The Spectrum of Sulfur and its Allotropes


Mar 5, 1971 - shows the absorption edge of a 160-/x thick film at various temperatures .... 0. 0. 719.98 σ 7Γ π σ c'. 2 4 1 0 c. %+. 2 4 1 0. D. (...
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The Spectrum of Sulfur and its Allotropes

B. MEYER, M. GOUTERMAN, D. JENSEN, T. V. OOMMEN, K. SPITZER, and T. STROYER-HANSEN

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University of Washington, Seattle, Wash. 98105

Orthorhombic sulfur absorbs at 285 and 265 nm because of S ; polymeric sulfur absorbs at 360 nm; liquid sulfur has an absorption edge which shifts from 400 nm at 120°C to 700 nm at 700°C. The shift is a result of changes in molecular composition and simultaneous absorption by cyclo-S , polymeric sulfur, S , and S . The spectrum of sulfur vapor changes with temperature and pressure. Blue absorption is caused by S , green absorption by S , absorption at 400 nm by S , and absorption at 520 nm by S . The comparison of spectra of allotropes in various phases, including solutions and matrices, with extended Hückel calculations shows that the spectrum of chains converges towards the near infrared with increasing molecular size, while sulfur rings all absorb in the near ultraviolet. 8

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Ο ulfur exists in a variety of metastable molecular forms and polymorphs ^ (I). For most of these allotropes the color is known well and is used often as an identifying tool. However, the spectrum of these species, with few exceptions, is not established well. This article reviews the spectra observed for polymeric sulfur, S , S , S , S , and S and discusses and predicts the electronic energy levels, the spectrum, and color of these and some other allotropes. 8

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This paper consists offivesections. In thefirst,the absorption spec­ trum of solid sulfur is discussed; the second deals with liquid sulfur, the third reviews spectra of the vapor, and the fourth describes spectra of sulfur allotropes in solution, glasses, and matrices. Thefifthsection is a short review of some extended Huckel computations of energy levels of various allotropes in different structural configurations. 53

In Sulfur Research Trends; Miller, D., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1972.

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Orthorhombic S , the stable form of sulfur at room temperature, forms yellow crystals which are transparent above 400 nm. In thick crystals, absorption occurs above 400 nm. The spectrum has been re­ corded by Clark and Simpson (2) by reflection and absorption spectros­ copy. Figure 1 shows the absorption spectrum, with a maximum at 285 nm which is explained as being a result of the first allowed transition of cyclo-S . The energy levels of S are discussed later. The first absorption peak at 285 nm is so strong that crystals are opaque below 350 nm. Thus, the known ultraviolet spectrum of sulfur consists of an absorption edge. The vacuum ultraviolet spectrum of solid sulfur is not known. Fukuda (3) studied the ultraviolet absorption of sulfur and discovered that the absorption edge shifts with temperature at a rate of 0.2 nm/°K between 0 ° C and the melting point. Our observations (4) indicate that the shift between 77°K and 25°C is 0.23 nm/°K. This shift explains why the color of sulfur fades reversibly upon cooling. At 77° Κ orthorhombic sulfur is snow white like table salt (5). The shift of the absorption edge and, thus, the color change is caused by change in thermal population of the vibrational levels of the electronic ground state of S (3). A quantitative correlation between the shift of the absorption edge and kT has been developed by Gilleo (6) for sele­ nium. This model has been applied to sulfur by Bass (7). Knowledge of electronic energy levels of sulfur allotropes is still rather meager. It is amazing that no great effort has been made to record and analyze the spectrum of S . For example, S must have a low lying triplet state in the visible. This triplet state is not known although its 8

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TRENDS

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In Sulfur Research Trends; Miller, D., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1972.

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transition energy and lifetime are important keys to understanding the photolysis of the S-S bond. S-S scission occurs in the presence of day­ light and is responsible for many perplexing properties of S in chemistry, radiation biology, and physiology. Thus, the discovery of triplet state properties of S should be one of our foremost goals. The spectra of the unstable solid allotropes of sulfur are not known well. S , S , S , S12, and other species (S, 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° Κ 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 S , and an absorption peak at 550 nm which is a result of S . The spectrum of trapped vapor con­ sists 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 S . The study of the spectrum of solid sulfur is not only complicated by metastable allotropes but also by impurities. Even 99.999% sulfur con­ tains traces of organic matter which upon melting react with sulfur and 8

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In Sulfur Research Trends; Miller, D., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1972.

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color it brown. This brown color has been mistaken frequently as stem­ ming from metastable allotropes. However, the formation of brown polysulfides is irreversible and can be distinguished easily from unstable allotropes which at room temperature all convert into yellow orthorhombic sulfur. Freshly poured vats of Frasch sulfur have a blue hue (II) which disappears within a few days. We believe that this color is partly a result of the reflection spectrum of S and S . Although liquid Frasch sulfur can contain only minute traces of these species, their concentration is sufficient to explain this hue. The slow speed of fading is caused by the slow kinetics of recombination of S and S trapped in a matrix of S ; it does not indicate that S or S is stable at room temperature. 3

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At the melting point, liquid sulfur is transparent and has the same yellow color as the crystals. At the boiling point it is deep red and opaque. The absorption coefficient in the ultraviolet is so high that the spectrum of even a very thin film consists only of an absorption edge. Figure 3 shows the absorption edge of a 160-/x thick film at various temperatures. The spectra above 444°C, the normal boiling point, were taken at high pressure in sealed quartz cells. Figure 4 shows the shift rate as a function

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Figure 3.

Red absorption edge of liquid sulfur at various tem­ peratures between 120°-700°C

In Sulfur Research Trends; Miller, D., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1972.

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of temperature (4). Of the earlier workers, Mondain-Monval (12) observed a shift of 0.294 nm/°K below 160°C and 0.74 nm/°K above 160°C; Bass (7) found an average value of 0.62 nm/°K between the melting point and 300°C compared with our value of 0.576 nm/°K. Bass and earlier workers explained the shifts in terms of thermal population of ground state vibrational modes. Such population explains the temperature dependence of the spectrum of solid sulfur. In the case of liquid, however, the shift rate is more than twice that of the solid, and Figure 4 shows that the shift is not smooth. The discontinuity at 160 °C reminds one of the extreme viscosity change at that temperature (13). This viscosity change is caused by polymerization of S and change in molecular composition. We have restudied recently the spectrum of liquid sulfur and concluded that at the melting point the color is mainly a result of thermally broadened S . At 160 °C, it is caused by overlap of S and polymer absorption. At the boiling point the color is caused by at least four species: the absorption of thermally broadened S , thermally broadened polymeric sulfur, an absorption peak caused by an electronic transition of S at 400 nm, and absorption by S at 550 nm. The assignment of the visible peaks to S and S is based on comparison of spectra of the liquid with those of sulfur vapor, sulfur solutions, as well as glasses and matrices containing selected sulfur allotropes (14). From these we conclude that liquid sulfur contains 0.1 to 3% S and S . Since neither the 8

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In Sulfur Research Trends; Miller, D., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1972.

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composition of liquid sulfur nor the spectra of all potentially important allotropes is known fully, it is possible that we have overlooked other less important sources for the color, such as S , S , or Si . The discovery of shoulders and peaks in the visible spectrum of hot liquid sulfur (Figure 5) and the assignment to S and S have shown how little research has been done and how little is known about the composition of liquid sulfur. There is no question that this phase contains a wealth of unearthed treasures for physicists, physical chemists, as well as synthetic chemists of all types. The spectrum yields some important insights into the type of species the liquid contains and into their reactivity. The fifth section shows that the red absorption indicates that liquid sulfur contains reactive chains because sulfur rings in analogy with alkanes are expected to be yellow and to absorb in the near ultraviolet, while sulfur chains in analogy with alkyl diradicals are expected to be colored deeply. e

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RESEARCH

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Sulfur Vapor Below 250°C the spectrum of saturated vapor has been described by Bass (7). It consists of unresolved maxima at 210, 265, and 285 nm which are caused by electronic transitions of cyclo-S . If traces of impurities are present, a long vibrational progression appears between 185-210 nm with an origin at 45950 cm" as a result of C S which is formed in the reaction between any hydrocarbon and sulfur. This spectrum is an even 8

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