Masiker Marilyn C, Mayne Charles L, Eyring Edward M
Department of Chemistry, University of Utah, Salt Lake City, Utah 84112, USA.
Magn Reson Chem. 2006 Mar;44(3):220-9. doi: 10.1002/mrc.1787.
NMR chemical shift titration has been widely used as a method for the determination of stability constants. Systems involving metal-ligand complexation have been investigated using a number of methodologies. There are significant differences in the values reported for stability constants obtained by different experimental methods, such as calorimetry and ion selective electrode (ISE) titrations; nor has NMR chemical shift titration always yielded consistent results. Different researchers have obtained different results for the same system with results differing by as much as an order of magnitude. The chemical shift data are generally plotted against the concentration ratio of the metal and ligand for a set of solutions. A nonlinear least squares fitting method using an analytical solution of the cubic equation for the equilibrium concentration of the free ligand is used in this study and compared with methods used in the literature. Second-order association constants for the LiClO(4):12-crown-4 system in acetonitrile and the NaClO(4):12-crown-4 system in methanol are reported. Formation of both 1:1 and 1:2 metal-ligand complexes are considered. The LiClO(4):12-crown-4 acetonitrile system had been investigated previously by NMR titration but only 1:1 complexation was considered in that study. This study provides convincing evidence that both 1:1 and 1:2 complexes are important, at least, in the lithium system. A Monte Carlo investigation of the propagation of errors from the chemical shifts to the stability constants shows that the choice of data analysis methods may, in part, contribute to discrepancies and that the nonlinear nature of the model can dramatically affect the error limits on the stability constants.
核磁共振化学位移滴定法已被广泛用作测定稳定常数的一种方法。涉及金属 - 配体络合的体系已通过多种方法进行了研究。通过不同实验方法（如量热法和离子选择性电极（ISE）滴定）获得的稳定常数报告值存在显著差异；核磁共振化学位移滴定也并非总能得到一致的结果。不同研究人员对同一体系得到了不同的结果，结果相差高达一个数量级。通常将化学位移数据与一组溶液中金属和配体的浓度比作图。本研究使用了一种非线性最小二乘法拟合方法，该方法采用了自由配体平衡浓度的三次方程解析解，并与文献中使用的方法进行了比较。报道了乙腈中LiClO₄:12 - 冠 - 4体系和甲醇中NaClO₄:12 - 冠 - 4体系的二级缔合常数。考虑了1:1和1:2金属 - 配体络合物的形成。LiClO₄:12 - 冠 - 4乙腈体系此前已通过核磁共振滴定法进行了研究，但该研究仅考虑了1:1络合。本研究提供了令人信服的证据，表明至少在锂体系中，1:1和1:2络合物都很重要。对从化学位移到稳定常数的误差传播进行的蒙特卡罗研究表明，数据分析方法的选择可能在一定程度上导致差异，并且模型的非线性性质会极大地影响稳定常数的误差范围。
