Entropy effects in the fragmentation of 1,1-dimethylhydrazine ions

The 1,1-dimethylhydrazine ion ((CH 3 ) 2 NNH 2 + °) has two low-energy dissociation channels, the loss of a hydrogen atom to form the fragment ion m/z 59, (CH 3 )(CH 2 )NNH 2 + , and the loss of a methyl radical to form the fragment ion m/z 45, the methylhydrazyl cation, CH 3 NNH 2 + . The dissociation of the 1,1-dimethylhydrazine ion has been investigated using threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy, in the photon energy range 8.25-31 eV, and tandem mass spectrometry. Theoretical breakdown curves have been obtained from a variational transition state theory (VTST) modeling of the two reaction channels and compared to those obtained from experiment. Seven transition states have been found at the B3-LYP/6-31+G(d) level of theory for the methyl radical loss channel in the internal energy range of 2.32-3.56 eV. The methyl loss channel transition states are found at R N-C = 4.265, 4.065, 3.965, 3.165, 2.765, 2.665, and 2.565 A over this internal energy range. Three transition states have been found for the hydrogen atom loss channel: R H-C = 2.298, 2.198, and 2.098 A. The ΔS‡(45) value, at an internal energy of 2.32 eV and a bond distance of R N-C = 4.265 A, is 65 J K -1 mol -1 . As the internal energy increases to 3.56 eV the variational transition state moves to lower R value so that at R N-C = 2.565 A, the ΔS‡ decreases to 29 J K -1 mol -1 . For the hydrogen atom loss channel the variation in ΔS‡ is less than that for the methyl loss channel. To obtain agreement with the experimental breakdown curves, ΔS‡(59) = 26-16 J K -1 mol -1 over the studied internal energy range. The 0 K enthalpies of formation (Δ f H 0 ) for the two fragment ions m/z 45 and m/z 59 have been calculated from the 0 K activation energies (E 0 ) obtained from the fitting procedure: A f H 0 [CH 3 NNH 2 + ] = 906 ± 6 kJ mol -1 and Δ f H 0 [(CH 3 )(CH 2 )NNH 2 + ] = 822 ± 7 kJ mol -1 . The calculated G3 values are Δ f H 0 -[CH 3 NNH 2 + ] = 911 kJ mol -1 and Δ f H 0 [(CH3)(CH2)NNH2+] = 825 kJ mol -1 . In addition to the two low-energy dissociation products, 21 other fragment ions have been observed in the dissociation of the 1,1-dimethylhydrazine ion as the photon energy was increased. Their appearance energies are reported.