Paul Scherrer Institute, 5232 Villigen, Switzerland.
Hefei National Laboratory for Physical Sciences at the Microscale, Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, China.
J Phys Chem A. 2021 Apr 29;125(16):3327-3340. doi: 10.1021/acs.jpca.1c00876. Epub 2021 Apr 19.
We studied the valence photoionization of vanillin by photoelectron photoion coincidence spectroscopy in the 8.20-19.80 eV photon energy range. Vertical ionization energies by EOM-IP-CCSD calculations reproduce the photoelectron spectral features. Composite method calculations and Franck-Condon simulation of the weak, ground-state band yield the adiabatic ionization energy of the most stable vanillin conformer as 8.306(20) eV. The lowest energy dissociative photoionization channels correspond to hydrogen atom, carbon monoxide, and methyl losses, which form the dominant CHO (/ 151) and the less intense CHO (/ 124) and CHO (/ 137) fragment ions in parallel dissociation channels at modeled 0 K appearance energies of 10.13(1), 10.40(3), and 10.58(10) eV, respectively. On the basis of the breakdown diagram, we explore the energetics of sequential methyl and carbon monoxide loss channels, which dominate the fragmentation mechanism at higher photon energies. The 0 K appearance energy for sequential CO loss from the / 151 fragment to CHO (/ 123) is 12.99(10) eV, and for sequential CH loss from the / 123 fragment to CHO (/ 108), it is 15.40(20) eV based on the model. Finally, we review the thermochemistry of the bi- and trifunctionalized benzene derivatives guaiacol, hydroxybenzaldehyde, anisaldehyde, and vanillin. On the basis of isodesmic functional group exchange reactions, we propose new enthalpies of formations, among them Δ°(vanillin, g) = -383.5 ± 2.9 kJ mol. These mechanistic insights and ab initio thermochemistry results will support analytical works to study lignin conversion involving vanillin.
我们通过光电离光电子符合谱研究了香草醛在 8.20-19.80 eV 光子能量范围内的价光电离。EOM-IP-CCSD 计算的垂直电离能再现了光电子光谱特征。弱的基态带的复合方法计算和 Franck-Condon 模拟给出最稳定的香草醛构象的绝热电离能为 8.306(20) eV。最低能量的解离光电离通道对应于氢原子、一氧化碳和甲基的损失，它们在模拟的 0 K 出现能为 10.13(1)、10.40(3)和 10.58(10) eV 的平行解离通道中形成主要的 CHO(/151)和较弱的 CHO(/124)和 CHO(/137)碎片离子。基于分解图，我们探讨了连续甲基和一氧化碳损失通道的能量学，这些通道在较高光子能量下主导着碎片机制。从 /151 碎片到 CHO(/123)的连续 CO 损失的 0 K 出现能为 12.99(10) eV，而从 /123 碎片到 CHO(/108)的连续 CH 损失的 0 K 出现能为 15.40(20) eV，这是基于模型的。最后，我们综述了双官能化和三官能化苯衍生物愈创木酚、羟基苯甲醛、苯甲醛和香草醛的热化学。基于等电子基团交换反应，我们提出了新的生成焓，其中Δ°(香草醛，g)=-383.5±2.9 kJ mol。这些机理见解和从头计算热化学结果将支持研究涉及香草醛的木质素转化的分析工作。
