Adamson B D, Skeen S A, Ahmed M, Hansen N
Combustion Research Facility , Sandia National Laboratories , Livermore , California 94551 , United States.
Chemical Sciences Division , Lawrence Berkeley National Laboratory , Berkeley , California 94720 , United States.
J Phys Chem A. 2018 Dec 6;122(48):9338-9349. doi: 10.1021/acs.jpca.8b08947. Epub 2018 Nov 27.
This paper provides experimental evidence for the chemical structures of aliphatically substituted and bridged polycyclic aromatic hydrocarbon (PAH) species in gas-physe combustion environments. The identification of these single- and multicore aromatic species, which have been hypothesized to be important in PAH growth and soot nucleation, was made possible through a combination of sampling gaseous constituents from an atmospheric pressure inverse coflow diffusion flame of ethylene and high-resolution tandem mass spectrometry (MS-MS). In these experiments, the flame-sampled components were ionized using a continuous VUV lamp at 10.0 eV and the ions were subsequently fragmented through collisions with Ar atoms in a collision-induced dissociation (CID) process. The resulting fragment ions, which were separated using a reflectron time-of-flight mass spectrometer, were used to extract structural information about the sampled aromatic compounds. The high-resolution mass spectra revealed the presence of alkylated single-core aromatic compounds and the fragment ions that were observed correspond to the loss of saturated and unsaturated units containing up to a total of 6 carbon atoms. Furthermore, the aromatic structures that form the foundational building blocks of the larger PAHs were identified to be smaller single-ring and pericondensed aromatic species with repetitive structural features. For demonstrative purposes, details are provided for the CID of molecular ions at masses 202 and 434. Insights into the role of the aliphatically substituted and bridged aromatics in the reaction network of PAH growth chemistry were obtained from spatially resolved measurements of the flame. The experimental results are consistent with a growth mechanism in which alkylated aromatics are oxidized to form pericondensed ring structures or react and recombine with other aromatics to form larger, potentially three-dimensional, aliphatically bridged multicore aromatic hydrocarbons.
本文提供了气相燃烧环境中脂肪族取代和桥连多环芳烃（PAH）物种化学结构的实验证据。这些单核和多核芳烃物种被认为在PAH生长和烟灰成核过程中很重要，通过从乙烯的常压反向同轴流扩散火焰中采集气态成分并结合高分辨率串联质谱（MS-MS），得以鉴定这些物种。在这些实验中，火焰采样的成分使用10.0 eV的连续真空紫外灯进行电离，随后离子在碰撞诱导解离（CID）过程中与氩原子碰撞而碎片化。使用反射式飞行时间质谱仪分离得到的碎片离子，用于提取有关采样芳烃化合物的结构信息。高分辨率质谱揭示了烷基化单核芳烃化合物的存在，观察到的碎片离子对应于总共含6个碳原子的饱和和不饱和单元的损失。此外，构成较大PAHs基本结构单元的芳烃结构被鉴定为具有重复结构特征的较小单环和稠合芳烃物种。为了说明目的，提供了质量数为202和434的分子离子的CID细节。通过对火焰的空间分辨测量，深入了解了脂肪族取代和桥连芳烃在PAH生长化学反应网络中的作用。实验结果与一种生长机制一致，即烷基化芳烃被氧化形成稠合环结构，或与其他芳烃反应并重组形成更大且可能为三维的脂肪族桥连多核芳烃。
