Joshua L. Goodman

Radiative and nonradiative electron transfer in contact radical-ion pairs

Abstract The relationship between radiative and nonradiative electron transfer is explored for return electron transfer processes in the contact radical-ion pairs formed by excitation of ground state CT complexes. Using a conventional nonadiabatic theory of electron transfer, absolute rate constants for nonradiative return electron transfer, varying over more than two orders of magnitude, can be predicted from information obtained from analyses of the corresponding radiative processes. The effects of solvent polarity, driving force and molecular dimension on the rates of nonradiative return electron transfer are studied.

Photolysis of alkylhalodiazirines: Two pathways for vinyl halide formation

Abstract Photoacoustic calorimetry and product studios on the photochemical decomposition of methylhalodiazirines indicate two pathways for the formation of vinyl halides: 1) a [1,2] intramolecular hydrogen shift of the initially formed methylhalocarbene, 2) an excited state pathway which circumvents the ground state carbene. Results suggest that carbene/olefin or carbene/alcohol complexes are not involved in vinyl halide formation.

The energetics and kinetics of relaxed alkene triplet states as determined by pulsed time-resolved photoacoustic calorimetry

Abstract The energetics and lifetimes of a number of substituted olefin and diene triplets have been measured using photoacoustic calorimetry. All the acyclic triplets studied have triplet energies less than their spectroscopic values. This suggests they prefer a twisted relaxed geometry.

Photooxidative Cleavage of Organosilanes

Abstract Avoiding energy wasting return electron transfer reactions remains a central challenge in the design of high efficiency photoinduced electron transfer processes. One strategy for achieving this goal is to utilize compounds which as a result of electron transfer undergo rapid and irreversible chemical reaction in competion with the return electron process. Organosilanes represent a promising class of compounds that may fulfill this requirement. We have found that some organosilanes undergo rapid carbon-silicon bond cleavage when oxidized to their radical cations in photoinduced electron transfer reactions. Interestingly, these radical cation cleavage reactions occur by a rare nucleophile-assisted mechanism. We have investigated the reactivity of different classes of nucleophiles, and have determined the activation parameters for several of these reactions.

Intramolecular nucleophile-assisted cleavage of organosilane cation radicals

Abstract Organosilane cation radicals which have tethered nucleophiles are found to undergo rapid intramolecular nucleophilic substitution in both high and low polarity solvents. This reaction competes with both separation and return electron transfer within the primary ion radical pair generated by photoinduced bimolecular electron transfer. These results demonstrate the ability to effect ion radical substitutions in low polarity solvents.

Mechanistic studies on azoalkane cation radical decomposition

Abstract We report on the oxidative decomposition of two azoalkanes, azo-bis-2-methyl-2-propane and 3,3,6,6-tetramethyl-1, 2-diazacyclohex-2-ene. These azoalkane cation radicals decompose at rates 10 18 times faster than their neutral precursors. This can be attributed to a decrease of 24 kcal/mol in the CN bond dissociation energy on oxidation.

The determination of the heats of formation of carbonyl and nitrile ylides by photoacoustic calorimetry

Abstract Using photoacoustic calorimetry, the heats of formation of the ylides 1 and 2, formed from the reaction of methylene and acetone and acetonitrile, respectively, are 4.5 and 70.8 kcal mol . The heats of reaction for fragmentation of the ylides 1 and 2 are estimated to be 45 and 49 kcal mol .

Associative return electron transfer. A bond-coupled electron transfer in the photoreactions of cyclopropylaminesThis paper is dedicated to Professor Fred Lewis on the event of his 60th birthday.

The dynamics of the geminate radical-ion pairs formed by electron transfer to the excited states of cyanoanthracenes from 2-phenylcyclopropylamines are dominated by exothermic bond cleavage of the amine radical cations. Quantitative studies of product formation as a function of the energetics of the photochemical and corresponding thermal reactions provide support for a novel mechanism in which return electron transfer in the geminate pair occurs in concert with bond formation from the ring-opened radical cations. This bond-coupled electron transfer process is referred to as an associative return electron transfer reaction. The important features of the associative electron transfer process that explain the experimental observations are described in terms of potential energy surfaces and competition between adiabatic and non-adiabatic deactivation paths.

Reaction volumes of excited state photoprocesses

Abstract Photoacoustic calorimetry is employed to measure reaction volumes for the formation and decay of the excited states of several coordination complexes. This information can be used to calculate partial volumes of excited states and estimate structural changes that occur upon excitation. In addition, the excited state energies and lifetimes are obtained.

The contribution of quantum mechanical tunneling to the 1,2-hydrogen rearrangement of methylbromocarbene

The rate constants for the 1,2-hydrogen rearrangement of methyl- and methyl-d3-bromocarbene have been determined as a function of temperature. The Arrhenius plots are curved, and the intermolecular isotope effect is small and may increase with increasing temperature. We believe that although the rearrangement proceeds classically at high temperatures, as suggested by theory, quantum mechanical tunneling contributes significantly to the reaction at low temperatures. Alternative explanations are presented and discussed.