Non-adiabatic behavior in the homolytic and heterolytic bond dissociation of protonated hydrazine: A guided ion beam and theoretical investigation.

Threshold collision-induced dissociation using a guided ion beam tandem mass spectrometer was performed on protonated hydrazine and its perdeuterated variant. The dominant dissociation pathways observed were endothermic homolytic and heterolytic cleavages of the N-N bond. The data were analyzed using a statistical model after accounting for internal and kinetic energy distributions, multiple collisions, and kinetic shifts to obtain 0 K bond dissociation energies. Comparison with literature thermochemistry demonstrates that both channels behave non-adiabatically. Heterolytic bond cleavage yields NH + NH products, but the NH fragment is in the spin-restricted excited A state and not in the spin-forbidden ground B state, whereas homolytic bond cleavage leads to dissociation to the NH + NH product asymptote with NH in its excited A state rather than the energetically favored B state. The rationale for the non-adiabatic behavior observed in the homolytic bond cleavage is revealed by detailed theoretical calculations of the relevant potential energy surfaces and the relevant occupied valence molecular orbitals. These calculations suggest that the non-adiabatic behavior results from conservation of the σ and π character of the binding and lone pair electrons on the nitrogen atoms.