Monomer shift, dimer shift and dimerization constant for self-association determined graphically from the NMR dilution shift: Pyrrolidone in CDCl3 as an example

We describe a graphical method for analysis of dilution NMR shift data, which permits the self-consistent determination of the monomer shift, dimer shift, and dimerization constant for systems involving a monomer-dimer equilibrium. The new method specifically avoids the manual extrapolation to zero concentration conventionally used to obtain the monomer shift. In addition, it provides quantitative measures for both the consistency and error of the determination. Measurements of the dilution shift for pyrrolidone in CDCl3 over a series of temperatures are used to demonstrate the applicability of the graphical method. The enthalpy and entropy of the dimerization reaction are determined via a van't Hoff plot of the temperature dependence of the dimerization constant.