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Publication - Professor Andrew Orr-Ewing

    Picosecond to Millisecond Tracking of a Photocatalytic Decarboxylation Reaction Reveals the Lifetime of a Reactive Carboxyl Radical in Solution

    Citation

    Bhattacherjee, A, Sneha, M, Lewis-Borrell, L, Tau, O, Clark, IP & Orr-Ewing, A, 2019, ‘Picosecond to Millisecond Tracking of a Photocatalytic Decarboxylation Reaction Reveals the Lifetime of a Reactive Carboxyl Radical in Solution’. Nature Communications.

    Abstract

    The photochemical decarboxylation of carboxylic acids is a versatile route for the generation of free radical intermediates for chemical synthesis and engineering. However, the sequential nature of this multi-step reaction makes the elementary steps as well as the over all mechanistic pathway challenging to probe in a single time-resolved experiment. Here, we employ a high repetition-rate (100 kHz) mid-infrared probe in a transient absorption spectroscopy experiment to track the mechanism of the decarboxylation of cyclohexane carboxylic acid (CHCA) in acetonitrile-d3 using a photooxidant system (phenanthrene, PHEN and 1,4-dicyanobenzene, DCB), over eight orders of magnitude in time. At 256 nm, selective excitation of PHEN in the reaction mixture leads to diffusive photoinduced electron transfer to DCB with rate-coefficient 1.9(3) x 1010 M-1s-1. Reduction of the PHEN cation by CHCA in an alkaline medium shows the growth of carbon dioxide within a few microseconds after photoexcitation. A measured time offset in the rise of the byproduct with respect to the decay of the oxidizing arene cation directly reports on the timescale for the unimolecular decay of the transient carboxyl radical in solution. The lifetime determination (520 120 ns) of a reactive carboxyl radical in situ sheds light on the timescale of an activated C-C bond fission in photocatalytic decarboxylation reactions.

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