Photoelectron-Photofragment Coincidence Studies of the Dynamics of Decarboxylation

在这个由化学部化学结构、动力学和机理-A （CSDM-A）项目资助的项目中，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各种有机分子（包括羧酸、脂肪酸和酯）的脱羧反应。脱羧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。醋酸脱羧生成CO2和甲烷（CH4）。脂肪酸是具有很长的碳+氢链的羧酸。脂肪酸是构成动物脂肪和植物油的物质。脱羧过程在许多复杂的现象中起着关键作用，从许多生物体用来释放储存能量的克雷布斯酸循环，到大气气溶胶化学中新粒子的形成。尽管它很重要，但人们对脱羧过程中二氧化碳的消除机制知之甚少。Continetti研究小组使用了一种称为光电子-光电碎片重合（PPC）光谱的技术。在PPC实验中，一束羧酸（实际上是带负电荷的形式）被引导到真空室中，并暴露在强激光束下，从酸分子中射出一个电子。生成的分子是不稳定的，会分解成二氧化碳和碳氢化合物碎片。PPC光谱学测量喷射电子、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光能是如何被酸性分子吸收并导致碎片化的。这个研究项目不仅为脱羧的化学过程提供了基本的见解，而且为一般的分子碎片化提供了基本的见解。它也可能有助于我们对生物燃料燃烧和其他复杂化学现象的理解。除了基本的科学问题外，参与这项研究项目的研究人员还在设计和操作一种新型实验装置，分析多方面数据以及分子离子运动的计算机模拟方面获得了宝贵的经验。该研究项目为学生和博士后提供了在设计和构建新型实验仪器和复杂数据分析方面的培训机会。脱羧是由CO2的热力学稳定性驱动的化学基本过程，对生物学、燃烧和大气化学具有重要意义。本课题以羧酸前体为原料，通过质子提取合成前体阴离子，利用光解离性分离前体阴离子进行脱羧研究。重离子PPC光谱仪提供了直接检查解离途径的全部范围的机会，包括阴离子光解离（PD）和解离光脱离（DPD）的大感兴趣的物种。PPC光谱测量的重点是一系列复杂的阴离子，这些阴离子预计会经历脱羧，包括羧酸阴离子、二羧酸阴离子和脂肪酸，探测光吸收后可达到的低能分解途径的全部范围。该项目旨在揭示分子内氢键在相对能量学和解离动力学中的作用的新见解。最后，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也正在研究中，提供基准能量学数据，有助于验证复合离子和自由基的理论计算。这项工作的更广泛的影响包括从了解与大气和生物学相关的脱羧动力学中获得的潜在社会效益，以及为该计划的研究生和博士后提供培训机会。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.