The 77Se NMR spectrum of l,2-Se2S5 exhibits one singlet at 1077.3 ppm indicating pseudorotation in solution; the same conclusion is reached from the solution Raman spectrum of S7 showing characteristic line broadening compared to the solid state spectrum

Jensen, Detlef

Papavassiliou, Maria

Seppelt, Konrad

Steudel, Ralf

Zeitschrift für Naturforschung B

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Notizen 245 Spectroscopic Evidence for Pseudorotation of Seven-Membered Chalcogen Rings in Solution [1] Ralf Steudel*, Maria Papavassiliou, and Detlef Jensen Institut für Anorganische und Analytische Chemie, Technische Universität Berlin, D-1000 Berlin 12, F R G Konrad Seppelt Institut für Anorganische und Analytische Chemie, Freie Universität Berlin, D-1000 Berlin 33, F R G Z. Naturforsch. 43b, 245-247 (1988); received October 7/October 12, 1987 77Se NMR Spectra, Raman Spectra, Elemental Sulfur, Selenium Sulfides The 77Se NMR spectrum of l,2-Se2S5 exhibits one singlet at 1077.3 ppm indicating pseudorotation in solution; the same conclusion is reached from the solution Raman spectrum of S7 showing characteris-tic line broadening compared to the solid state spec-trum. The homocyclic S7 molecule is a constituent of gase-ous and liquid elemental sulfur, of solidified liquid sulfur, of irradiated sulfur solutions, and of many chemically prepared sulfurs [2]. Evidence for pseudorotation of S7 in the vapor phase at elevated temperatures has first been derived from the molar entropy calculated by statistical methods; the calcu-lated and experimental values agree only when pseudorotation is postulated [3], Later the height of the torsional barrier of SS bonds in sulfur rings has been estimated from the heat of formation of S7 as =S24 kJ/mol [4] which would result in pseudorotation at 20 °C. We now report direct evidence for pseudorotation of seven-membered chalcogen rings at room temperature by which process all ring atoms become equivalent within a certain time interval due to a simultaneous rotation (torsion) about several of the ring bonds. The chair-like heterocycle l,2-Se2S5 according to its Raman spectrum has two neighboring Se atoms in positions 4 and 6 of Fig. 1 resulting in non-equivalent selenium atoms [5]. We have now prepared chro-matographically pure l,2-Se2S5 from titanocene pen-tasulfide and Se2Cl2, and recorded the 77Se NMR * Reprint requests to Prof. Dr. R. Steudel. Verlag der Zeitschrift für Naturforschung, D-7400 Tübingen 0932 - 0776/88/0200 - 0243/$ 01.00/0 Fig. 1. Molecular structure of the chalcogen rings S7 [6a] and l,2-Se5S2 [6b] and most probably also of l,2-Se2S5 [5], spectrum in CS2 solution (natural abundance of 77Se, Jeol FX90Q spectrometer, 17.03 MHz, 6 /us pulse width). After 2 h (ca. 20,000 scans) the spectrum showed only one peak at 1077.3 ppm [(CH3)2Se ext. standard]; see Fig. 2. No other signal was observed in the range 1235—65 ppm. We therefore assign the singlet at 1077.3 to l,2-Se2S5. After 12 h (ca. 120,000 scans) three additional peaks at 784.5, 653.4, and 559.9 ppm with an approximate intensity ratio of 1:2:1 were observed (see Fig. 2). HPLC analysis of this solution showed the presence of SeS5 and l,2,3-Se3S5 in addition to l,2-Se2S5 as the only species [5]. The six-membered ring SeS5 previously not observed by NMR and the eight-membered ring l,2,3-Se3S5 result from disproportionation of l,2-Se2S5 according to equation (1) [5], 2 1,2-Se2S5 —» SeS5 + l,2,3-Se3S5 (1) The 77Se NMR spectrum of l,2,3-Se3S5 is known [7] (signals at 654 and 561 ppm, intensity ratio 1:2); con-sequently we assign the peaks at 653.4 and 559.9 ppm to 1,2,3-Se3S5 and the remaining peak at 784.5 ppm to SeS5. The intensity ratio fits equation (1). The NMR signals of SeS5 and l,2-Se2S5 do not coincide with the signals of 14 eight-membered rings of type Se„S8_„ (n = 1---8) which have been found in the region 560-729 ppm [7]. It should therefore be possible to detect six- and seven-membered selenium sulfide rings even in the presence of a large excess of Se„S8_„ molecules as in extracts of quenched melts of the two elements. Our spectra show that in solution the Se atoms of l,2-Se2S5 are magnetically equivalent on the NMR time scale which can only be understood assuming pseudorotation. It seems very unlikely that in solu-tion the Se atoms just move to the equivalent posi-tions 6 and 7 (see Fig. 1) but after crystallization are found again only in positions 4 and 6. Therefore, we conclude that pseudorotation of the entire ring takes place thus averaging also the two different Se posi-tions. To further demonstrate the pseudorotation of seven-membered chalcogen rings at 20 °C we have recorded the Raman spectrum of S7 dissolved in CS2 Bereitgestellt von | Technische Universität BerlinAngemeldetHeruntergeladen am | 02.01.19 11:382 4 6 Notizen 1200 1000 800 600 400 200 ppm Fig. 2. 77Se NMR spectra of freshly dissolved l,2-Se2S5 in CS2 (bottom) and of the same solution measured with data accumulation for 12 h and showing the decomposition products SeSs (784.5 ppm) and l,2,3-Se3S5 (653.4 and 559.9 ppm) in addition (top). The two small satellites of the Se2S5 signal (see inset) have been identified as spinning side bands. and compared it to the published solid state spectrum [3] (see Fig. 3). Pseudorotation is known to influence the half-widths of certain stretching modes dramati-cally due to coupling with low lying torsional modes and in particular due to the splitting of vibrational energy levels of the double minimum potential re-sulting in a number of allowed transitions of very similar energy (see the analyses for cyclopentane [8] and cycloheptane [9] and their derivatives). In solu-tion the expected closely neighboring lines are not resolved and broad Raman signals will result. The spectra in Fig. 3 show that the half-width of the two SS stretching modes of S7 near 360 and 400 cm"1 increases from 8 and 4 cm - 1 , respectively, in solid S7 to 24 and 31 cm - 1 , respectively, for the CS2 solution. On the other hand, the half-width of the totally sym-metrical bending mode at 239 cm - 1 although split into two lines in the solid state does not differ signifi-cantly in the two spectra indicating that neither temperature nor collisional (relaxation time) effects are responsible for the broad Raman lines in the solution spectrum. Pseudorotation therefore pro-vides the best if not the only explanation. Financial support by the Deutsche Forschungsgemein-schaft and the Verband der Chemischen Industrie is grate-fully acknowledged. Bereitgestellt von | Technische Universität BerlinAngemeldetHeruntergeladen am | 02.01.19 11:38Notizen 247 100 150 200 250 300 350 400 450 500 550 1 /cm Fig. 3. Raman spectra of crystalline S7 at -100 °C (top. slit widths 1.9 cm"') and of S7 dissolved in CS2 at 20 °C (bottom, slit widths 3.19 cm"1) recorded using 647.1 nm laser excitation ( ISA Raman spectrometer with double monochromator [10]). [1] Sulfur Compounds, Part 113; for Part 112 see R. Steudel, Th. Göbel, and G. Holdt, Z. Naturforsch. 43 b, 203 (1988). [2] R. Steudel, Elemental Sulfur and Related Homocyclic Compounds and Ions, in A. Müller and B. Krebs (eds): Sulfur - The Significance for Chemistry, for the Geo-, Bio- and Cosmosphere and Technology, p. 3—37, Elsevier, Amsterdam (1984); Top. Curr. Chem. 102, 149 (1982). [3] R. Steudel and F. Schuster, J. Mol. Struct. 44, 149 (1978). [4] R. Steudel, Z. Naturforsch. 38b, 543 (1983). [5] R. Steudel and E.-M. Strauss, Angew. Chem. 96, 356 (1984); Angew. Chem., Int. Ed. Engl. 23, 362 (1984). [6] a) R. Steudel, J. Steidel, J. Pickardt, F. Schuster, and R. Reinhardt, Z. Naturforsch. 35b, 1378 (1980); b) R. Steudel, M. Papavassiliou, E.-M. Strauss, and R. Laitinen, Angew. Chem. 98, 81 (1986); Angew. Chem., Int. Ed. Engl. 25, 99 (1986). [7] R. Laitinen and T. Pakkanen, Inorg. Chem. 26, 2598 (1987); J . Chem. Soc. Chem. Commun. 1986, 1381. [8] C. Rafilipomanana, D. Cavagnat, R. Cavagnat, J. C. Lassegues, and C. Biran, J. Mol. Struct. 127, 283 (1985); C. Rafilipomanana, D. Cavagnat, and J. C. Lassegues, J. Mol. Struct. 129, 215 (1985); T. Shimanouchi, J. Chem. Phys. 47, 5018 (1967). [9] D. F. Bocian and H. L. Strauss, J. Am. Chem. Soc. 99, 2866, 2876 (1977); D. F. Bocian, H. M. Pickett, T. C. Rounds, and H. L. Strauss, J. Am. Chem. Soc. 97, 687 (1975). [10] For details of the Raman equipment see R. Steudel and B. Holz, Z. Naturforsch. 42b, 691 (1987). Bereitgestellt von | Technische Universität BerlinAngemeldetHeruntergeladen am | 02.01.19 11:38Nachdruck — auch auszugsweise - nur mit schriftlicher Genehmigung des Verlages gestattet Satz und Druck: Allgäuer Zeitungsverlag GmbH, Kempten Bereitgestellt von | Technische Universität BerlinAngemeldetHeruntergeladen am | 02.01.19 11:38

Spectroscopic Evidence for Pseudorotation of Seven-Membered Chalcogen Rings in Solution [1]

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Spectroscopic Evidence for Pseudorotation of Seven-Membered Chalcogen Rings in Solution [1]

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