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Using ab Initio MO Calculations To Understand the Photodissociation Dynamics of CH 2 CCH 2 and CH 2 C 2

1997; American Chemical Society; Volume: 101; Issue: 36 Linguagem: Inglês

10.1021/jp970597d

1520-5215

William M. Jackson, Alexander M. Mebel, S. H. Lin, Y. T. Lee,

Atmospheric chemistry and aerosols

Potential energy surfaces (PES) of the ground and excited states of allene C3H4 and vinylidenecarbene C3H2 have been studied by ab initio CCSD(T) and MRCI methods. The three lowest singlet excited states of allene, 1A2, 1B1, and 1E, are calculated to have the vertical excitation energies of 6.10, 6.55, and 6.94 eV, respectively. Three local minima are found on the excited S1 surface, 2b (1Ag, D2h), 5 (1A'', Cs), and 10 (1B2, C2v'), and their adiabatic excitation energies are 3.02, 3.05, and 4.70 eV, respectively. The PES of the ground and excited states are shown to cross when the geometry of allene changes by twisting the CH2 groups and bending the CCC angle or along the pathway that leads to H2 detachment. For vinylidenecarbene the lowest singlet excited states are 1A2 and 1B1 with the respective vertical excitation energies of 1.88 and 2.44 eV and the adiabatic excitation energies of 1.77 and 2.05 eV. The endothermicity of the C3H4 → C3H2 + H2 reaction is predicted to be ∼83 kcal/mol with the barrier of ∼92 kcal/mol on the S0 surface. The calculations suggest the most likely mechanism for photodissociation of allene at 193 nm to produce C3H2 + H2 involves a Franck−Condon transition to the 1B1 excited state. This is followed by a twisting of the CH2 groups and then conversion to the vibrationally excited ground state through the seam of crossing. Once the vibrationally excited allene molecule is in the ground electronic state it dissociates to produce C3H2 + H2.