ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
The photodissociation of acetophenone (C6H5COCH3) at 193 and 248 nm has been studied using the time-of-flight mass spectrometric technique. For hν=193 nm, two major primary channels, C6H5COCH3+hν→C6H5CO+CH3 [channel (1)] and C6H5+CH3CO [channel (2)], are observed with comparable cross sections. Data analysis shows that (approximate)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 (1) and (2) at 193 nm, (approximate)25%–30% of the available energy is channeled into kinetic energies of the primary photofragments. Measurements at hν=248 nm reveal that the branching ratio of channel (2) to channel (1) is (approximate)0.01. For channel (1) at hν=248 nm, (approximate)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 (D0). The photofragment angular distributions are found to be isotropic for both channels (1) and (2) at hν=193 nm and for channel (1) at hν=248 nm. A minor photodissociation channel C6H5COCH3+hν→C6H5CH3+CO is identified at both hν=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 (ΔfH°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 ΔfH°0 values for C6H5CO and C6H5 are likely to be low by 3–4 kcal/mol. These theoretical ΔfH° values for C6H5CO and C6H5 yield a theoretical D0(C6H5CO–CH3) value of 85.1±1.4 kcal/mol, which is in excellent accord with the experimental results obtained in the present study. © 1997 American Institute of Physics.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.474964
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