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Accueil > Publications > Recherche par années > Années 1990 > 1999

Szilagyi, E ; Toth, E ; Brucher, E ; Merbach, AE

Lanthanide(III)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid complexes in acidic medium : significant decrease in water exchange rate

Journal of The Chemical Society-Dalton Transactions (15) 2481-2486

par Administrateur - publié le

Abstract :

UV-Vis and lanthanide-induced O-17 shift measurements on the complex [Eu(DOTA)](-) (H(4)DOTA=1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) have shown that the inner co-ordination sphere of the Ln(3+) ion is not affected on protonation which suggests that the proton is attached to a non-co-ordinated oxygen atom of a carboxylate group. Proton NMR measurements performed as a function of the H+ concentration revealed that the protonation slightly accelerates the intramolecular dynamic processes : the enantiomerization for [La(DOTA)](-) and the enantiomerization and interconversion between the major and minor isomer for [Eu(DOTA)](-). Contrary to first glance expectations, the water exchange rate on [Gd(DOTA)(H2O)](-) decreases significantly with increasing extent of protonation, and at 1.0 M H+ concentration is about ten times lower than in neutral media. In 1.0 M acidic solution the proton relaxivities were found to be higher than expected solely on the basis of the water exchange rates. This finding is interpreted with a faster proton exchange in acidic solutions which is the consequence of the catalytic effect of H+ ions.