S1 Anion-π Interactions in Four-Membered Rings Carolina Estarellas, M. Carmen Rotger, Magdalena Capó, David Quiñonero, Antonio Frontera,* Antoni Costa and Pere M. Deyà Departament de Química, Universitat de les Illes Balears, Crta. de Valldemossa km 7,5, 07122 Palma, Spain Fax: (+) 34 971173426 E-mail: toni.frontera@uib.es Supporting Information (13 pages including this page) Experimental details (pages S2-S6) List of the CSD reference codes obtained from the search of anion-π interaction in four-membered rings (page S6) Selected structures exhibiting anion–π interactions (pages S7-S9) Atoms-in-Molecules analysis (page S9) Cartesian coordinates of the RI-MP2/aug-cc-pVTZ optimized structures 3-11 (pages S10-S13)