INFRARED INDUCED REACTION OF ATOMIC CHLORINE WITH SOLID PARAHYDROGEN
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Date
2006
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Ohio State University
Abstract
The Cl + H$_{2}\to $HCl + H reaction has been studied in solid parahydrogen (pH$_{2})$ via FTIR spectroscopy of the HCl product. Atomic Cl is generated in solid pH$_{2}$ from the \textit{in situ} photodissociation of Cl$_{2}$ at 355 nm. The trapped Cl atoms are too cold to react with the surrounding pH$_{2}$ matrix at liquid helium temperatures given the reaction has a barrier of $\sim $ 5 kcal mol$^{-1}$. The hydrogen abstraction reaction can not even proceed via quantum mechanical tunneling since the reaction is endothermic by approximately 360 cm$^{-1}$, which is too great at temperatures of 2-4 K. The reaction can be triggered by allowing IR light from the FTIR spectrometer to impinge upon the Cl atom doped pH$_{2}$ solid. Spectra can be recorded with and without a long bandpass filter present in the IR beam which filters out light above 3900 cm$^{-1}$. With the filtered light, repeated scans show no evidence for growth of the HCl R$_{1}$(0) feature and thus no reaction. However, when the filter is removed the HCl absorption grows in intensity signaling the onset of reaction. The integrated absorbance of the HCl feature as a function of time can be fit to a simple first-order rate equation that indicates the Cl atoms have a half-life of 100 minutes under these conditions. We speculate that the reaction is due to solid pH$_{2}$ absorptions that occur above 4100 cm$^{-1}$, launching H$_{2}$ vibrational excitons (vibrons) into the solid which scatter off the Cl atoms and induce reaction. These experiments are ongoing and the most recent results will be presented at the meeting.
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Author Institution: Department of Chemistry,; University of Wyoming, Laramie, WY 82071-3838