A general nuclear magnetic resonance analysis of hetero-association of aromatic molecules in aqueous solution

Abstract

A general nuclear magnetic resonance analysis of a statistical-thermodynamical model of hetero-association of aromatic molecules in solution has been developed to take “edge effects” into consideration, i.e., the dependence of proton chemical shifts on the position of the molecule situated inside or at the edge of the aggregate. This generalized approach is compared with a previously published model, where an average contribution to proton shielding is considered irrespective of the position of the molecule in the stack. Association parameters have been determined from experimental concentration and temperature dependences of 500 MHz proton chemical shifts of the hetero-association of the acridine dye, proflavine, and the phenanthridinium dye, ethidium bromide, in aqueous solution. Differences in the parameters in the range 10%–30% calculated using the basic and generalized approaches have been found to depend substantially on the magnitude of the equilibrium hetero-association constant Khet—the larger the value of Khet, the higher the discrepancy between the two methods.