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 al lene C3H4 and vinylidenecarbene C3H2 have been studied by ab initio CC SD(T) and MRCI methods. The three lowest singlet excited states of all ene, (1)A(2), B-1(1), and E-1, are calculated to have the vertical exc itation 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 ene rgies 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 cha nges by twisting the CH2 groups and bending the CCC angle or along the pathway that leads to H-2 detachment. For vinylidenecarbene the lowes t singlet excited states are (1)A(2), and B-1(1) with the respective v ertical excitation energies of 1.88 and 2.44 eV and the adiabatic exci tation 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 t he barrier of similar to 92 kcal/mol on the S-0 surface. The calculati ons 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 g roups and then conversion to the vibrationally excited ground state th rough the seam of crossing. Once the vibrationally excited allene mole cule is in the, ground electronic state it dissociates to produce C3H2 + H-2.