Photoelectron-Photofragment Coincidence Studies of the Dynamics of Decarboxylation

在这项由化学系化学结构、动力学和机制-A(CSDM-A)计划资助的项目中，加州大学圣地亚哥分校的Robert Continetti教授正在使用激光光学技术研究包括羧酸、脂肪酸和酯在内的各种有机分子的脱羧基反应。术语脱羧基是指从分子中去除二氧化碳(CO2)。一种常见的羧酸是醋酸(C2H4O2)，它是醋的独特成分。醋酸脱羧基生成二氧化碳和甲烷(CH4)。脂肪酸是具有非常长的碳氢链的羧酸。脂肪酸是构成动物脂肪和植物油的物质。从许多生物体用来释放储存能量的Krebs酸循环，到大气气溶胶化学中新粒子的形成，脱羧化过程在一系列复杂的现象中发挥着关键作用。尽管这很重要，但人们对脱酸过程中二氧化碳的消除机制知之甚少。Continetti研究小组使用一种名为光电子-光碎片符合(PPC)光谱学的技术。在PPC实验中，一束羧酸(实际上是带负电荷的形式)被引导到真空室，并将光束暴露在强激光下，从酸分子中射出一个电子。生成的分子是不稳定的，分解成二氧化碳和碳氢化合物碎片。PPC光谱测量喷出的电子以及二氧化碳和碳氢化合物碎片的速度(和动能)，因此可以深入了解激光能量是如何被酸性分子吸收并导致碎片的。这项研究项目不仅为脱羧化的化学过程提供了基本的见解，而且也为一般的分子裂解提供了基本的见解。这也可能有助于我们对生物燃料燃烧和其他复杂的化学现象的理解。除了基本的科学问题外，从事这一研究项目的研究人员在新型实验装置的设计和操作、多方面数据的分析以及分子离子运动的计算机模拟方面获得了宝贵的经验。这一研究项目为学生和博士后提供了设计和建造新型实验仪器和复杂数据分析的机会。脱羧化是由二氧化碳的热力学稳定性驱动的化学中的一个基本过程，对生物、燃烧和大气化学具有重要意义。在本项目中，通过从羧酸前体中提取质子合成的前驱体阴离子的解离光解离来研究脱羧基。重离子PPC光谱仪提供了直接检查所有解离途径的机会，包括对感兴趣的大物种的阴离子光解离(PD)和解离光解离(DPD)。PPC光谱测量的重点是一系列预计会发生脱羧化的复杂阴离子，包括羧酸盐阴离子、二羧酸盐阴离子和脂肪酸，探索光吸收后可访问的所有低能分解途径。这个项目试图揭示分子内氢键在相对能量学和解离动力学中所起的作用。最后，还研究了与生物燃料相关的长链饱和脂肪酸和不饱和脂肪酸的甲酯和乙酯的解离动力学，提供了基准能量学数据，有助于验证复杂离子和自由基的理论计算。这项工作的更广泛的影响包括了解与大气和生物学相关的脱羧化动力学的潜在社会效益，以及为项目中的研究生和博士后助理提供培训机会。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 -A (CSDM-A) program of the Chemistry Division, Professor Robert Continetti of the University of California, San Diego is using laser optical techniques to study the decarboxylation reactions of various organic molecules including carboxylic acids, fatty acids, and esters. The term decarboxylation refers to the removal of carbon dioxide (CO2) from the molecule. A common carboxylic acid is acetic acid (C2H4O2), the distinctive component of vinegar. Decarboxylation of acetic acid gives CO2 and methane (CH4). Fatty acids are carboxylic acids with very long carbon+hydrogen chains. Fatty acids are what constitute animal fat and vegetable oils. The process of decarboxylation plays a key role in complex phenomena ranging from the Krebs acid cycle used by many organisms to release stored energy, to new particle formation in the chemistry of atmospheric aerosols. In spite of the importance, little is known about the mechanism of the elimination of carbon dioxide that occurs in decarboxylation. The Continetti research group uses a technique called photoelectron-photofragment coincidence (PPC) spectroscopy. In the PPC experiment, a beam of the carboxylic acid (actually a negatively charged form) is directed into a vacuum chamber, and the beam is exposed to a strong laser beam, which ejects an electron from the acid molecule. The resulting molecule is unstable, and falls apart into CO2 and hydrocarbon fragments. PPC spectroscopy measures the speed (and kinetic energy) of the ejected electron and the CO2 and hydrocarbon fragments, and therefore provides insights into how the laser light energy is absorbed by the acid molecule and leads to fragmentation. This research project is providing fundamental insights not only into the chemical process of decarboxylation, but to molecular fragmentation in general. It is also likely to contribute to our understanding of biofuel combustion and other complex chemical phenomena. In addition to the basic science questions, researchers engaged in this research project are gaining valuable experience in the design and operation of a novel experimental apparatus, the analysis of multifaceted data, and the computer simulation of molecular ion motions. This research project provides opportunities for the training of students and postdoctoral associates in the design and construction of novel experimental instrumentation and complex data analysis.Decarboxylation is a fundamental process in chemistry driven by the thermodynamic stability of CO2, with implications for biology, combustion and atmospheric chemistry. In this project, decarboxylation is studied using the dissociative photodetachment of precursor anions synthesized by proton abstraction from carboxylic acid precursors. The heavy ion PPC spectrometer provides the opportunity to directly examine the full gamut of dissociation pathways including anionic photodissociation (PD) and dissociative photodetachment (DPD) for large species of interest. The PPC spectroscopy measurements are focusing on a series of complex anions that are expected to undergo decarboxylation, including the carboxylate anions, dicarboxylate dianions and fatty acids, probing the full range of low energy decomposition pathways accessible following photoabsorption. This project seeks to reveal new insights into the role of intramolecular hydrogen bonding on the relative energetics and dissociation dynamics. Finally, the dissociation dynamics of the methyl- and ethyl-esters of long chain saturated and unsaturated fatty acids of relevance to biofuels are also being studied, providing benchmark energetics data that can aid in the validation of theoretical calculations in complex ions and radicals. The broader impacts of this work include potential societal benefits from understanding decarboxylation dynamics of relevance to the atmosphere and biology, as well as providing training opportunities for graduate students and postdoctoral associates in the program.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.