AI) Crown-Ethers

12) Xray-derived Structure of the Complex Lanthanum Nitrate and 1,9-Diaza-18crown6

Alla K. Shestakova, Vyacheslav A. Chertkov, Hans-Jörg Schneider, Konstantin A. Lysenko, Org. Letters, 2001, 3, 325-327

The metal ion has the total coordination number of twelve, including 3x2 sites from 3 nitrate anions and 6 from the macrocycle (check the distances!). The shape of the macrocycle can be characterized as a sequence of B-C-B--C-B conformations, where B is a boatlike connection and C chair-like connection (check the torsional angels!, for answers see the reference).