Product branching ratios in photodissociation of phenyl radical: A theoretical ab initio/Rice-Ramsperger-Kassel-Marcus study
- Department of Chemistry and Biochemistry, Florida International University, Miami, Florida 33199 (United States)
Ab initio CCSD(T)/CBS//B3LYP/6-311G** calculations of the potential energy surface for possible dissociation channels of the phenyl radical are combined with microcanonical Rice-Ramsperger-Kassel-Marcus calculations of reaction rate constants in order to predict statistical product branching ratios in photodissociation of c-C{sub 6}H{sub 5} at various wavelengths. The results indicate that at 248 nm the photodissociation process is dominated by the production of ortho-benzyne via direct elimination of a hydrogen atom from the phenyl radical. At 193 nm, the statistical branching ratios are computed to be 63.4%, 21.1%, and 14.4% for the o-C{sub 6}H{sub 4}+ H, l-C{sub 6}H{sub 4} ((Z)-hexa-3-ene-1,5-diyne) + H, and n-C{sub 4}H{sub 3}+ C{sub 2}H{sub 2} products, respectively, in a contradiction with recent experimental measurements, which showed C{sub 4}H{sub 3}+ C{sub 2}H{sub 2} as the major product. Although two lower energy pathways to the i-C{sub 4}H{sub 3}+ C{sub 2}H{sub 2} products are identified, they appeared to be kinetically unfavorable and the computed statistical branching ratio of i-C{sub 4}H{sub 3}+ C{sub 2}H{sub 2} does not exceed 1%. To explain the disagreement with experiment, we optimized conical intersections between the ground and the first excited electronic states of C{sub 6}H{sub 5} and, based on their structures and energies, suggested the following photodissociation mechanism at 193 nm: c-C{sub 6}H{sub 5} 1{yields} absorption of a photon {yields} electronically excited 1{yields} internal conversion to the lowest excited state {yields} conversion to the ground electronic state via conical intersections at CI-2 or CI-3{yields} non-statistical decay of the vibrationally excited radical favoring the formation of the n-C{sub 4}H{sub 3}+ C{sub 2}H{sub 2} products. This scenario can be attained if the intramolecular vibrational redistribution in the CI-2 or CI-3 structures in the ground electronic state is slower than their dissociation to n-C{sub 4}H{sub 3}+ C{sub 2}H{sub 2} driven by the dynamical preference.
- OSTI ID:
- 22098886
- Journal Information:
- Journal of Chemical Physics, Vol. 136, Issue 23; Other Information: (c) 2012 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); ISSN 0021-9606
- Country of Publication:
- United States
- Language:
- English
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