LI-7, LI-6, NA-23 AND CS-133 MULTINUCLEAR NMR-STUDIES OF ADDUCTS FORMED WITH SHIFT-REAGENT, TMDOTP5

Interactions between Li+, Na+, Cs+, Ca2+ and Mg2+ and the shift reagen t (SR), TmDOTP5-, were studied by Li-7, Li-6, Na-23 and Cs-133 multinu lear NMR spectroscopy. The unusually large paramagnetic shifts (>>100 ppm) observed in the cation NMR resonances at low alkali metal ion to TmDOTP5- ratios indicated that the cations bind near the 4-fold symmet ry axis of the complex. The geometric parameters for the adducts forme d between Li-7, Li-6, Na-23 and Cs-133 cations and TmDOTP5- were obtai ned from shift and relaxation rate data and compared with those calcul ated using MMX energy minimization techniques. It is found that the la rger Cs+ ion lies closest to the 4-fold axis of symmetry and displays the largest binding constant. The smaller Li+ ion deviates most from t he 4-fold symmetry axis and has a less favorable binding interaction t han Cs+. Na+ lies somewhere between these two extremes but has the lar gest limiting shifts due to its favorable distance from the paramagnet ic metal center. A significant reversal of the Na-23 shift upon additi on of Ca2+ and Mg2+ indicate that these two divalent cations form very stable 1:1 adducts with TmDOTP5- (log K (Ca2+)=5.69 and log K-1 (Mg2)=3.85). An analysis of the Mg2+ titration data shows that the 2Mg(2+) :TmDOTP5- adduct becomes dominant at high Mg2+/TmDOTP5- ratio (log K-2 (Mg2+)=2.17). The significant influence of NH4+ on the TmDOTP5--induc ed Li-6 shifts suggests that the NH4+ ion may form multiple H-bonds wi th the SR along the 4-fold axis of symmetry.