Loss of H

₂ from CH₃NH₃⁺, CH₃OH₂⁺ and CH₃FH⁺. Reaction mechanisms and dynamics from observation of metastable ion fragmentations and ab initio calculations

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Eur. J. Mass Spectrom. 1, 121 - 129 (1995)

Loss of H₂ from CH₃NH₃⁺, CH₃OH₂⁺ and CH₃FH⁺. Reaction mechanisms and dynamics from observation of metastable ion fragmentations and ab initio calculations

Elisabeth Leere Øiestad, Åse Marit Leere Øiestad, Haakon Skaane, Kenneth Ruud, Trygve Helgaker and Einar Uggerud^*
Department of Chemistry, University of Oslo, PO Box 1033, Blindern, N-0315 Oslo, Norway
Tore Vulpius
Department of Chemistry, H.C. Ørsted Institute, University of Copenhagen, DK-2100 Copenhagen, Denmark

ABSTRACT:

The distributions of the translational energy (T) released during loss of H₂ from metastable CH₃NH₃⁺ and CH₃OH₂⁺ ions have been measured. For both reactions the most probable T value accounts for approximately 3/4 of the reaction's reverse critical energy. Subject to the same experimental conditions CH₃FH⁺ ions do not give rise to any measurable signal for H₂ loss. The relevant parts of the potential energy surfaces of all three reactions were investigated using various ab initio quantum chemical computational schemes. Ab initio direct dynamics calculations were performed to obtain representative reaction trajectories. Translational energy releases computed at the end of these trajectories (where the fragments have separated) agree with the corresponding experimental figures. The three reactions follow a common polar mechanism which involves an initial transfer of a proton from the most basic centre (N, O or F) towards one of the hydrogen atoms of the methyl group. During this stage the proton polarizes the electrons around the methyl hydrogen to give it some hydride character, and in the transition state this has resulted in an embryonic H-H bond. Further electron reorganization during the concerted bond breaking and bond making process leads to a strong repulsive force along the reaction coordinate as the two fragments depart. This accounts for the highly non-statistical partitioning of the available potential energy into relative translation between the two fragments formed.

Keywords:

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