Boulanger Anne-Marie, Rennie Emma E, Holland David M P, Shaw David A, Mayer Paul M
Chemistry Department, University of Ottawa, Ottawa, Ontario, Canada.
J Phys Chem A. 2007 Jun 28;111(25):5388-98. doi: 10.1021/jp067863k. Epub 2007 Jun 1.
The 1,1-dimethylhydrazine ion ((CH3)2NNH2+*) has two low-energy dissociation channels, the loss of a hydrogen atom to form the fragment ion m/z 59, (CH3)(CH2)NNH2+, and the loss of a methyl radical to form the fragment ion m/z 45, the methylhydrazyl cation, CH3NNH2+. 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 DeltaS++(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 DeltaS++ decreases to 29 J K-1 mol-1. For the hydrogen atom loss channel the variation in DeltaS++ is less than that for the methyl loss channel. To obtain agreement with the experimental breakdown curves, DeltaS++(59) = 26-16 J K-1 mol-1 over the studied internal energy range. The 0 K enthalpies of formation (DeltafH0) for the two fragment ions m/z 45 and m/z 59 have been calculated from the 0 K activation energies (E0) obtained from the fitting procedure: DeltafH0[CH3NNH2+] = 906 +/- 6 kJ mol-1 and DeltafH0[(CH3)(CH2)NNH2+] = 822 +/- 7 kJ mol-1. The calculated G3 values are DeltafH0[CH3NNH2+] = 911 kJ mol-1 and DeltafH0[(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.
1,1 - 二甲基肼离子（(CH3)2NNH2+*）有两个低能解离通道，失去一个氢原子形成碎片离子m/z 59，即(CH3)(CH2)NNH2+，以及失去一个甲基自由基形成碎片离子m/z 45，即甲基肼阳离子CH3NNH2+。利用阈值光电子 - 光离子符合（TPEPICO）光谱，在8.25 - 31 eV的光子能量范围内，以及串联质谱法，对1,1 - 二甲基肼离子的解离进行了研究。通过对两个反应通道的变分过渡态理论（VTST）建模获得了理论分解曲线，并与实验所得曲线进行了比较。在理论水平B3 - LYP/6 - 31+G(d)下，对于甲基自由基损失通道，在内能范围2.32 - 3.56 eV内发现了七个过渡态。在该内能范围内，甲基损失通道的过渡态在R(N - C) = 4.265、4.065、3.965、3.165、2.765、2.665和2.565 Å处被发现。对于氢原子损失通道，发现了三个过渡态：R(H - C) = 2.298、2.198和2.098 Å。在内能为2.32 eV且键距R(N - C) = 4.265 Å时，DeltaS++(45)值为65 J K-1 mol-1。随着内能增加到3.56 eV，变分过渡态移向更低的R值，使得在R(N - C) = 2.565 Å时，DeltaS++降至29 J K-1 mol-1。对于氢原子损失通道，DeltaS++的变化小于甲基损失通道。为了与实验分解曲线相符，在研究的内能范围内，DeltaS++(59) = 26 - 16 J K-1 mol-1。根据拟合程序得到的0 K活化能（E0），计算了两个碎片离子m/z 45和m/z 59的0 K生成焓（DeltafH0）：DeltafH0[CH3NNH2+] = 906 ± 6 kJ mol-1，DeltafH0[(CH3)(CH2)NNH2+] = 822 ± 7 kJ mol-1。计算得到的G3值为DeltafH0[CH3NNH2+] = 911 kJ mol-1，DeltafH0[(CH3)(CH2)NNH2+] = 825 kJ mol-1。除了这两个低能解离产物外，当光子能量增加时，在1,1 - 二甲基肼离子的解离过程中还观察到了另外21个碎片离子。报告了它们的出现能。
