With macrocycle 3 the amide protons Hc exhibit strong NOEs with protons Ha and Hb, consistent with the NHs pointing both inside and outside the cavity, as observed in the model (Figure ) as well as in the X-ray crystal structure of the macrocycle (Figure ), which corroborated the H-bond interactions between two of the carbonyls with the two opposite NH groups. Anion binding. The interaction of macrocycles 1-3 with the three anionic guests Cl -, H2PO4 -, and H2P2O7 2-was studied through 1 H-NMR titrations with the corresponding tetrabutylammonium salts in DMSO-d6 with 0.5% H2O added. For macrocycles 1 and 2, the 1 H NMR signals of the protons pointing into the cavity showed clear downfield shifts upon addition of TBACl (Figures and), while the signals for protons Hc and Hb of macrocycle 3 were shifted less (Figure ). The displacements of the 1 H NMR signals of the protons pointing into the cavity were fitted with a 1:1 binding model and K1:1 values for Cl -of 1.2•10 2 M -1 and 2.1•10 3 M -1 were obtained for 1 and 2 respectively (Table ).