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 (PI) and a 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.

Keywords: photoionization, PEPICO, ion thermochemistry, heat of formation, proton affinity, C₃H₇Cl, C₃H₇Br, ion dissociation onset, time- of-flight, threshold electron spectroscopy