Using ab initio MO calculations to understand the photodissociation dynamics of CH2CCH2 and CH2C2

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, (1)A(2), B-1(1), and E-1, 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 S-1 surface, 2b ((1)A(g), Du), 5 ((1)A '', C-s), and 10 (B-1(2), C-2v'), 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 H-2 detachment. For vinylidenecarbene the lowest singlet excited states are (1)A(2), and B-1(1) 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 + H-2 reaction is predicted to be similar to 83 kcal/mol with the barrier of similar to 92 kcal/mol on the S-0 surface. The calculations suggest the most likely mechanism for photodissociation of allene at 193 nm to produce C3H2 + H-2 involves a Franck-Condon transition to the B-1(1) 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 + H-2.