Andrew Duffin and Dr. Eric Sevy, Chemistry and Biochemistry
Gas phase atoms and molecules constantly exchange energy in collisions. These energy transfer events are the rate limiting processes in reactions that are important for understanding atmospheric and combustion chemistry. While a great deal is known about collisional energy transfer, there is no good theory for predicting the outcome of energy transfer collisions. A study of energy transfer as a function of vibrational frequency would aid in formulating a model that would accurately describe energy transfer events.
To study collisional energy transfer, large donor molecules, such as benzene, and small bath molecules, such as carbon dioxide, are allowed to flow through a long collision cell. An excimer laser is used to excite the donor molecules into highly vibrationally excited states. The donor molecules them collide with the bath molecules and transfer energy in the process. An ultra high-resolution diode laser is used to probe the CO2 molecules to determine the amount of energy transferred to the bath. The diode laser probes the rotational quantum states of the carbon dioxide and can also measure Doppler broadening in order to obtain translational energy information.
Experimental work as well as computational studies suggest that there is a correlation between donor molecule vibrational mode character and the energy transfer cross section. In order to experimentally determine the relationship we planned to change the frequency of vibrations in benzene donor molecules by isotopically substituting H atoms for D, Cl, and F. Unfortunately, we found that chlorinated benzene has negligible vapor pressure near room temperature. Consequently, we were unable to experimentally determine a relationship between donor vibrational mode frequency and energy transfer.
However, we were able to use the laboratory time to further determine the photochemistry of pyrazine. When 248 nm light, wavelength output of the excimer laser, irradiates pyrazine, the pyrazine molecules absorb the energy. The absorbed energy can be re-radiated, dissipate through collisions, or in some cases it can break the pyrazine molecule apart. Previous work had shown pyrazine (C4H4N2) photoproducts to include hydrogen cyanide (HCN), acetylene (C2H2), and an unknown C3H3N product. We were able to use both FT-IR and UV-VIS spectroscopy to determine the identity of the unknown photoproduct as acrylonitrile.
A 20 cm gas cell was filled with pyrazine and fired on using the excimer laser. The cell was then placed in a FT-IR or UV-VIS spectrophotometer. After some time finding the correct parameters, we were able to obtain spectra that indicated the presence of acrylonitrile.