Wu Xiangkun, Zhou Xiaoguo, Hemberger Patrick, Bodi Andras
Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemical Physics, National Synchrotron Radiation Laboratory, University of Science and Technology of China , Hefei 230026, China.
Laboratory for Femtochemistry and Synchrotron Radiation, Paul Scherrer Institute , 5232 Villigen, Switzerland.
J Phys Chem A. 2017 Apr 13;121(14):2748-2759. doi: 10.1021/acs.jpca.7b00544. Epub 2017 Mar 30.
The dissociation of internal energy selected dimethyl carbonate (DMC) cations was studied by imaging photoelectron photoion coincidence spectroscopy (iPEPICO) in the 10.3-12.5 eV photon energy range. Vibrational fine structure is observed in the ground state band of the threshold photoelectron spectrum up to the dissociation threshold, and Franck-Condon simulations identify the O-C-O bend mode, in particular in combination with the C═O stretch mode, to be active. The DMC ionization energy was determined to be 10.47 ± 0.01 eV. The first dissociative photoionization product, CHOH, dominates the breakdown diagram from 11.0 to 11.8 eV and is also the lightest fragment ion at m/z = 31. Statistical modeling of the PEPICO data yields a 0 K appearance energy of E = 11.14 ± 0.01 eV. At higher photon energies, parallel dissociation channels are observed, leading to the daughter ions CHO, probably OHCHCHOH (m/z = 62, E = 11.16 eV), CHCHOH (m/z = 45, E = 11.46 eV), and CHOCO (m/z = 59, E = 11.47 eV). The m/z = 62 ethylene glycol ion may lose a CHO fragment and yield CHOH (m/z = 33) in a sequential dissociation at E = 11.54 ± 0.03 eV. Reaction path calculations explain the observed dissociation processes, except for the formation of the ethylene glycol cation. Composite method calculations of isodesmic and isomerization reaction energies are used to obtain the enthalpy of formation of dimethyl carbonate at ΔH(DMC(g)) = -548.3 ± 1.5 kJ mol. This puts the heat of formation of the cation at ΔH(DMC(g)) = 461.9 ± 1.8 kJ mol.
在10.3 - 12.5 eV光子能量范围内，通过成像光电子光离子符合光谱（iPEPICO）研究了内禀能量选择的碳酸二甲酯（DMC）阳离子的解离。在阈值光电子能谱的基态带中观察到振动精细结构，直至解离阈值，并且弗兰克 - 康登模拟确定O - C - O弯曲模式，特别是与C═O伸缩模式结合时，是活跃的。确定DMC的电离能为10.47±0.01 eV。第一个解离光离子化产物CHOH在11.0至11.8 eV的分解图中占主导地位，并且也是m/z = 31时最轻的碎片离子。对PEPICO数据的统计建模得出0 K出现能量为E = 11.14±0.01 eV。在更高的光子能量下，观察到平行的解离通道，产生子离子CHO，可能还有OHCHCHOH（m/z = 62，E = 11.16 eV）、CHCHOH（m/z = 45，E = 11.46 eV）和CHOCO（m/z = 59，E = 11.47 eV）。m/z = 62的乙二醇离子可能在E = 11.54±0.03 eV的顺序解离中失去一个CHO片段并产生CHOH（m/z = 33）。反应路径计算解释了观察到的解离过程，但乙二醇阳离子的形成除外。使用等键和异构化反应能量的复合方法计算得出碳酸二甲酯的生成焓为ΔH（DMC（g））= -548.3±1.5 kJ mol。这使得阳离子的生成热为ΔH（DMC⁺（g））= 461.9±1.8 kJ mol。
