DISSOCIATION OF NH3 TO NH+H-2

The dissociation paths NH3 --> NH + H-2 have been investigated for the X approximately 1A', A approximately 1A2" and a approximately 3A2" 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 1-DELTA (NH) + 1-SIGMA-g+ (H-2) and X 3-SIGMA- (NH) + 3-SIGMA-u+ (H + H); the A approximately 1A2" state with a 1-DELTA, b 1-SIGMA+, c 1-PI (NH) + 1-SIGMA-g+ (H-2) and X 3-SIGMA- (NH) + 3-SIGMA-u+ (H + H); the a approximately 3A2" with X 3-SIGMA- (NH) and A 3-PI (NH) + 1-SIGMA-g+ (H-2), respectively. Parts of these dissociative paths of the potential-energy functions have been mapped using CASSCF ab initio wavefunctions. In the C2v structures the X 1A1' state possesses a high barrier reaching into the energy region of the A approximately 1A2" and B approximately 1E" states. No such barriers exist for the A approximately 1A2" and a approximately 1A2" states. The X approximately 1A1', A approximately 1A2" and the B approximately 1E" states as well as the a approximately 3A2" and b approximately 3E" states of ammonia possess common conical intersections, leading to vibronic coupling effects among those states. The A approximately 1A2" dissociates mainly into NH2(X approximately 2B1) + H, the a approximately 3A2" state into NH(X 3-SIGMA-) + H-2(X 1-SIGMAg+) products.