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) an d 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 1) near their dissociation thresholds. The breakdown diagr ams 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) fo r 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 C H3Br in high-n (greater than or equal to 100) Rydberg states at energies sl ightly above the dissociation threshold occurs in a time scale of less than or equal to 10(-7) s. The observed step is a confirmation of the 0 K AE(CH 3+) from CH3Br determined in the PFI-PEPICO study. The adiabatic ionization energies (IEs) for the CH3Br+ ((X) over tilde E-2(3/2,1/2)) spin-orbit sta tes 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 I E[CH3Br+ ((X) over tilde E-2(3/2))] value obtained here, when combined with the known IE of CH3 (9.8380 +/- 0.0004 eV) and IE[CH3I+ ((X) over tilde E- 2(3/2))] (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 h eats of formation (Delta H-f(0)0) for Br (117.93 +/- 0.13 kJ/mol), I (107.1 6 +/- 0.04 kJ/mol), and CH3+ (1099.05 +/- 0.33 kJ/mol), we have obtained mo re precise Delta H-f(0)0 demonstrated that highly reliable Delta H-f(0)0 va lues for a range of molecules with error limits comparable to those for som e of the most precisely measured values, such as Delta H-f(0)0(CH4), can be obtained by PFI-PE and PFI-PEPICO measurements. (C) 2001 American Institut e of Physics.