In this project supported by the Chemical Structure, Dynamics and Mechanisms Program of the Chemistry Division, Prof. Terry A. Miller of Ohio State University and his research group will utilize high resolution cavity ring-down spectroscopy (HR-CRDS) to obtain and analyze the spectra of a number of reactive chemical intermediates relevant to tropospheric chemistry, including organic peroxy radicals, nitrate radicals, and hydroxy alkoxy radicals. A new technique developed in the Miller laboratory called far-infrared/mid-infrared resonance enhanced multiphoton ionization double-resonance spectroscopy (FIR/MIR-REMPI-DRS) will be used to extend spectral studies to longer wavelengths and obtain high resolution spectra of species like carbon chains (relevant to astrochemistry) and biologically relevant molecules such as amino acids. These studies will provide diagnostic markers for radicals and other molecules that are important in combustion and atmospheric chemistry, and extend high resolution spectroscopy to low frequency regions of the spectrum that can provide characteristic information on large molecules. The analyses of the spectra will also provide precise molecular parameters that can serve as experimental ?gold standards? to guide and benchmark electronic structure calculations.
The broader scientific impacts of the proposed studies include valuable spectral data and diagnostics relevant to tropospheric chemistry, astrochemistry, and biochemistry. In addition, the young researchers working on these projects will acquire excellent training in physical chemistry - gaining invaluable experience in both sophisticated experimental and theoretical methods. Prof. Miller will continue the development of research-based graduate courses that emphasize the broad applications of molecular spectroscopy, as well as develop a new module on the same theme appropriate to introductory chemistry courses.
This project focused on the development and application of advanced spectroscopic techniques, both experimental and theoretical, to the study of reactive chemical intermediates relevant to the oxidation of organic molecules, particularly those involved in atmospheric chemistry, but also ones important to astrochemistry and combustion chemistry. These spectra constitute the basis for molecular diagnostics which can be used to monitor those species as they are involved in chemical reactions in the lab or the real world. The spectral analyses also characterize the geometric and electronic structure of the molecules, and serve to benchmark both sophisticated electronic structure calculations as well as our chemical intuition with respect to the nature of chemical bonding and reactivity. Spectroscopic studies have been carried out on several reactive species using two very sensitive spectrometers that we have developed. One (cavity ringdown spectroscopy) uses the principle of photon absorption to record spectra. The other (laser-induced fluorescence) utilizes photon emission to observe spectra. Together these apparatuses cover a spectral region from deep-UV (~200 nm) to mid-IR ~5000 nm) with resolution as high as 100 MHz. Using these very powerful spectrometers allows the most information to be recovered from the observed molecular spectra. Spectra were observed and analyzed during the grant period for several specific reactive species of particular interest to atmospheric chemistry and pollution. Various organic peroxy radicals (ROO) play a key role in atmospheric reactions leading to the formation of ozone as well as OH and HO2 radicals. For example, annually enormous quantities of unsaturated organic molecules, e.g. ethene, propene, butadiene, isoprene, etc. are injected from biogenic and anthropogenic sources into the troposphere where they are subject to oxidative processes involving peroxy radicals. For such unsaturated molecules, OH adds to one side of a double bond followed by O2 to the other side forming ß-hydroxy alkylperoxy radicals, of which the simplest is ß-hydroxyethylperoxy, ß-HEP. Our HR-CRDS apparatus has recorded the first spectra of ß-HEP (HOCH2CH2OO) and three deuterated isoptopologues: ß-DHEP (DOCH2CH2OO), ß-HEP-d4 (HOCD2CD2OO), and perdeutero ß-DHEP-d4 (DOCD2CD2OO). This work provides an unambiguous assignment of the observed 7390 cm-1 band to the origin of the G1G2G3 conformer of ß-HEP which could be valuable for diagnostic purposes. The spectral analysis also determines molecular parameters that benchmark electronic structure calculations for the molecule. In polluted urban atmospheres where the NO concentration is high, it readily reacts with peroxy radicals to form alkoxy radicals, RO, and nitrogen dioxide NO2. This reaction is critical to the production of tropospheric ozone since NO2 is photolyzed by solar radiation producing O atoms which combine with O2 to form O3. We have investigated the detailed spectroscopy of simple alkoxy radicals: methoxy, CH3O, ethoxy, (CH3) CH2O, and isopropoxy, (CH3)2CHO. Our work has provided rotationally resolved spectra of both methyl substituted methoxy radicals as well as ones for the 3 deuterium isotopomers of methoxy. Analyses of these spectra have quantified unusual vibronic interactions between their electronic states, which serve as excellent experimental benchmarks for evaluating the successes and limitations of recent electronic structure calculations for these molecules. The nitrate radical (NO3) is a key reactant in atmospheric chemistry leading to the formation of acid rain and is the primary oxidant in the night sky. It reacts with NO2 to form N2O5, which undergoes hydrolysis to nitric acid contributing to acid rain. NO3 can also add to alkenes in the atmosphere to produce nitrates, which are stable enough to spread the effects of NOx pollution from urban to pristine environments. Spectroscopic diagnostics can be very useful for unraveling the complex chemistry of NO3 in the atmosphere. However its electronic structure is remarkable in many respects and significant gaps remain in its understanding and concomitantly in its spectroscopy. Our work focused on the A2E"-X2A' electronic transition which is electric dipole forbidden but for certain sublevels is vibronically allowed. We have identified over 20 vibronic bands of its spectrum and have obtained rotationally resolved spectra for more than half of them. The on-going analysis of these spectra will provide an extensive database for NO3, which will be important both for diagnostic monitoring purposes and for validating electronic structure calculations. These and related discoveries have been reported in 7 peer-reviewed articles in very well respected journals and 2 more are in preparation. They have also been presented in some 33 lectures given at scientific conferences. This grant has supported 3 postdoctoral and 4 graduate students, and the supported research has formed the basis for all or part of three Ph.D. dissertations of the latter.
