Dissociation of NH3 to NH2+H

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Abstract
Potential energy, dipole moment, and electronic transition moment surfaces for the lowest dissociative pathways of the singlet X̃ and à states of NH3 yielding NH2 (X̃ 2B1,à 2A1) +H(2S) products have been calculated using complete active space MCSCF ab initio wave functions. The à state dissociation proceeds via a minimum barrier at the following planar geometry: αHNH =113°, rNH =1.042 Å (in the NH2 fragment), and RNH =1.323 Å (in the dissociation coordinate). The barrier height is calculated to be 3226 cm−1 with an expected accuracy of about 300 cm−1. The barrier height increases with increasing out-of-plane angle. Close to the barrier there are strong variations of the shapes of the dipole moment and transition moment surfaces. The minimum energy path through the X̃–à conical intersection follows planar geometries. Along this pathway the angle αHNH decreases, but the distance rNH in the NH2 fragment hardly changes. The crossing distance RcNH of the X̃ and à states in planar structures depends strongly on αHNH and varies from about 1.68 Å (60°) to infinity (180°). The photodissociation process NH3(Ã) →NH2(X̃ 2B1) +H(2S) is discussed on the basis of the calculated potential energy surfaces.