Tetranitrogen

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Tetranitrogen is a neutrally charged polynitrogen allotrope of the chemical formula N
4 and consists of four nitrogen atoms. The tetranitrogen cation is the positively charged ion, N+
4, which is more stable than the neutral tetranitrogen and is thus more studied. The structure, stability and properties of these molecules have been of great interest to research scientists in the last ten years.[when?]

History

Polynitrogen compounds have been well known and characterized by chemists for many years. Molecular nitrogen (N
2) was first isolated by Daniel Rutherford in 1772[1] and the azide ion (N−
3) was discovered by Theodor Curtius in 1890.[2] Discoveries of other related nitrogenous allotypes during the twentieth century include the aromatic molecule pentazole and the radical molecule N•
3. However, none of these complexes could be isolated or synthesized on a macroscopic scale like N
2 and azide; it was not until 1999 that a large scale synthesis was devised for a third nitrogen allotrope, the pentazenium (N+
5) cation.[3] This increased interest in polynitrogen compounds in the late twentieth century was due to the advance of computational chemistry which predicted that these types of molecules could be used as potential high energy density matter (HEDM) sources.[4]

The N+
4 cation was first discovered in 1958 upon analysis of anomalous background peaks of molecular weight 56+ and 42+ in the mass spectra of molecular nitrogen, which corresponded with formation of N+
4 and N+
3, respectively.[5] Explicit synthesis of N+
4 was first carried out in 2001 by a similar mechanism of electron bombardment of N
2.[6] Theoretical chemistry predicted several possible synthesis mechanisms for N
4 including reaction of a neutral N atom with a N•
3 radical, binding of two N
2 molecules in the excited state, and extrusion from polycyclic compounds, none of which could be accomplished experimentally. However, in 2002 a method for synthesis of tetranitrogen was devised from the deionization of N+
4 through neutralization-reionization mass spectrometry (NRMS).[7] In the synthesis, N+
4 (which was first formed in the ionization chamber of the mass spectrometer) underwent two high energy collision events. During the first collision, N+
4 contacted a target gas, CH
4, to yield a small percentage of neutral N
4 molecules.[7]

N_4^+ \xrightarrow[-CH_4^+]{+CH_4} N_4

A deflecting electrode was used to remove any unreacted N+
4 ions as well as the target gas, CH
4, and any additional unintended reaction products, leaving a stream of N
4 molecules. In order to affirm the synthesis and isolation of N
4, this stream then underwent a second collision event, contacting a second target gas, O
2, reforming the N+
4 cation.[7]

N_4 \xrightarrow[-e^-]{+O_2} N_4^+

The disappearance and reemergence of this "recovery peak" confirms the completion of both reactions, providing ample evidence for the synthesis of N
4 by this method. Because the "flight time" between the two reactions, carried out in separate chambers of the mass spectrometer, was on the order of 1 μs, the N
4 molecule has a lifetime of at least this long.[7]

Characteristics

Since its discovery, N
4 has not been well studied. It is a gas at room temperature (298 K). It also has a lifetime in excess of 1 μs, though it is predicted to be characterized as metastable.[7]

The structure of N+
4 has been predicted by theoretical experiments and confirmed by experimental techniques involving collisionally activated dissociation mass spectrometry (CADMS). This technique bombards N+
4 producing fragments which can then be analyzed by tandem mass spectrometry. Based on the fragments observed, a structure was determined invlvolving two sets of nitrogen atoms triple bonded to each other (two N
2 units), and associated with each other with a longer, weaker bond. This is expected to be a similar configuration for N
4, disproving a proposed tetrahedral (Td) model in which all of the nitrogen atoms are equivalent. This process is very exothermic, releasing ~800 kJ mol−1 of energy.[7]

Applications

Tetranitrogen and other similar polynitrogen compounds are predicted to be good candidates for use as high energy density matter (HEDM), high energy fuel sources with small weight in comparison with traditional liquid and fuel cell based energy sources.[8][9] The N≡N triple bond of N
2 is much stronger (energy of formation of 229 kcal/mole) than either an equivalent one-and-a-half N=N double bonds (100 kcal/mole, i.e. 150 kcal/mole total) or an equivalent three N-N single bonds (38.4 kcal/mole, i.e. 115 kcal/mole total). Because of this, polynitrogen molecules are expected to readily break down into harmless N
2 gas, in the process releasing large amounts of chemical energy. This is in contrast to carbon containing compounds which have lower energies of formation for an equivalent number of single or double bonds than for a C≡C triple bond, allowing for the thermodynamically favorable formation of polymers.[8] It is for this reason that the only allotropic form of nitrogen found in nature is molecular nitrogen (N
2) and why novel strategies of synthesizing polynitrogen allotropes in a cost-efficient manner are so highly sought after.

See also

References

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