Direct Observation of Ion-Pair Dynamics

Article abstract of DOI:10.1021/ja00312a014

The intimate ion pair is spontaneously generated by the change-transfer excitation of the electron donor-acceptor complex of anthracene donors (A) and tetranitromethane (TNO2) with a 25-ps laser pulse.The kinetics of the subsequent ion-pair decay to the adduct (i.e., -> AT) is followed spectroscopically over the separate picosecond, nanosecond, and microsecond time domains, each with a different laser-flash system.Three distinctive rate profiles are observed: (a) the picosecond decay following first-order kinetics (k_I), the nanosecond decay also following first-order but slower kinetics (k_II), and the microsecond decay following second-order kinetics (k_III).The experimental rate constants k_I, k_II, and k_III are associated with the relaxation of the intimate or tight ion pair , the solvent-separated or loose ion pair , and the free ions , respectively, as originally formulated by Winstein and co-workers for solvolysis mechanisms.These kinetics data together with the measurements of the fractional partitioning of ion pairs allow all the microscopic rate constsnts relevant to ion pair dynamics to be separately evaluated.The Winstein ion-pair formulation is substantiated by the observation and quantitative treatment of the "common-ion" and "special" salt effects.The role of solvent is underscored by the unique kinetics responses of reactive and persistent cations derived from various 9-substituted and 9,10-disubstituted anthracenes, respectively, with changes in the polarity of the medium from benzene and dichloromethane to acetonitrile.The overall importance of charge annihilation and separation in the microdynamics of transient ion pairs is underscored by a comparison with the bimolecular kinetics of radical-radical interaction measured under comparable conditions.