Photodissociation and photoisomerization of α -fluorotoluene and 4-fluorotoluene in a molecular beam

Cheng Liang Huang*, Jyh Chiang Jiang, Yuri A. Dyakov, Ming Fu Lin, Chien-Ming Tseng, S. H. Lin, Yuan T. Lee, Chi Kung Ni

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

3 Scopus citations

Abstract

The photodissociation of jet-cooled α -fluorotoluene and 4-fluorotoluene at 193 and 248 nm was studied using vacuum ultraviolet (vuv) photoionization/multimass ion imaging techniques as well as electron impact ionization/photofragment translational spectroscopy. Four dissociation channels were observed for α -fluorotoluene at both 193 and 248 nm, including two major channels C6 H5 C H2 F→ C6 H5 C H2 (or C7 H7) +F and C6 H5 C H2 F→ C6 H5 CH (or C7 H6) +HF and two minor channels C6 H5 C H2 F→ C6 H5 CHF+H and C6 H5 C H2 F→ C6 H5 +C H2 F. The vuv wavelength dependence of the C7 H7 fragment photoionization spectra indicates that at least part of the F atom elimination channel results from the isomerization of α -fluorotoluene to a seven-membered ring prior to dissociation. Dissociation channels of 4-fluorotoluene at 193 nm include two major channels C6 H4 FC H3 → C6 H4 FC H2 +H and C6 H4 FC H3 → C6 H4 F+C H3 and two minor channels C6 H4 FC H3 → C6 H5 C H2 (or C7 H7) +F and C6 H4 FC H3 → C6 H5 CH (or C7 H6) +HF. The dissociation rates for α -fluorotoluene at 193 and 248 nm are 3.3× 107 and 5.6× 105 s-1, respectively. The dissociation rate for 4-fluorotoluene at 193 nm is 1.0× 106 s-1. An ab initio calculation demonstrates that the barrier height for isomerization from α -fluorotoluene to a seven-membered ring isomer is much lower than that from 4-fluorotoluene to a seven-membered ring isomer. The experimental observed differences of dissociation rates and relative branching ratios between α -fluorotoluene and 4-fluorotoluene may be explained by the differences in the six-membered ring to seven-membered ring isomerization barrier heights, F atom elimination threshold, and HF elimination threshold between α -fluorotoluene and 4-fluorotoluene.

Original languageEnglish
Article number133305
JournalJournal of Chemical Physics
Volume125
Issue number13
DOIs
StatePublished - 12 Oct 2006

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