In this series of posts about the electronic effects in small sulfur rings[cite]10.59350/rzepa.28615[/cite] I have explored increasingly large induced geometric effects. Here is the largest so far, for the compound S7I1+[cite]10.1021/ic50225a048[/cite]
Archive for May, 2025
S7I1+: The largest anomeric effect exhibited by sulfur.
Wednesday, May 21st, 20255-Imino-5λ4-heptathiepane 3-oxide. More exuberent anomeric effects.
Tuesday, May 20th, 2025The two previous posts[cite]10.59350/rzepa.28515[/cite],[cite]10.59350/rzepa.28407[/cite] on the topic of anomeric effects in 7-membered sulfur rings illustrated how orbital interactions between the lone pairs in the molecules and S-S bonds produced widely varying S-S bond lengths in the molecules, some are shorter than normal (which is ~2.05Å for e.g. the S8 ring) by ~ 0.1Å and some are longer by ~0.24Å. Here we extend this to the unknown molecule shown below.
Cycloheptasulfur sulfoxide, S7O – Anomeric effects galore!
Monday, May 19th, 2025The monosulfoxide of cyclo-heptasulfur was reported along with cycloheptasulfur itself in 1977,[cite]10.1002/anie.197707161[/cite] along with the remarks that “The δ modification of S7 contains bonds of widely differing length: this has never been observed before in an unsubstituted molecule. and “the same effect having also been observed in other sulfur rings (S8O, S7I1+ and S7O).” Here I take a look at the last of these other molecules, the monosulfoxide of S7, as a follow up to the commentary on S7 itself.[cite]10.59350/rzepa.28407[/cite]
Cyclo-Heptasulfur, S7 – a classic anomeric effect discovered during a pub lunch!
Friday, May 16th, 2025Way back in 1977, the crystal structure of the sulfur ring S7 was reported.[cite]10.1002/anie.197707151[/cite] 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[cite]10.14469/hpc/15228[/cite] 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?