The predissociation mechanism of the (B)over-tilde(2)A ' state of HCO via the conical intersection with the (X)over-tilde(2)A ' state

We use large-scale multiconfiguration self-consistent field/configuration i nteraction calculations to characterize the predissociation mechanism of th e (B) over tilde (2)A' state of HCO through conical intersection with the ( X) over tilde (2)A' ground state. We locate two regions of intersection: th e first represents a highly bent HCO that is 8 kcal/mol energetically lower than the (B) over tilde state minimum, with a barrier height of 26 kcal/mo l. Energy points on the (B) over tilde (2)A' potential surface connecting t hese extrema were also calculated. This region emphatically illustrates the feasibility of a nonradiative decay mechanism consistent with latest exper imental findings of purely vibronic coupling mechanism. The second region o f intersection represents a confluence of three linear (2)Sigma(+)-(2)Pi st ates crossings, 53 kcal/mol below the (B) over tilde state minimum. A barri er about 21 kcal/mol above the (B) over tilde state equilibrium structure i s located and assigned to the entrance channel of H-CO((3)Pi). (C) 2000 Ame rican Institute of Physics. [S0021-9606(00)30420-2].