Bipyridylium quaternary salts and related compounds. Part 3.—Weak intermolecular charge-transfer complexes of biological interest, occurring in solution and involving paraquat

Abstract

Complex formation between paraquat dichloride and (a) a series of π-donor molecules, and (b) a series of donors capable of nπ participation, has been studied by u.-v. and n.m.r. spectroscopy. In addition, the acceptor properties of p-benzoquinone with the same donors have been examined. A linear correlation was found between the charge-transfer band frequencies for the two sets of complexes. By correlating band energies with the calculated highest occupied orbital energies of the donor molecules, a value of 1.24 eV has been deduced for the electron affinity of paraquat. Association constant values for the complexes have been determined, falling in the range of 2–20 l. mole^–1. In the typical case of hydroquinone as the donor, enthalpy, entropy and free energy changes involved in complexing have been determined. The values are within the range commonly found in biological systems.