Dielectric relaxation studies of charge-transfer complexes in solution

摘要

According to Mulliken's theory,1 charge-transfer complexes are formed by interaction between molecules acting as electron donors (e.g., aromatic hydrocarbons, dioxan) and other molecules acting as electron acceptors (e.g., quinones, aromatic nitrο-compounds). The formation of charge-transfer complexes is accompanied by the appearance of an ultraviolet absorption which is absent in the spectra of the individual components. The intensity of this characteristic ultraviolet absorption is used to calculate the equilibrium constant in the reaction Donor + Acceptor ⇌ Complex assuming a 1:1 stoichiometry for the complex. The wave function of the complex is written as ψn = aψ0 + bψ1 where ψ0 is the “no-bond” wave function and contains the “classical” inter molecular interactions (e.g., dispersion forces, dipole-induced dipole forces) between donor and acceptor, ψ1 is the “dative-bond” wave function where an electron has been transferred from the donor to the acceptor, and the coefficients a and b determine the relative contributions of the “no-bond” and the “dative-bond” structures. In the ground electronic state of the complex, the “nobond” wave function predominates (a b),2 and electrostatic interactions provide the major contribution to the interaction energy between acceptor and donor. The calculated electrostatic energy between trinitrobenzene and benzene at an intermolecular distance of 3.5 Å is 7.6 kJ mole−1, 3 and the observed enthalpy of formation of the complex in carbon tetrachloride is 7.1 kJ mole−1, 4 Complexes between nonpolar components exhibit a finite orientation polarization. 2 Part of this polarization arises from any charge transfer between the donor and acceptor, but contributions from induced dipoles are very significant.5 In the equilibrium Donor + Acceptor ⇌ Complex between a nonpolar donor and a nonpolar acceptor, the formation and subsequent decomposition of the complex provides a relaxation mechanism for the orientation polarization. The relaxation time observed in weak complexes is then a function of the lifetime of the complex. The results describe some dielectric measurements on such weak complexes.