High-resolution energy-selected study of the reaction CH[sub 3]X[sup +]→CH[sub 3][sup +]+X: Accurate thermochemistry for the CH[sub 3]X/CH[sub 3]X[sup +] (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 (CH[sub 3][sup +]) from the dissociation of energy-selected CH[sub 3]X[sup +] (X=Br and I) near their dissociation thresholds. The breakdown diagrams for CH[sub 3]X 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 CH[sub 3][sup +] from CH[sub 3]Br and CH[sub 3]I, respectively. Similar to the observation in PFI-PE studies of CH[sub 4], C[sub 2]H[sub 2], and NH[sub 3], the PFI-PE spectrum for CH[sub 3]Br exhibits a step at the 0 K AE for CH[sub 3][sup +], indicating that the dissociation of excited CH[sub 3]Br in high-n (>=100) Rydberg states at energies slightly above the dissociation threshold occurs in a time scale of ≤10[sup -7] s. The observed step is a confirmation of the 0 K AE(CH[sub 3][sup +]) from CH[sub 3]Br determined in the PFI-PEPICO study. The adiabatic ionization energies (IEs) for the CH[sub 3]Br[sup +](X˜ [sup 2]E[sub 3/2,1/2]) 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(CH[sub 3][sup +]) values from CH[sub 3]Br and CH[sub 3]I and the IE[CH[sub 3]Br[sup +](X˜ [sup 2]E[sub 3/2])] value obtained here, when combined with the known IE of CH[sub 3] (9.8380±0.0004 eV) and IE[CH[sub 3]I[sup +](X˜ [sup 2]E[sub 3/2])] (9.5381±0.0001 eV), have allowed accurate determination of the 0 K bond dissociation energies for CH[sub 3]–Br (2.996±0.002 eV), CH[sub 3][sup +]–Br (2.291±0.002 eV), CH[sub 3]–I (2.431±0.003 eV), and CH[sub 3][sup +]–I (2.731±0.003 eV). Using the AE(CH[sub 3][sup +]) from CH[sub 3]Br and CH[sub 3]I, together with the known 0 K he... [ABSTRACT FROM AUTHOR]