Iodine atom loss kinetics in internal energy selected 1-iodoalkane cations by imaging photoelectron photoion coincidence spectroscopy

•1-Iodoalkanes dissociatively photoionize to produce 2-alkyl cations.•Appearance energies from propyl to heptyl iodide show a shift to lower energies.•In heptyl iodide, alkyl-chain fragmentation happens shortly after the iodine loss.•Isodesmic reaction calculations allow the derivation of thermochemical onsets.•Reverse barriers are derived from the experimental data and the calculations.

Imaging photoelectron photoion coincidence (iPEPICO) spectroscopy has been used to determine 0 K appearance energies for the unimolecular dissociation reactions of several energy selected straight chain alkyl iodide cations 1-CnH2n+1I+ → CnH2n+1+ + I, (n = 3–7). The 0 K appearance energy of iodine atom loss, yielding in fact the 2-alkyl radical cation up to n = 6, was determined to be 9.836 ± 0.010, 9.752 ± 0.010, 9.721 ± 0.010, 9.684 ± 0.010 and 9.688 ± 0.015 eV in 1-C3H7I, 1-C4H9I, 1-C5H11I, 1-C6H13I, and 1-C7H15I, respectively. In 1-iodohexane and the smaller molecules, these correspond to the transition state along the 1-iodoalkane cation → 2-iodoalkane cation reaction path, and can be used in conjunction with isodesmic reaction energies to determine the reverse barriers to dissociative photoionization. The small kinetic shift is indicative of little H tunneling during isomerization. Directly computed reverse barriers show that run-of-the-mill computational approaches are of limited use when applied to open shell systems containing period 5 elements. Hindered rotors were found to play a minor role in the internal energy distribution and the dissociation rate constants.

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