Knowledge Bank

The Cl + H${2}\to $HCl+Hreactionhasbeenstudiedinsolidparahydrogen(pH${2})$ via FTIR spectroscopy of the HCl product. Atomic Cl is generated in solid pH$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ from the \textit{in situ} photodissociation of Cl$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ at 355 nm. The trapped Cl atoms are too cold to react with the surrounding pH$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ matrix at liquid helium temperatures given the reaction has a barrier of $\sim $ 5 kcal mol$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. The hydrogen abstraction reaction can not even proceed via quantum mechanical tunneling since the reaction is endothermic by approximately 360 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$, 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$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ solid. Spectra can be recorded with and without a long bandpass filter present in the IR beam which filters out light above 3900 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. With the filtered light, repeated scans show no evidence for growth of the HCl R$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$(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$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ absorptions that occur above 4100 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$, launching H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ 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.