QUANTITATIVE 2-DIMENSIONAL EXSY SPECTROSCOPY AND DYNAMIC BEHAVIOR OF A PARAMAGNETIC LANTHANIDE MACROCYCLIC CHELATE - YBDOTA (DOTA = 1,4,7,10-TETRAAZACYCLODODECANE-N,N',N'',N'''-TETRAACETIC ACID)

YbDOTA (DOTA: 1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid) is a rigid lanthanide chelate that adopts two conformations in slow exchange. The intra- and intermolecular exchanges within and between the two isomeric forms of YbDOTA have been investigated by a quantitative analysis of the pure absorption mode two-dimensional exchange H-1 spectra. The dynamic matrix was completely solved and the activation parameters for all exchanges were deduced from variable temperature measurements. Useful information on the dynamic processes were obtained despite the paramagnetism of Yb(III) and despite the short relaxation times and the very large NMR shifts induced by this ion. As suggested previously (Aime et al. Inorg. Chem. 1992, 31, 4291), the inter-and intramolecular exchange peaks were assigned respectively to a reorientation of the acetate substituents and to a complete inversion of the conformation of the chelate. The activation energy for the major --> minor species exchange is higher than for the minor --> major species exchange (DELTAGdouble dagger = 65.6 vs 61.4 kJ mol-1) presumably because of the greater steric crowding of the minor isomer. The accuracy of the measurements is discussed and the data are compared with previously published analyses.