Complexation stoechiometry determined by application of chemometrics to time domain nuclear magnetic resonance signals

Abstract
The objective of this study is to demonstrate the feasibility of using multivariate statistical techniques applied to Time Domain - Nuclear Magnetic Resonance (TD-NMR) signals to determine the stoechiometry of complexation reactions. The complex used here is the well known EDTA-manganese complex. The measurement is based on the change in the interactions between the paramagnetic manganese cation and the solvent depending on whether it is free in solution or chelated with EDTA. Free manganese significantly increases the relaxation rate of the solvating water protons while chelated manganese has a weaker effect. The complexation is studied using Job's Continuous Variations Method. The system is analysed in two ways. In a first step R1 and R2 relaxation rates are calculated by non-linear regression and are shown to be sensitive parameters for determining the stoechiometry of the complex. In a second step, the normalised TD-NMR signals are directly analysed using Principal Components Analysis (PCA) and Evolving Factor Analysis (EFA). These multivariate statistical analyses of the signals are also shown to be good methods for detecting the complexation.