ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
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+(X˜ 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+(X˜ 2E3/2)] value obtained here, when combined with the known IE of CH3 (9.8380±0.0004 eV) and IE[CH3I+(X˜ 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. © 2001 American Institute of Physics.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.1391268
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