Chemistry, Spectroscopy, and Isotope-Selective Infrared Photochemistry of a Volatile Uranium Compound Tailored for 10-μm Absorption: U(OCH₃)₆

This contribution reports on the chemical, vibrational spectroscopic, and infrared multiphoton photochemical properties of uranium hexamethoxide, U(OCH₃)₆, a prototype molecule for laser-induced uranium isotope separation with a carbon dioxide laser. Uranium hexamethoxide can be prepared from UC1₄ by conversion to Li₂U(OCH₃)₆, followed by oxidation with lead tetraacetate. Vapor pressure studies on U(OCH₃)₆ indicate that ΔH_sub° = 23 ± 3 kcal/mol and ΔS_sub° = 76 ± 4 eu; at 33 °C, the vapor pressure is 17 mtorr. The vibrational spectra of U(¹⁶OCH₃)₆ and U(¹⁸OCH₃)₆ have been assigned by using infrared and laser Raman data. Under idealized Oh symmetry, the U(¹⁶OCH₃)₆ U-O stretching fundamentals are assigned at 505.0 (A_1g), 464.8 (T_1u), and 414.0 cm^-1 (Eg). Tentative assignments are also made for several of the overtone and combination transitions evidence for possible lowering of the symmetry is presented. In gas-phase infrared photochemical experiments, the predominant U(OCH₃)₆ photoproducts isolated are U(OCH₃)₅, methanol, and formaldehyde. These are suggestive of multiphoton U-O bond homolysis to produce uranium pentamethoxide and methoxy radicals. The enrichment of unreacted U(OCH₃)₆ in ²³⁵U is maximum at ca. 927 cm^-1 (near what may be a U-O stretching overtone transition) and exhibits both a low fluence threshold and diminution at high fluence.