Spectroscopic Interrogation of Reactive Intermediates Implicated in Hydrocarbon Combustion and Pyrolysis

University Of Massachusetts Boston, Dorchester MA

Investigators

Abstract

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.