Extended-molecular allotropes

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. 

Elements. Those elements that form extended covalent (as opposed to metallic) arrays are boron, all the Group IV elements except lead, also phosphorus, arsenic, selenium and tellurium. All other elements form either only metallic phases or only molecular ones. Some of the above elements, of course, have allotropes of metallic or molecular type in addition to the phase or phases that are extended covalent arrays. For example, tin has a metallic allotrope (white tin) in addition to that with the diamond structure (grey tin), and selenium forms two molecular allotropes containing Se8 rings, isostruc-... 

An allotrope of a chemical element is defined as a solid phase (of the pure element) which differs by its crystal structure and therefore by its X-ray diffraction pattern from the other allotropes of that element. This definition can be extended to microcrystalline and amorphous phases which may be characterized either by their diffraction pattern or by suitable molecular spectra. 

The recent discovery of a family of large, solid cait)on molecules with great stability, the so-called "fullerenes , has considerably extended the scope and variety of carbon molecules known to exist and is opening an entirely new chapter on the physics and chemistry of carbon, with many potential applications. The fullerenes can be considered as another major allotrope of carbon and its first stable, finite, discrete molecularform. They are different, in that respect, from the other two allotropes, graphite and diamond, which eire not molecular but infinite-network solids. The other known carbon molecules, C2 to are unstable and found only in the vapor... 

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