CONTACT ION-PAIRS - PICOSECOND DYNAMICS OF SOLVENT SEPARATION, INTERNAL RETURN, AND SPECIAL SALT EFFECT

Direct observation by time-resolved picosecond spectroscopy of the contact ion pair (CIP) allows the microdynamical rate constants for its (a) annihilation by internal return, (b) relaxation to the solvent-separated ion pair (SSIP), and (c) ionic exchange in the special salt effect to be quantitatively evaluated for the solvolysis mechanism (Scheme I) originally formulated by Winstein and co-workers. The relevant contact ion pair [Ar+,T-] is spontaneously generated by the 20-ps laser pulse excitation of the EDA complex from tetranitromethane (TNO2) and anthracene (Ar). The use of a series of 9-Y-anthracenes with graded substituents (Y = nitro > cyano > formyl > bromo > hydrogen > vinyl > methyl) provides the structurally related cations (Ar+) possessing diagnostic absorption bands and of significantly differing reactivities to modulate ion-pair dynamics. The temporal relaxation of each CIP follows uniquely from the time-resolved spectroscopy of Ar+. The varied (picosecond) decay patterns of the cation absorbance are characterized by first-order kinetics (k(obsd)) to limiting residuals (R)-all in solvents varying from acetonitrile (polar), chloroform, dichloromethane, benzene, to n-hexane (nonpolar). The rigorous dissection of k(obsd) and R for the contact ion pair in Scheme I yields the individual rate constants for internal return (k1) and solvent separation into SSIP (k2). The variation in ion-pair microdynamics (k1, k2) with changes in cation reactivity (given by the Hammett substituent parameter for Y) and solvent polarity (taken as the Kosower Z-value) is discussed in terms of CIP structure and the driving force for internal return. The interception of the contact ion pair by added salt is measured by the second-order rate constant (k(x)) for ion-pair exchange (eq 15). The time constant for k(x)[salt] identifies the temporal operation of the special salt effect on the contact ion pair to be as short as 10 ns at salt concentrations as low as 5 X 10(-3) M. Ion-pair reversibility CIP half arrow right over half arrow left SSIP is exergonic by approximately 2 kcal mol-1 in dichloromethane, and thus external ion-pair return (k-2) is shown to be too slow to affect the picosecond dynamics of the contact ion pair.