A laser photofragmentation time-of-flight mass spectrometric study of acetophenone at 193 and 248 nm
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AbstractThe photodissociation of acetophenone (C6H5COCH3) at 193 and 248 nm has been studied using the time-of-flight mass spectrometric technique. For h nu=193 nm, two major primary channels, C6H5COCH3+liv-->C6H5CO+CH3 [channel (1)] and C6H5+CH3CO [channel (2)], are observed with comparable cross sections. Data analysis shows that approximate to 30%-50% of primary C6H5CO and CH3CO radicals further decomposes, yielding secondary products C6H5+CO and CH3+CO, respectively. The translational energy release measurements indicate that for both channels (I) and (2) at 193 nm, approximate to 25%-30% of the available energy is channeled into kinetic energies of the primary photofragments. Measurements at h nu=248 nm reveal that the branching ratio of channel (2) to channel (1) is approximate to 0.01. For channel (1) at h nu=248 nm, approximate to 42% of the available energy is directed as the kinetic energy of the photofragments. The observed maximum kinetic energy release for channel (1) at 248 nm yields a value of 85.0 +/- 2.2 kcal/mol for the C6H5CO-CH3 bond dissociation energy at 0 K (D-0). The photofragment angular distributions are found to be isotropic for both channels (1) and (2) at h nu=193 nm and for channel (1) at h nu=248 nm. A minor photodissociation channel C6H5COCH3+h nu-->C6H5CH3+CO is identified at both h nu=193 and 248 nm. The energetics for the dissociation reactions of acetophenone have also been investigated using ab initio Gaussian-2-type procedures, The heats of formation at 0 K (Delta(f)H degrees(0)) for C6H5CO and C6H5 calculated using the isodesmic reaction scheme are 33.9 +/- 1.3 and 87.6 +/- 1.0 kcal/mol, respectively. These results suggest that the literature Delta(f)H degrees(0) values for C6H5CO and C6H5 are likely to be low by 3-4 kcal/mol. These theoretical Delta(f)H degrees values for C6H5CO and C6H5 yield a theoretical D-0(C6H5CO-CH3) value of 85.1 +/- 1.4 kcal/mol, which is in excellent accord with the experimental results obtained in the present study. (C) 1997 American Institute of Physics.
All Author(s) ListZhao HQ, Cheung YS, Liao CL, Liao CX, Ng CY, Li WK
Journal nameJournal of Chemical Physics
Volume Number107
Issue Number18
Pages7230 - 7241
LanguagesEnglish-United Kingdom
Web of Science Subject CategoriesChemistry; Chemistry, Physical; Physics; Physics, Atomic, Molecular & Chemical; PHYSICS, ATOMIC, MOLECULAR & CHEMICAL

Last updated on 2020-09-08 at 03:08