4, N5, Ne, N7, Ng, and larger molecules . 

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