Thermochemistry of N−N Containing Ions:  A Threshold Photoelectron Photoion Coincidence Spectroscopy Study of Ionized Methyl- and Tetramethylhydrazine

The unimolecular dissociation reactions of the methylhydrazine (MH) and tetramethylhydrazine (TMH) radical cations have been investigated using tandem mass spectrometry and threshold photoelectron photoion coincidence spectroscopy in the photon energy ranges 9.60−31.95 eV (for the MH ion) and 7.74−29.94 eV (for the TMH ion). Methylhydrazine ions (CH₃NHNH₂^+•) have three low-energy dissociation channels:  hydrogen atom loss to form CH₂NHNH₂⁺ (m/z 45), loss of a methyl radical to form NHNH₂⁺ (m/z 31), and loss of methane to form the fragment ion m/z 30, N₂H₂^+•. Tetramethylhydrazine ions only exhibit two dissociation reactions near threshold:  that of methyl radical loss to form (CH₃)₂NNCH₃⁺ (m/z 73) and of methane loss to form the fragment ion m/z 72 with the empirical formula C₃H₈N₂^+•. The experimental breakdown curves were modeled with Rice−Ramsperger−Kassel−Marcus theory, and it was found that, particularly for methyl radical loss, variational transition state theory was needed to obtain satisfactory fits to the data. The 0 K enthalpies of formation (Δ_fH₀) for all fragment ions (m/z 73, m/z 72, m/z 45, m/z 31, and m/z 30) have been determined from the 0 K activation energies (E₀) obtained from the fitting procedure:  Δ_fH₀[(CH₃)₂NNCH₃⁺] = 833 ± 5 kJ mol^-1, Δ_fH₀ [C₃H₈N₂^+•] = 1064 ± 5 kJ mol^-1, Δ_fH₀[CH₂NHNH₂⁺] = 862 ± 5 kJ mol^-1, Δ_fH₀[NHNH₂⁺] = 959 ± 5 kJ mol^-1, and Δ_fH₀[N₂H₂^+•] = 1155 ± 5 kJ mol^-1. The breakdown curves have been measured from threshold up to hν ≈ 32 eV for both hydrazine ions. As the photon energy increases, other dissociation products are observed and their appearance energies are reported.