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

摘要

Interactions between Li+, Na+, Cs+, Ca2+ and Mg2+ and the shift reagent (SR), TmDOTP5-, were studied by Li-7, Li-6, Na-23 and Cs-133 multinulear 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 symmetry axis of the complex. The geometric parameters for the adducts formed between Li-7, Li-6, Na-23 and Cs-133 cations and TmDOTP5- were obtained from shift and relaxation rate data and compared with those calculated using MMX energy minimization techniques. It is found that the larger Cs+ ion lies closest to the 4-fold axis of symmetry and displays the largest binding constant. The smaller Li+ ion deviates most from the 4-fold symmetry axis and has a less favorable binding interaction than Cs+. Na+ lies somewhere between these two extremes but has the largest limiting shifts due to its favorable distance from the paramagnetic metal center. A significant reversal of the Na-23 shift upon addition 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--induced Li-6 shifts suggests that the NH4+ ion may form multiple H-bonds with the SR along the 4-fold axis of symmetry. [References: 35]