Robert Continetti, PhD, University of California (San Diego)
This research program is focused on the characterization of the energetics and dynamics of reactive intermediates important in the combustion of biofuels. Fuels derived from biogenic sources pose new challenges for existing liquid-fuel combustion technology, and detailed data on the isolated molecules that are present in these fuels is required for first-principles modeling of combustion processes involved in their use. In particular, little gas phase data exists for molecules of the size of species like the ester methyl linoleate (C19H32O2) that typically form major fractions of biodiesel fuels, much less reactive radicals derived from these parent molecules. The goal of this project is to provide new information on this class of molecules, including the heats of formation, electron affinities and dissociation dynamics of the reactive radicals produced by hydrogen abstraction from the parent molecules. Experimentally, this will be achieved using negative ion photoelectron spectroscopy and photoelectron-photofragment coincidence (PPC) spectroscopy by incorporating an electrospray ionization (ESI) ion source and an ion funnel onto an existing PPC spectrometer. The experimental efforts are being guided by DFT calculations on the structure and energetics of these species.
During the first year of this grant the new ESI source and a novel printed-circuit-board-based ion funnel have been designed, built and are currently in the final testing stages. Ion currents of a few picoamps have been observed for test molecules and we are continuing to work to improve this. Two significant issues have been encountered so far. First, the pressure in the ion funnel region may be too high, leading to reduced collection and transmission efficiency for the ions. Second, we have had some challenges with our home-built radiofrequency power supply for the ion funnel, and are assessing modifications and improvements to it. Once we have improved the brightness of the ion beam we will examine the efficacy of adding a hexapole trap after the ion funnel to further improve the intensity of the pulsed ion beam that will be used in the PPC spectrometer. Then there will be no barrier to the studies of the photodetachment dynamics of the closed shell anions formed by proton abstraction from the biofuel precursor molecules and we will proceed with the planned measurements.
Quantum chemistry calculations have also been carried out to support and guide this experimental program. The molecular structures and electron affinities of biodiesel combustion intermediates were evaluated using density functional theory (B3LYP/6311+G(d,p)) on anions produced by the different proton abstraction channels from a series of esters such as methy linoleate and fatty acids such as oleic acid, as well as the corresponding radicals formed by photodetachment of the excess electron. Adiabatic electron affinities (AEA’s) for the acyl radicals were found to be approximately 1.8 eV, while the saturated carboxylate radicals the AEA’s were closer to 3.3 eV. Interestingly, we found that even for molecules of this size the carboxylate radicals are often predicted to eliminate carbon dioxide when produced by photodetachment. Based on these findings, the photoelectron and PPC spectroscopy experiments on these biofuel-relevant species should provide important and fundamental information on the gas-phase reactivity of this class of molecules. We look forward to reporting on this progress in the second year of the grant.