sequences. Hydrazide derivatives 1, which has two intramolecular S(6) RO.H-N hydrogen bonds, and 2 complex to afford two fastly exchanging isomeric heterodimers 1.2 and 1.2' in chloroform, as a result of two different conformational arrangements of 2. An average binding constant K(assoc) of 4.7 x 10(4) M(-)(1) was determined for heterodimer 1.2 and 1.2' by (1)H NMR titration of 1 with changing 2 in chloroform-d