Allotropes Allowed’ transitions

It is conceivable that a parallel bundle of linear polyyne chains (the carbon allotrope named chaoite or carbyne) may form, via thermally-allowed (4 + 2)-n-electron cycloadditions, six-membered rings which lead to graphene sheets and/or nanotubes. However, the mechanism of nanotube formation in a plasma produced by arc discharge appears to require the presence of a transition metal atom at the rim of the growing nanotube [77 j. 

The spectra of the unstable solid allotropes of sulfur are not known well. Sg, St, Sg, Si2, and other species (8, 9, 10) form yellow to white solids. Their solution spectra are discussed later. Quenched liquid sulfur or vapor produces yellow polymeric sulfur. Its spectrum shows a shoulder at 360 nm, as shown later. If liquid sulfur or sulfur vapor is quenched rapidly to 77 °K or below, deep colored films are formed. The spectrum of red sulfur produced by quenching a boiling liquid film is shown in Figure 2. The color is a result of three spectral features the absorption edge of polymeric sulfur, an absorption peak at 400 nm which is caused by the lowest allowed electronic transition of Sa, and an absorption peak at 550 nm which is a result of S4. The spectrum of trapped vapor consists of several broad peaks it changes with the composition of the vapor source and depends on speed of deposition and many other factors. The deep color of all trapped metastable species fades at — 90°C to deep yellow. Above this temperature, the spectrum shows only polymeric sulfur and Sg. 

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