PhD Research
Juliane L. Fry
2001-2005, California Institute of Technology

Vibrational spectroscopy and kinetics of peroxynitrous acid (HOONO)

The molecule that is the major focus of my thesis research, HOONO, is a weakly-bound isomer of nitric acid produced in the OH + NO2 reaction. The OH + NO2 reaction is the major sink of both HOx (OH + HO2) and NOx (NO + NO2) families of radicals, important reactants in polluted air. The short thermal lifetime of HOONO makes it a temporary reservoir of OH and NO2 rather than a permanent sink. Thus, understanding the extent and kinetics of HOONO formation is crucial to evaluating the OH + NO2 reaction and the atmospheric concentrations of OH and NO2. Laboratory studies apply spectroscopic techniques to study HOONO formation, and hence an understanding of HOONO spectroscopy is an essential foundation.

We have studied the kinetics and vibrational spectroscopy of peroxynitrous acid (HOONO) in the region of the first OH stretch overtone experimentally and theoretically, using action spectroscopy and cavity ringdown spectroscopy, and statistical master equation and two-dimensional coupled OH stretch torsion theoretical models.

Action spectroscopy and isomerization kinetics
with Sergey Nizkorodov, Mitchio Okumura, and Paul Wennberg (Caltech)

In our lab, two conformers of HOONO are distinguished based on their differing time and temperature behavior, and assigned as cis-cis and trans-perp HOONO. The isomerization from the less stable trans-perp HOONO to cis-cis HOONO is observed directly in the range 223 - 238 K to determine the isomerization barrier of 33 12 kJ/mol. Statistical calculations are consistent with the experimental measurement. These studies indicate that the only atmospherically relevant conformer is cis-cis HOONO. A direct absorption cavity ringdown spectrum of the first OH overtone region of cis-cis HOONO is measured for comparison to the action spectrum. The complex spectroscopy of the cis-cis HOONO first OH overtone region is then elucidated using a two-dimensional OH stretch torsion coupling model, which accurately predicts the major features in both the absorption and action spectra of cis-cis HOONO. With this model, we are able to assign the major features in the cis-cis HOONO overtone spectrum and draw conclusions relevant to measurement of the branching ratio of HOONO to HNO3, the quantity of importance in atmospheric modeling.
The poster at right (click for a larger version) describes our isomerization kinetics studies.

Two-dimensional model to assign the cis-cis HOONO action spectrum
with Anne McCoy (Ohio State University), Mitchio Okumura, and Joe Francisco (Purdue)

A joint theoretical and experimental investigation was undertaken to study the effects of OH stretch/HOON torsion coupling and of quantum yield on the first overtone action spectrum of cis-cis HOONO. The minimum energy path along the HOON dihedral angle is computed. Then the two-dimensional ab initio potential energy and dipole moment surfaces for cis-cis HOONO are calculated as functions of the HOON torsion and OH bond length about the minimum energy path. The OH stretch vibration depends strongly on the torsional angle, and the torsional potential possesses a broad shelf at ~ 90, the cis-perp conformation. The calculated electronic energies and dipoles are fit to simple functional forms and absorption spectra in the region of the OH fundamental and first overtone are calculated from these surfaces. While the experimental and calculated spectra of the OH fundamental band are in good agreement, significant differences in the intensity patterns are observed between the calculated absorption spectrum and the measured action spectrum in the 2νOH region. These differences are attributed to the fact that several of the experimentally accessible states do not have sufficient energy to dissociate to OH + NO2 and therefore are not detectable in an action spectrum. Scaling of the intensities of transitions to these states, assuming D0=82.0 kJ/mol, is shown to produce a spectrum that is in good agreement with the measured action spectrum. Based on this agreement, we assign two of the features in the spectrum to Δn = 0 transitions (where n is the HOON torsion quantum number) that are blue shifted relative to the origin band, while the large peak near 7000 cm-1 is assigned to a series of Δn = +1 transitions, with predominant contributions from torsionally excited states with substantial cis-perp character. The direct absorption spectrum of cis-cis HOONO (6300-6850 cm-1) is recorded by cavity ringdown spectroscopy in a discharge flow cell. A single band of HOONO is observed at 6370 cm-1 and is assigned as the origin of the first OH overtone of cis-cis HOONO. These results imply that the origin band is suppressed by over an order of magnitude in the action spectrum, due to a reduced quantum yield. The striking differences between absorption and action spectra are correctly predicted by the calculations.

Cavity ringdown spectroscopy
with Andrew Mollner and Mitchio Okumura (Caltech)

A direct absorption cavity ringdown experiment confirmed that the action and absorption spectra are different for cis-cis HOONO, which is evidence that the dissociation threshold for cis-cis HOONO is in the midst of the first OH overtone spectrum. The action spectroscopy technique gains its high sensitivity by employing laser induced fluorescence detection of OH photofragments of HOONO dissociation, after energy has been resonantly coupled into the molecule via a vibrational mode. Thus, the action spectrum is the absorption spectrum convoluted with the quantum yield for photodissociation. The bottom panel at right shows the experimental (interfering species subtracted) direct absorption spectrum of cis-cis HOONO, showing only one major band. At far right is shown the variation in quantum yield over the spectrum (middle panel), and the action spectrum is shown in the bottom panel (at various temperatures). If you can mentally deconvolute well enough, you'll see that deconvoluting the quantum yield from the action spectrum would yield something like the noisy direct absorption spectrum.

Rotational spectroscopy of cis-cis HOONO

In the course of our HOONO studies, we became quite proficient at generating this elusive, short-lived molecule in flow systems. So we threw the book at it spectroscopically, to obtain the most complete characterization of this molecule possible, to facilitate searches for HOONO in the Earth's atmosphere. We were able to generate HOONO in low-pressure submillimeter and microwave spectrometers and characterize the rotational and distortion constants of cis-cis HOONO.

Rotational constants, molecular structure, and dipole moment of cis-cis HOONO
with Brian Drouin and Chip Miller (JPL), ongoing also with Geoff Blake and Susanna Widicus (Caltech)

The pure rotational spectrum of cis-cis HOONO has been observed. Over 220 transitions, sampling states up to J'= 67 and Ka'= 31, have been fitted with an rms uncertainty of 48.4 kHz. The experimentally determined rotational constants agree well with ab initio values for the cis-cis conformer, a five-membered ring formed by intramolecular hydrogen bonding. The small, positive inertial defect Δ=0.075667(60) amu 2 and lack of any observable torsional splittings in the spectrum indicate that cis-cis HOONO exists in a well-defined planar structure at room temperature.
Work is ongoing to determine the molecular structure and dipole moment of cis-cis HOONO by isotopomer studies and Stark effect studies, respectively.

Photodissociation of organic hydroperoxides

Peroxides are an important and ill-understood class of molecules present in the Earth's atmosphere. Hydroperoxides in particular provide reservoirs for hydroxyl radical, the most important oxidative species in the lower atmosphere. Interaction with ultraviolet and infrared photons from the sun is the predominant mechanism by which a hydroperoxides release OH to the atmosphere. In the case of hydrogen peroxide, H2O2, the cross-sections for UV photolysis are well-established. This is not the case for the two next most abundant peroxides, methyl hydroperoxide (MHP, CH3OOH) and hydroxymethyl hydroperoxide (HMHP, HOCH2OOH), which are more difficult to synthesize and maintain in a stable state.

UV photolysis cross sections of methyl hydroperoxide and hydroxymethyl hydroperoxide
with Coleen Roehl, Zsuzsa Marka, and Paul Wennberg (Caltech)

We have completed UV cross section measurements, relative to H2O2, in the range 305 - 365 nm (not all shown here), of methyl hydroperoxide. We are currently in the process of conducting the same analysis of hydroxymethylhydroperoxide. We can then estimate the magnitude of the atmospheric OH source from UV photolysis of these organic peroxides. These experiments are done by action spectroscopy, measuring the OH product from photodissociation. With UV photons, however, we expect that the quantum yield is always unity, and hence this is effectively a very sensitive absorption spectroscopy.

Overtone action spectroscopy of hydroxymethyl hydroperoxide
with Coleen Roehl and Paul Wennberg (Caltech), Jamie Matthews and Amit Sinha (UCSD), and Jo Lane and Henrik Kjaergaard (Otago)

We have measured the overtone action spectrum of HMHP in the 4vOH and 5vOH regions, and have predicted and interpreted these spectra using ab initio calculations. The dissociation energy for HMHP falls midway through the 4vOH spectrum. This indicates that overtone photodissociation processes will not be atmospherically important for this molecule, since the dissociation threshold is so high.

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