Dissociation of NH3 to NH + H2

Abstract

The dissociation paths NH₃→ NH + H₂ have been investigated for the X¹A^′₁, ùA^″₂ and ã³A^″₂ electronic states of ammonia. Depending on the point group used to describe the dissociation, the NH₃ electronic states correlate with different asymptotes: the electronic ground state with the a ¹Δ(NH)+¹Σ⁺_g(H₂) and X ³Σ^–(NH)+³Σ⁺_u(H + H); the ùA^″₂ state with a ¹Δ, b ¹Σ⁺, c ¹Π(NH)+¹Σ⁺_g(H₂) and X ³Σ^–(NH)+³Σ⁺_u(H + H); the ã³A^″₂ with X ³Σ^–(NH) and A ³Π(NH)+¹Σ⁺_g(H₂), respectively. Parts of these dissociative paths of the potential-energy functions have been mapped using CASSCF ab initio wavefunctions. In the C_2v structures the X ¹A^′₁ state possesses a high barrier reaching into the energy region of the ùA^″₂ and B ¹E″ states. No such barriers exist for the ùA^″₂ and ã¹A^″₂ states. The X ¹A^′₁, ùA^″₂ and the B ¹E″ states as well as the ã³A^″₂ and b ³E″ states of ammonia possess common conical intersections, leading to vibronic coupling effects among those states. The ùA^″₂ dissociates mainly into NH₂(X ²B₁)+ H, the ã³A^″₂ state into NH(X ³Σ^–)+ H₂(X ¹Σ⁺_g) products.