Spectroscopy and Photochemistry of Matrix Isolated Species

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Normally inaccessible chemical species including free radicals, hydrogen-bonded complexes, unusual inorganic species, transition metal containing intermediates, metal hydrides, and molecular ions are under investigation using infrared matrix-isolation spectroscopy. These transient molecules are prepared by reactions of suitable atoms and/or molecules prediluted in argon by precursor photolysis, or by microwave discharge of the reagents during condensation at 7 K in a vacuum chamber. Solid argon isolates the reaction or photolysis products and preserves them for spectroscopic study at 7 K. Neon is used to isolate new reaction products on a 4 K substrate. Pure H2 and D2 are also be employed as a reagent and matrix for the investigation of novel metal hydrides. A new source of atoms that require high temperatures to generate, namely pulsed laser ablation from solid samples, has been developed for use in matrix-infrared spectroscopy in this laboratory. This method exploits two advantages: first, the ablated atoms contain excess energy which can activate reactions with small molecules, and second, collisions with matrix atoms during the condensation process relax energetic product molecules and allow them to be trapped in the solid matrix for spectroscopic study. In addition, laser-ablation also produces cations and electrons for reactions to make charged products.

The experimental apparatus used in hydrogen matrix investigations is sketched in the figure. Infrared spectra are recorded after co-depositing reagent and host matrix gases on the cold sample window and rotating by 90 degrees to face the infrared light beam.

These infrared spectroscopic matrix-isolation studies characterize the bonding and structure of chemical intermediates, interesting new inorganic molecules and complexes and molecular ions and often provide a useful complement and guide to high resolution gas phase work. Stable isotopes are used to determine assignments of the observed infrared absorptions to fundamental vibrational frequencies. Comparison of vibrational energies within related chemical species provides conclusions about the bonding of these newly observed chemical intermediates. Selection rules based upon molecular symmetry and vibrational analysis help determine the molecular geometry. Ab initio electronic structure calculations are done to find molecular structures compatible with the infrared spectrum. Agreement between calculated and observed isotopic vibrational spectra provides further evidence for the discovery of new transient molecular species.

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