A LASER PHOTOFRAGMENTATION TIME-OF-FLIGHT MASS-SPECTROMETRIC STUDY OFACETOPHENONE AT 193 AND 248 NM

The photodissociation of acetophenone (C6H5COCH3) at 193 and 248 nm ha s been studied using the time-of-flight mass spectrometric technique. For h nu=193 nm, two major primary channels, C6H5COCH3+liv-->C6H5CO+CH 3 [channel (1)] and C6H5+CH3CO [channel (2)], are observed with compar able cross sections. Data analysis shows that approximate to 30%-50% o f primary C6H5CO and CH3CO radicals further decomposes, yielding secon dary products C6H5+CO and CH3+CO, respectively. The translational ener gy release measurements indicate that for both channels (I) and (2) at 193 nm, approximate to 25%-30% of the available energy is channeled i nto 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 th e photofragments. The observed maximum kinetic energy release for chan nel (1) at 248 nm yields a value of 85.0 +/- 2.2 kcal/mol for the C6H5 CO-CH3 bond dissociation energy at 0 K (D-0). The photofragment angula r 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 photodiss ociation channel C6H5COCH3+h nu-->C6H5CH3+CO is identified at both h n u=193 and 248 nm. The energetics for the dissociation reactions of ace tophenone have also been investigated using ab initio Gaussian-2-type procedures, The heats of formation at 0 K (Delta(f)H degrees(0)) for C 6H5CO and C6H5 calculated using the isodesmic reaction scheme are 33.9 +/- 1.3 and 87.6 +/- 1.0 kcal/mol, respectively. These results sugges t 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 degr ees values for C6H5CO and C6H5 yield a theoretical D-0(C6H5CO-CH3) val ue of 85.1 +/- 1.4 kcal/mol, which is in excellent accord with the exp erimental results obtained in the present study. (C) 1997 American Ins titute of Physics.