Estimation of Dimerisation Constants from Complexatin‐Induced Displacements of 1H‐NMR Chemical Shifts: Dimerisation of Caffeine

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Abstract
The determination of the dimerisation constant (KD) for the weak self‐association of a compound C in dilute solution according to the equilibrium, 2C⇌C2 is described. The method uses chemical shifts measured on a series of solutions of C at different concentrations: the optimum KD is defined by a linear regression best‐fit procedure, which simultaneously determines optimum values for δo and also for δ∞, the intrinsic chemical shifts for nuclei in the monomer and dimer species. The dimerisation of caffeine in D2O is used as a model to demonstrate the working of the method and the quality of results obtained. The most probable value of KD for caffeine at 30.5° is found in the range 5.5–6.0 kg solution · mol−1, and the enthalpy and entropy of dimerisation are found to be ΔH⊖ = −15.1 kJ · mol−1 and ΔS⊖ = −35.3 J · °C−1 · mol−1, respectively. The influence of small errors in δo on the confidence limits of KD is discussed.