Spectroscopic Interrogation of Reactive Intermediates Implicated in Hydrocarbon Combustion and Pyrolysis

在这个由化学系化学结构、动力学和机理(CSDM-A)计划资助的项目中，马萨诸塞大学波士顿分校的尼尔·赖利教授正在使用敏感的激光技术来研究碳氢分子的燃烧(也称为燃烧或热解)。他感兴趣的是描述从反应物到产物的反应过程中发生的分子碎片的结构。这些“中间”物种寿命短，很难研究，但捕捉到关于它们结构的信息可以告诉我们这些反应(以及一般的化学反应)是如何发生的细节。该项目还提供了与碳氢分子在从内燃机到地球大气层和遥远行星的卫星的各种环境中的行为相关的知识。从事该项目的博士后、研究生和本科生正在接受激光光谱学和真空技术方面的培训。研究的内容被纳入分析仪器的研究生和本科生课程。波士顿公立学校地区的高中生通过研讨会学习化学分析，在研讨会上，他们使用现成的材料建造自己的光谱仪(告诉分子如何吸收或发射不同波长的光的仪器)。这项研究提供了对化石燃料燃烧、生物质焚烧和有机分子的大气氧化的基本理解。该项目的重点是共振稳定的自由基(RSR)基序，包括取代的苄基、环戊二烯基、烯丙基、炔丙基和乙烯基生色团，在化石燃料燃烧、生物质焚烧以及人类和生物排放的烯烃的大气氧化的早期阶段都很重要。Reilly教授和他的研究小组通过一系列方法产生目标自由基，包括对精心选择的分子前体进行脉冲放电和激光光解，并在激光询问光学波段之前在超音速膨胀中冷却它们。以激光诱导荧光和分散荧光(LIF/DF)和共振增强多光子电离(REMPI)为核心的一系列协同光谱方法被用来鉴定和表征新观察到的物种。REMPI用于记录复杂混合物中单一物种的质量分辨电子光谱和电离势，并为激光诱导荧光研究提供光谱“图谱”，而REMPI和LIF光谱共有的分子带的DF光谱则产生已知分子质量物种的基态振动频率。通过光学烧孔光谱对异构体进行了区分，并结合量子化学计算，从单振动能级发射光谱中获得了详细的光谱指认。这些研究在几个方面产生了广泛的影响：它们能够在广泛的环境中对目标物种进行明确的、特定于异构体的现场监测，促进从机理上提取洞察力，并帮助验证对能源生产重要的过程的模型化学。其中几个目标中间体具有基本的理论意义，并为对低对称性分子中的振动相互作用进行量子化学处理提供了机会。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

In this project funded by the Chemical Structure, Dynamics, and Mechanisms (CSDM-A) program of the Division of Chemistry, Professor Neil Reilly of the University of Massachusetts Boston is using sensitive laser techniques to study the burning of hydrocarbon molecules (also known as combustion or pyrolysis). He is interested in characterizing the structure of the molecular fragments that occur as the reaction proceeds from reactants to products. These "intermediate" species are short lived and difficult to study, but capturing information about their structure can tell us details about how these reactions (and chemical reactions in general) occur. The project is also providing knowledge that is relevant to hydrocarbon molecule behavior in environments that range from internal combustion engines to the atmospheres of Earth and of moons of distant planets. Post-doctoral, graduate, and undergraduate students working on this project are receiving training in laser spectroscopy and vacuum technology. Elements of the research are integrated into graduate and undergraduate courses on analytical instrumentation. High school students within the Boston Public Schools district are introduced to chemical analysis through workshops in which they build their own spectrometers (instruments that tell how molecules absorb or emit light at different wavelengths) using readily available materials. This research provides fundamental understanding of fossil fuel combustion, biomass incineration, and atmospheric oxidation of organic molecules. The project focuses on resonance-stabilized radical (RSR) motifs, including substituted benzyl, cyclopentadienyl, allyl, propargyl, and vinyl chromophores, with and without oxygen, that are important in the early stages of fossil fuel combustion, biomass incineration, and atmospheric oxidation of anthropogenically and biogenically emitted alkenes. Professor Reilly and his research group generate the target radicals by a range of methods, including pulsed electric discharge and laser photolysis of judiciously chosen molecular precursors, and cool them in supersonic expansions prior to laser interrogation in the optical band. A suite of synergistic spectroscopic methods is employed to identify and characterize newly observed species, centering on laser-induced fluorescence and dispersed fluorescence (LIF/DF) and resonance-enhanced multiphoton ionization (REMPI). REMPI is used to record mass-resolved electronic spectra and ionization potentials of single species present in complex mixtures, and provides a spectral "atlas" for LIF surveys, while DF spectroscopy of molecular bands common to REMPI and LIF spectra yields ground-state vibrational frequencies of species of known molecular mass. Isomers are distinguished by optical hole-burning spectroscopy and detailed spectroscopic assignments are obtained from single-vibronic-level emission spectra, in concert with quantum chemical calculations. These investigations are of broad impact in several ways: they enable unambiguous, isomer-specific, in situ monitoring of the targeted species in a wide variety of environments, facilitate the extraction of mechanistic insight, and aid the validation of model chemistries of processes important to energy production. Several of the targeted intermediates are of fundamental theoretical interest and provide opportunities for benchmarking quantum chemical treatments of vibronic interactions in molecules of low symmetry.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.