The C3H7+ Appearance Energy from 2-Iodopropane and 2-Chloropropane Studied by Threshold Photoelectron Photoion Coincidence

Abstract

The dissociation onset for halogen atom loss was investigated for 2-iodo and 2-chloropropane ions using a newly developed threshold photoelectron photoion coincidence (TPEPICO) technique that suppresses the effects of hot electrons. This study was undertaken in order to resolve current discrepancies concerning these onsets in the literature. The 0 K dissociation thresholds were determined to be 9.818 ± 0.010 and 11.036 ± 0.010 eV for 2-C3H7I and 2-C3H7Cl, respectively. These numbers agree quite well with photoionization and a mass analyzed threshold ionization (MATI) study, but are significantly lower than onsets measured by the much higher resolution pulsed field ionization PEPICO method. The derived C3H7+ heat of formation at 298 K is 803.9 ± 1.5 kJ mol−1, and a more precise value for the 298 K 2-iodopropane heat of formation is −40.8 ± 1.3 kJ mol−1. The derived proton affinity of propene is 746.1 kJ mol−1. We have also performed a high-level ab initio calculation on the 0 K dissociation limit, E0, for C3H7+ from C3H7Cl at the CCSD(T)/CBS level of theory with high-level corrections. The theoretical prediction of 11.048–11.061 eV for E0 (C3H7+) from C3H7Cl is found to be consistent with the current experimental value.