1) Four-membered ring 
Trimethylene sulfide | Section1 = {Chembox Identifiers | CASNo = 287-27-4 | CASNo_Ref = {{cascite|changed|??} | PubChem = 9251 | PubChem_Ref = {Pubchemcite|correct|PubChem} | ChemSpiderID = 8895 | ChemSpiderID_Ref = {chemspidercite|correct|chemspider} | EINECS = 206-015-0 | UNNumber = 1993 | Beilstein = 102383 | SMILES = C1CSC1 | StdInChI = 1S/C3H...