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

Abstract

Imaging photoelectron photoion coincidence (iPEPICO) spectroscopy has been used to determine 0K 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 0K 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±0.015eV 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.