Lisa M. Goss
Associate Professor of Chemistry
Ph.D. Physical Chemistry,
University of Colorado at Boulder -- 1998
|Research Area:||High resolution vibrational spectroscopy and atmospheric chemistry|
|Student Experience required for research:||Chem 111, Chem 112, enrolled in Chem 301 for project 1, enrolled in Chem 351 for projects 2 & 3|
|Student Experience gained from research:||Synthetic methods, NMR and IR spectrometry for project 1, rotational and vibrational spectroscopy, quantum mechanics for projects 2 & 3|
|Ideal Preparation for:||Graduate school in chemistry, employment in analytical labs or synthesis|
1) Vibrational spectroscopy of the atmospheric pollutant peroxy acetyl nitrate and related molecules
Photochemical air pollution affects cities around the world, with impacts ranging from decreased visibility to increased respiratory distress in humans. It is characterized by the formation of secondary pollutants including ozone, nitrogen dioxide, peroxy acetyl nitrate (PAN), aldehydes and ketones from the oxidation of atmospheric hydrocarbons in the presence of sunlight and nitric oxide. PAN is formed from reaction of acetyl peroxy radicals and nitrogen dioxide in photochemical air pollution:
CH3COOO• + NO2 → CH3COOONO2 (1)
PAN is phytotoxic and a powerful lachrymator and thus an important secondary pollutant to understand. An additional characteristic of the chemistry of PAN is to act as a reservoir for odd nitrogen species which allows them to be transported away from the source of the pollution. Odd nitrogen is the sum of NO and NO2 and in pollution is formed in combustion reactions such as:
N2 + O2 → 2 NO• (elevated temperature) (2)
When PAN decomposes, it reforms nitrogen dioxide:
CH3COOONO2 → NO2 + CH3COOO• (3)
Odd nitrogen is an important component in photochemical air pollution because its concentration determines the amount of ozone formed during a photochemical air pollution episode. Photolysis of NO2 produces ozone in the troposphere. Nitrogen dioxide photodissociates at λ < 420 nm to give nitric oxide and an oxygen atom and the resulting oxygen atom reacts with O2 and another body (M) within the troposphere to form ozone:
NO2 + hν (λ < 420 nm) → O + NO• (4)
O + O2 + M → O3 + M (5)
Since ozone is produced from the photolysis of nitrogen dioxide, the amount of nitrogen dioxide in the troposphere is closely monitored.
The thermal decomposition and near-UV and mid-IR absorption spectra of PAN have been well characterized. This project will examine the near-IR absorption spectrum of PAN. It has been shown recently that near-IR absorption may contribute to the atmospheric chemistry of molecules such as sulfuric acid and pernitric acid. This project will begin with the synthesis of PAN and then look at the spectrum in a cell and then continue to related molecules with other R groups.
2) Rotationally resolved vibrational spectroscopy of atmospheric pollutants containing methyl rotors
Methyl nitrite is a gas phase molecule formed in the atmosphere by the reaction of methoxy radicals with nitric oxide:
CH3O• + NO• → CH3ONO (1)
In equation (1), the dots indicate that those molecules are radicals with unpaired electrons. This makes these radicals particularly reactive. In daylight, methyl nitrite rapidly photolyzes to reform these radicals:
CH3ONO + hν (λ ≤ 440 nm) → CH3O• + NO• (2)
Since the methoxy radical (CH3O•) and nitric oxide (NO•) are both key intermediates in photochemical air pollution, methyl nitrite serves as a night-time reservoir, or storage molecule, that contributes to the “aged smog” phenomenon. This is where a pollution episode is worse on a second day due to pollutants that have not been removed from the atmosphere overnight.
High resolution Fourier Transform infrared spectrometry has been used on a variety of reactive molecules to measure their abundance in the atmosphere. This measurement technique uses the Beer-Lambert law which says that the absorbance is proportional to the concentration of the absorbing molecule and the path length over which this absorption occurs:
A(λ) = - Ln ( I(λ) / Io(λ) ) = (α(λ))x(concentration)x(pathlength) (3)
Here I(λ) and Io(λ) are light (as a function of wavelength) with and without absorber. The absorption coefficient is a characteristic of the particular molecule being studied. In the atmosphere, the concentrations are typically so low that a long pathlength (100’s of meters) is typically required. The absorption coefficient is measured in a separate experiment, the pathlength is determined by the geometry of the experiment, and the absorbance is calculated from measurements of I(λ) and Io(λ) so that the concentration can then be determined. The role of laboratory studies such as in this project is to measure the absorption coefficient as a function of wavelength (λ), understand how it changes with pressure and temperature, and, most importantly, to understand why the absorption coefficient (or the “spectrum”) of the molecule looks the way it does. This understanding is particularly crucial when the spectrum of the molecule being measured overlaps with the spectrum of other molecules that are present in the atmosphere and therefore might interfere with determining the concentration of methyl nitrite. Figuring out which lines (features in the spectrum) belong to which molecules can be a challenging task.
In a simple linear molecule like CO2, the spacing between the lines is directly proportional to the moment of inertia. Using the masses of the atoms and the moment of inertia, the bond lengths can be determined. For a large, asymmetric molecule like methyl nitrite, the pattern of lines is more complex and figuring out which lines correspond to which transitions between energy levels ("assigning the spectrum") is an involved process. The amount of information that can be obtained is also larger as well. The methyl group in methyl nitrite can rotate and this internal rotation can be investigated. Methyl nitrite also possesses two isomers. These two isomers only differ by rotation about the O-N bond. Both of these isomers are present in a sample of methyl nitrite and are observed in the spectrum.
The objective of this project is to measure and analyze the vibrational bands of methyl nitrite. The measurements will be made in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL) which is operated by Pacific Northwest National Lab (PNNL) for the Department of Energy. The High Resolution Infrared Spectroscopy Laboratory within EMSL has a Bruker IFS120 spectrometer that is capable of measuring infrared spectra at 0.0015 cm-1 resolution. Once the spectra have been measured at high resolution, they will be assigned and a nonlinear least squares fit will be performed to extract the constants characterizing the structure, cis-trans isomerism, and internal rotation of the molecule. Additional methyl rotor containing molecules to be investigated include methyl glyoxal, CH3COCHO.
3) Rotationally resolved vibrational spectroscopy of CFC replacement molecules
CFC’s have been implicated in polar ozone depletion (“the ozone hole”) as the source of chlorine atoms. Replacements for CFC’s have focused on molecules which are inert enough to be used for the same purposes as CFC’s but reactive enough to break down in the troposphere rather than reaching the stratosphere before breaking down like CFC’s. Release of Cl in the decomposition of CFC’s in the lower stratosphere results in catalytic destruction of ozone:
Cl + O3 → ClO + O2 (1)
ClO + O → Cl + O2 (2)
sum: O3 + O → 2 O2 (3)
This project will measure and analyze the rotationally resolved spectrum of replacement molecules such as HFC-125 (C2F5H) for reasons similar to those in project 2. CFC replacements could potentially be used in large amounts and are more stable than many pollutants and therefore reach concentrations in the atmosphere greater than that of a pollutant like methyl nitrite. An additional concern with these CFC replacements beyond pollutants like methyl nitrite is their absorption of IR radiation in the atmosphere. Changes in the absorption of IR radiation in the atmosphere (the greenhouse effect) can affect global climate.
"Rotationally Resolved Spectroscopy of the ν8 Band of cis-Methyl Nitrite," L.M. Goss, C.D. Mortensen, T.A. Blake, Journal of Molecular Spectroscopy, 225(2), 182-188, 2004.
"A Demonstration of Acid Rain and Lake Acidification: Wet Deposition of Sulfur Dioxide," L.M. Goss, Journal of Chemical Education, 80, 39-40, 2003.
"Atmospheric absorption of near infrared and visible solar radiation by the hydrogen bonded water dimer," V. Vaida, J.S. Daniel, H.G. Kjaergaard, L.M. Goss, A. Tuck, Quarterly Journal - Royal Meteorological Society, 127 (575), 1627-1644, 2001.
"Sequential Two-Photon Dissociation of Atmospheric Water," L.M. Goss, V. Vaida, J.W. Brault, R.T. Skodje, Journal of Physical Chemistry A, 105 (1), 70, 2001.
"Direct Absorption Spectroscopy of Water Clusters," L.M. Goss, S.W. Sharpe, T.A. Blake, V. Vaida, J.W. Brault, Journal of Physical Chemistry A, 103 (43), 8020-8024, 1999.
"Spectroscopic Characterization of Supersonic Molecular Beams," V. Vaida, G.J. Frost, L.M. Goss, Israeli Journal of Chemistry, 37, 387-393, 1997.
"Measurements of High Resolution Ultraviolet-Visible Absorption Cross Sections at Stratospheric Temperatures: 1. Nitrogen Dioxide," G.J. Frost, L.M. Goss, V. Vaida, Journal of Geophysical Research, 101, 3869-3877, 1996.
"Measurements of High Resolution Ultraviolet-Visible Absorption Cross Sections at Stratospheric Temperatures: 2. Chlorine Dioxide," G.J. Frost, L.M. Goss, V. Vaida, Journal of Geophysical Research, 101, 3879-3884, 1996.
"The Matrix Isolation IR and UV Spectra of Conformers and Isomers of Oligo-Silanes," B. Albinsson, H. Teramae, H.S. Plitt, L.M. Goss, H. Schmidbaur, J. Michl, Journal of Physical Chemistry, 100, 8681-8691, 1996.
"Photooxidation of CS2 in the near-ultraviolet and its atmospheric implications," L.M. Goss, G.J. Frost, D.J. Donaldson, V. Vaida, Geophysical Research Letters, 22, 2609-2612, 1995.