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.
