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

The two previous  posts[1],[2] 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[3] 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[4], it seems unlikely that the probably even more unstable S7ONH could ever be isolated, but there is a challenge!

# Acceptor S-S bond Donor Lp NBO E(2) Energy
1 S4-S5 O8 31.9
2 S1-S2 N9 29.6
3 S1-S6 N9 27.0
4 S5-S6 O8 20.4
5 S4-S5 S7 16.8
6 S1-S2 S3 16.4
7 S3-S7 S2 15.4
8 S3-S7 S4 15.2

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.

Author

References

  1. H. Rzepa, "Cycloheptasulfur sulfoxide, S<sub>7</sub>O – Anomeric effects galore!", 2025. https://doi.org/10.59350/rzepa.28515
  2. H. Rzepa, "Cyclo-Heptasulfur, S<sub>7</sub> – a classic anomeric effect discovered during a pub lunch!", 2025. https://doi.org/10.59350/rzepa.28407
  3. H. Rzepa, "S7O2 NBO7 Cs symmetry", 2025. https://doi.org/10.14469/hpc/15235
  4. R. Steudel, R. Reinhardt, and T. Sandow, "Bond Interaction in Sulfur Rings: Crystal and Molecular Structure of <i>cyclo</i>‐Heptasulfur Oxide, S<sub>7</sub>O", Angewandte Chemie International Edition in English, vol. 16, pp. 716-716, 1977. https://doi.org/10.1002/anie.197707161

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