Figure 11.

(a) The schematic representation of four possible structures of cyclic heterotetramers, formed by two different molecules (blue and red in this case); letters a, b and c indicate chemically non-equivalent bridging protons. (b) The proposed structure of heterotetramer of acids 1 and 4 (matches the spectral pattern seen in Figure 7 and Figure 8). (c) The proposed structure of heterotetramer of acids 1 and 3 (matches the spectral pattern seen in Figure 5 and Figure 6).