Alexander A. Tishkov

Direct observation of the ionization step in solvolysis reactions: electrophilicity versus electrofugality of carbocations.

Rates and equilibria of the reactions of highly stabilized amino-substituted benzhydrylium ions (Ar2CH+) with carboxylate ions have been determined photometrically in acetone and acetonitrile solutions. Treatment of covalent benzhydryl carboxylates (Ar2CH-O2CR) with aqueous acetone or acetonitrile leads to the regeneration of the colored amino-substituted benzhydrylium ions Ar2CH+, which do not undergo subsequent reactions with the solvent. One can, therefore, directly measure the first step of S(N)1 reactions. The electrofugality order, i.e., the relative ionization rates of benzhydryl esters Ar2CH-O2CR with the same anionic leaving group, does not correlate with the corresponding electrophilicity order, i.e., the relative reactivities of the corresponding benzhydrylium ions Ar2CH+ toward a common nucleophile. Thus, benzhydrylium ions which are produced with equal rates by ionization of the corresponding covalent esters may differ by more than 2 orders of magnitude in their reactivities toward nucleophiles, e.g., carboxylate ions. Variable intrinsic barriers account for the breakdown of the rate-equilibrium relationships. Complete free-energy profiles for the ionization of benzhydryl carboxylates Ar2CH-O2CR are constructed, which demonstrate that the transition states of these ionizations are not carbocation-like. As a consequence, variation of the solvent-ionizing power Y has only a small effect on the ionization rate constant (m = 0.35 to 0.55) indicating that small values of m in the Winstein-Grunwald equation do not necessarily imply an S(N)2 type mechanism.

Compact laser flash photolysis techniques compatible with ultrafast pump-probe setups

Two new transient absorption measurement techniques are described which use commercially available pulsed laser diodes or high-power light-emitting diodes (LEDs) as monitoring beam. The semiconductor devices substitute the probe in a kilohertz-repetition-rate ultrafast pump-probe setup. A fully functional and highly compact laser flash photolysis system reaching the nanosecond to millisecond time scale is thereby added to a state-of-the-art femtosecond system. The sample is excited with UV-Vis tunable femtosecond pulses, and for the electronically synchronized probing light either subnanosecond pulsed laser diodes for selected wavelengths or LEDs covering the visible to near infrared and UV regions are used. The applicability and reliability of the devices are demonstrated for various probe wavelengths in the visible by the investigation of excited-state decay or photoinduced bimolecular reactions. The time resolution is found to be 400 ps for the pulsed laser diodes and a few nanoseconds for the LEDs. Th...