Way back in 1977, the crystal structure of the sulfur ring S7 was reported.[1] The authors noted that “The δ modification of S7 contains bonds of widely differing length: this has never been observed before in an unsubstituted molecule.” No explanation was offered, although they note that similar effects have been observed in S8O, S7I+ and S7O. The S7 molecule was yesterday brought to my attention (thanks Derek!) over a pub lunch and in the time honoured manner of scientists, sketched out on a napkin – with a pen obtained from the waitress!. As an “organic chemist”, I immediately thought “anomeric effects”. And so indeed it has proven. A calculation using the MN15L/Def2-TZVPP DFT method and analysis using the Weinhold NBO7 procedure[2] reveals the following structure (with Cs symmetry) and indeed the four unique S-S distances are all different (experimental values in parentheses). So how does this arise?
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References
- R. Steudel, R. Reinhardt, and F. Schuster, "Crystal and Molecular Structure of <i>cyclo</i>‐Heptasulfur (δ‐S<sub>7</sub>)", Angewandte Chemie International Edition in English, vol. 16, pp. 715-715, 1977. https://doi.org/10.1002/anie.197707151
- H. Rzepa, "Cyclo-Heptasulfur, S7 – a classic anomeric effect discovered during a pub lunch!", 2025. https://doi.org/10.14469/hpc/15228