Molecule where an sp3 chalcogen is connected to an sp2 atom I am looking for a small molecule where an $\mathrm{sp^3}$ atom from Group 16 (O, S, Se,...) is connected to an $\mathrm{sp^2}$ or resonant atom of another column?

Background: The universal force field defines the above case in its torsion
rules and I would like to find an example/application of such a structure
to understand it better.

Thanks!
Out of the blue, consider triphenyloxonium cation. It is non-planar thanks to bulkiness of the phenyl groups.

Another possible example is mono-epoxide of allene. Considering oxygen atom as $sp^2$ here is unconvincing since it would involve huge angular strain, much higher than for a structure with $sp^3$ oxygen.