Spherand molecules are more rigid than thier ether counterparts. These are rings of rings that form collar shapes (why spherand, I just don't know). For example a six benzyl ring molecule

c12cc(C)cc(c2O) c3cc(C)cc(c3O) c4cc(C)cc(c4O) c5cc(C)cc(c5O) c6cc(C)cc(c6O) c7cc(C)cc1(c7O)
binds a lithium ion but not sodium or potassium. The binding energy is lower than a flexible ring, because there is less entropy loss.