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Project Activities

This grant provided me with salaries for two students for two summers as well as my salary for three summers.  The equipment funds were used to acquire a Thermo Scientific 380 FT-IR spectrometer.  This spectrometer in combination with a Raman spectrometer that I purchased with a grant from the NSF, has been a tremendous asset to my research program as well as to several of my colleagues at UP, Portland State University, and a local company.  The research requires that a large number of spectra be collected for the solute-solvent/cosolvent systems under investigation and we have made excellent progress toward that goal primarily due to the efficiency of both the FT-IR and the FT-Raman systems.  Furthermore, I have been able to allow guests to collect spectra with a relatively small time commitment, thus allowing me to make it more available.

I began the summer 2008 research with a new group of three undergraduates.  The first two weeks were devoted to training the students on the equipment and explaining the overall scope of the project.  Given our success with observing molecular interactions in solute/cosolvent systems, I decided to investigate the interaction between CO₂ and H₂O. I chose this direction in response to the interest, within the scientific community, in understanding the molecular interactions between CO₂ and H₂O.  Further knowledge of this system may lead to the development of new technology to capture and sequester CO₂.  Our ability to observe changes in the vibrations of a molecule is a sensitive technique for probing the local environment.  However, CO₂ and H₂O are essentially opaque in the infrared region.  To solve this problem, we designed a cell to collect infrared spectra of high-density fluids, specifically those that have an extremely large absorption cross-section.  The design fits the ATR module that is part of our instrument.  This cell will withstand pressures up to 35 MPa; thereby, allowing us to duplicate the pressures and temperatures of the deep ocean, where it has been proposed to sequester CO₂.  The manuscript reporting this design has been submitted to the Journal Vibrational Spectroscopy.  We used the cell to collect a series of infrared spectra in addition to the Raman spectra that were collected simultaneously.  Table 2 lists the conditions at which spectra were collected.

Table 2:  Mixtures of CO₂ and H₂O for which Raman and infrared spectra have been collected at the specified temperatures and 1500 p.s.i. pressure.

Each entry consists of 10 spectra of 32 scans each at 1 cm^-1 resolution.  This allows us to calculate the standard error such that the observed peak shifts will be statistically meaningful.  Furthermore, for the 90% and 10% mixtures, spectra were collected at pressures from 1000 to 3000 p.s.i. in increments of 100 p.s.i. at the temperatures listed.

We culled the data set and repeated experiments in order to confirm that we were making clathrates.  The resulting data appear to be complete and the spectra are of publishable quality.  However, an article recently appeared Ripmeester, et al. (J. Phys. Chem. A 2009, 113, 6308-6313) reporting essentially the same study upon which we have been working; however, our spectra were collected at higher resolution and show rotational structure on the asymmetric stretch of CO₂.  Since that discovery, we have been concentrating on the rotational structure of the enclathrated CO₂.  We are currently preparing an experiment to collect x-ray diffraction data simultaneously with the Raman and FT-IR spectra to couple the experimental results reported by Ripmeester, et al. and Ikeda, et al. (J. Chem. Phys. 1998, 108 (4), 1352-1359).  This work will be done in collaboration with my colleague, Dr. Edward Valente, at the University of Portland with our x-ray diffractometer.

Findings

We were able to reproduce the Raman spectra obtained by Sum, et al. (J. Phys. Chem. B 1997, 101, 7371-7377) of CO₂ incorporated into clathrate hydrates and then use the identical conditions to collect infrared absorption spectra of these structures.  We found that the enclathrated CO₂ exhibited rotational structure similar to gas phase CO₂.  Calculation of the rotational constant suggests that the CO₂ freely rotates inside the clathrate.  We predicted this phenomenon based on a suggestion by Consani and Pimentel in their paper on enclathrated C₂D₂ (J. Phys. Chem., 1987, 91, 289-293).  Furthermore, we found that the infrared spectra revealed that CO₂ exists in several different environments that are highly dependent upon the concentration of CO₂ in H₂O, the temperature, and the mixing time.

In the midst of analyzing the data and writing the manuscript reporting our results, a paper appeared (J. Phys. Chem. A 2009, 113, 6308-6313) reporting essentially the same study that we conducted.  While we were encouraged to see that another group, with which we have had no contact, reached many of the same conclusions, it was disappointing to have missed the opportunity to publish our results.  However, upon careful reading of the article, I noticed that our spectra were collected at higher resolution and as a result, we were able to see rotational structure in the asymmetric stretch of CO₂ that was absent in the spectra presented Ripmeester, et al. (J. Phys. Chem. A 2009, 113, 6308-6313).  The rotation of CO₂ in the clathrate cage was suggested by Ikeda, et al. from their x-ray crystallographic data that they presented in the Journal of Chemical Physics (J. Chem. Phys. 1998, 108 (4), 1352-1359).  Our manuscript reporting this discovery is in preparation and we plan to submit it to the Journal of Physical Chemistry Letters before the end of the fall semester.