Simultaneous Determination of Several Charge-transfer Complex Association Constants using Nuclear Magnetic Resonance

Abstract

A METHOD for determining the association constants of organic charge–transfer complexes in solution using nuclear magnetic resonance has recently been described^1,2. For a series of solutions containing an electron donor species (D) and an electron acceptor species (A) which interact by a very fast reversible process to form a 1 : 1 charge–transfer complex (DA), if one of the reactants, say D, is in large excess over the second reactant, then² where [D]₀ is the total (free and complexed) concentration of the donor, Δ is the observed chemical shift of a nucleus in A in the complexing medium relative to the shift of this nucleus in uncomplexed A, and Δ₀ is the shift of this nucleus in the acceptor moiety of the pure complex, again relative to the shift in uncomplexed A. K_c is the association constant defined by K_c = [DA]/[D][A], where the concentrations represent the equilibrium values for the various species. A plot of Δ/[D]₀ against Δ should yield a straight line, the gradient of which is equal to −K_c and the intercept with the ordinate equal to K_cΔ₀.