5-Imino-5λ⁴-heptathiepane 3-oxide. More exuberent anomeric effects.

The 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.

The usual MN15L/Def2-TZVPP calculation[cite]10.14469/hpc/15235[/cite] gives the calculated geometry shown below. In parentheses are the calculated S-S vibrational wavenumbers (some are marked with ~ since these modes are contaminated by mixing with other parts of the molecules).

The interaction energies between the donor and acceptor, E(2), are shown below. Numbers 5-8 are the same as was identified for the parent molecule S₇, but the energies have increased substantially (previously 12.3/10.1 kcal/mol). The Wiberg bond index for the strongest bond (S2-S3) is 1.276 and the weakest (S1-S2) is 0.610, quite some variation! Given that the known S₇O was already very unstable[cite]10.1002/anie.197707161[/cite], it seems unlikely that the probably even more unstable S₇ONH could ever be isolated, but there is a challenge!

There are numerous compounds with six, seven and eight membered sulfur rings, and it would always be worth keeping an eye out for unusually short or long S-S bonds in them, since they may well be more manifestations of these sulfur anomeric effects.