Loss of H2 from CH2NH2+ and NHNH2+.: Reaction mechanisms and dynamics from observation of metastable ion fragmentations and ab initio calculations

On the basis of the measurement of the translational energy release distribution, previous isotope labelling experiments, extensive ab initio calculations of relevant parts of the potential energy surface and an ab initio direct dynamics calculation, it is concluded that the loss of H-2 from CH2NH2+ (1) occurs in two steps. The first step, which is irreversible, is a hydrogen rearrangement giving the high energy isomer CH3N+ (2). The existence of this isomer is transient and it decomposes "on the fly" to give the products CHNH+ (3) and H-2 (4) via the transition structure TS2/3+4. Passage of this transition structure brings the reacting system to a part of the potential energy surface which provides a strong repulsive force between the two fragments. This gives rise to the observed large translational energy release. The mechanism also explains why the reaction is a specific 1,2-H-2 elimination. NHNH2+ (5) decomposes in one step via TS5/6+4 to give NNH+ (6) and H-2 (4), also with a large translational energy release. There is an alternative higher energy route which occurs in two steps via the isomer NNH3+ (7). (C) 1997 Elsevier Science B.V.