Assignment of pH‐dependent NMR spectra of the scorpiand macrocycle: an application of titration profiles and spectral linewidths

Routine homo‐ and heteronuclear 2D NMR experiments (COSY, HMQC, TOCSY and ROESY) were performed for the complex macrocyclic pentaamine ligand 1‐(2′‐aminoethyl)‐1,4,8,11‐tetraazacyclotetradecane ( 1 , scorpiand), derived from 1,4,8,11‐tetraazacyclotetradecane (cyclam). However, some its ¹³C and ¹H resonances were distinguished only based on the similarity between profiles of relevant pH–chemical shift plots and/or linewidth considerations. Following this empirical method relying on the assumption that titration curves of nuclei in comparable chemical environments have comparable shapes, it was possible to assign all chemical shifts (¹³C, pH 0.25–13.4; ¹H, pH 10–11.5) for some different protonated species H_n 1 ⁿ⁺. A multiple experimental approach used for recognizing their NMR signals is offered as a general assignment procedure in the case of polyazamacrocycles. Copyright © 2002 John Wiley & Sons, Ltd.