Polynitrogen neutral molecules

The experimental aspect will be presented first, to emphasize the dearth of such results. We can put polynitrogens into four classes neutral molecules, extended-molecular allotropes, isolable ions, and ephemeral ions mainly of mass-spectrometric interest. This fourth class will not be discussed. 

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 . 

Isolable ions, i.e. polynitrogen cations and anions, have met with more synthetic success than have neutral molecules. Although there is experimental evidence (above) for neutral N4 and Ng species, none have been isolated in contrast, the azide ion Ns" has been known for well over a century, an N5 cation has been made in macroscopic amounts, and an N5 anion has been detected. 

The simplest neutral polynitrogen, the N3 radical (azide radical), has been generated as a fleeting molecule by flash photolysis of HN3 (hydrazoic acid) [1] and by reaction of HN3 with fluorine atoms [2], Being a simple free radical, there appears to be no question of its isolation as a stable substance. 

Since polyphosphorus species are far more common than polynitrogen compounds, in contrast to their order of appearance in the periodic table (N, P, As, Sb, Bi), here we discuss the neutral homopolyatomic phosphorus compounds first. As stated above, phosphorus occurs in three main forms white, black, and red. White phosphorus consists of tetrahedral P4 molecules (Fig. 1) and forms a molecular lattice that melts already at 44°C, and even the boiling point is rather low at 280°C. In accord with its molecular structure, P4 is soluble in many organic nonpolar solvents but is insoluble in water—indeed, P4 is usually stored under water because it is spontaneously flammable in air. 

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