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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