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96 Ralf Steudel 3.1.2 Raman Spectra of Quenched Sulfur Melts To avoid the problems with the rather broad Raman signals of S8 and S1 in the spectra of hot sulfur melts it seemed helpful to record spectra of liquid sulfur rapidly quenched in liquid nitrogen [80]. To prepare such samples the equilibrated melt of high-purity sulfur was allowed to flow into stirred liquid nitrogen in as thin a string as possible resulting in a fine yellow powder of solid sulfur provided the melt temperature was below 160 C. At higher melt temperatures thin wires of solidified sulfur were obtained. All samples were handled and measured at temperatures below 78 C. The spectra now showed clearly the presence of S6 (268 cm1) and S7 (175, 288, 360, 402, 518 cm1) by the lines given in parentheses. In addition, b-S8 was detected [80]. The time-dependent formation of S7 from S8 in liquid sulfur at 120 C was monitored using the two neighboring Raman lines of S7 (360 cm1) and S8 (196 cm1) in the spectra of the melt quenched at certain time intervals after the start of the melting procedure of pure S8 in the aluminum oven. The in- tensity ratio I(360)/I(196) continuously increased from zero to 0.28, the final value being reached after ca. 10 h. By independent experiments it had been shown before that this intensity ratio is proportional to the S7 concentration in liquid S8. The temperature dependence of I(360)/I(196) was used to demonstrate that the S7 concentration in liquid sulfur increases from 115 to 159 C approximately by a factor of 2 but absolute concentrations could not be determined in this way [80]. 3.1.3 Infrared and Raman Spectra of p-Sulfur Above it has been shown that the Raman spectra of liquid sulfur and of quenched sulfur melts are obscured to a large extend by the strong lines of the main component S8. Therefore, to obtain spectral information on the mi- nority species one has to reduce the S8 concentration in the quenched sam- ples by extraction with carbon disulfide at 25 C followed by cooling of the filtered extract to 78 C whereupon most of the S8 together with some S12·CS2 crystallizes out (see above). The a-S8 is obtained as large yellow crys- tals while S12·CS2 forms almost colorless small plates which can easily be separated from the S8 by flotation. These plate-like crystals have been char- acterized by X-ray structure analysis as well as by vibrational spectra [87]. Infrared and Raman spectra of the filtered solution of S8+Sp as well as of its evaporation residue showed once more the presence of S8, S7, and S6 but in addition a Raman line at 460 cm1 was observed which was assigned to the S-S stretching vibrations of soluble rings larger than S8 (in the following termed as Sx). All these components were observed in sulfur melts quenched from temperatures of 120, 140, 250, 350, and 445 C [80]. To obtain quantitative data on the composition of the S8+Sp solution cali- bration curves were recorded using solutions of pure S6, S7, and S8 in CS2.PDF Image | Topics in Current Chemistry
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