High-resolution energy-selected study of the reaction $CH3X+→CH3++X:$ Accurate thermochemistry for the $CH3X/CH3X+$ (X=Br, I) system

Using the high-resolution pulsed field ionization-photoelectron (PFI-PE) and PFI-PE-photoion coincidence (PFI-PEPICO) techniques, we have examined the formation of methyl cation $(CH3+)$ from the dissociation of energy-selected $CH3X+$ (X=Br and I) near their dissociation thresholds. The breakdown diagrams for $CH3X$ thus obtained yield values of 12.834±0.002 eV and 12.269±0.003 eV for the 0 K dissociative threshold or appearance energy (AE) for $CH3+$ from $CH3Br$ and $CH3I,$ respectively. Similar to the observation in PFI-PE studies of $CH4,$ $C2H2,$ and $NH3,$ the PFI-PE spectrum for $CH3Br$ exhibits a step at the 0 K AE for $CH3+,$ indicating that the dissociation of excited $CH3Br$ in high-n (⩾100) Rydberg states at energies slightly above the dissociation threshold occurs in a time scale of $⩽10−7 s.$ The observed step is a confirmation of the 0 K $AE(CH3+)$ from $CH3Br$ determined in the PFI-PEPICO study. The adiabatic ionization energies (IEs) for the $CH3Br+(X̃ 2E3/2,1/2)$ spin–orbit states were determined by PFI-PE measurements to be 10.5427±0.0010 and 10.8615±0.0010 eV, respectively, yielding the spin–orbit coupling constant to be 2571±4 cm⁻¹. The $AE(CH3+)$ values from $CH3Br$ and $CH3I$ and the $IE[CH3Br+(X̃ 2E3/2)]$ value obtained here, when combined with the known IE of $CH3$ (9.8380±0.0004 eV) and $IE[CH3I+(X̃ 2E3/2)]$ (9.5381±0.0001 eV), have allowed accurate determination of the 0 K bond dissociation energies for $CH3–Br$ (2.996±0.002 eV), $CH3+–Br$ (2.291±0.002 eV), $CH3–I$ (2.431±0.003 eV), and $CH3+–I$ (2.731±0.003 eV). Using the $AE(CH3+)$ from $CH3Br$ and $CH3I,$ together with the known 0 K heats of formation $(ΔfH00)$ for Br (117.93±0.13 kJ/mol), I (107.16±0.04 kJ/mol), and $CH3+$ (1099.05±0.33 kJ/mol), we have obtained more precise $ΔfH00$ values for $CH3Br$ (−21.30±0.42 kJ/mol) and $CH3I$ (22.43±0.50 kJ/mol). This experiment demonstrated that highly reliable $ΔfH00$ values for a range of molecules with error limits comparable to those for some of the most precisely measured values, such as $ΔfH00(CH4),$ can be obtained by PFI-PE and PFI-PEPICO measurements.