Infrared Spectra of UF_<i>n</i> (<i>n</i> = 1–4) in Solid Ne and Ar Matrices: Symmetry-Breaking <i>D</i>_2<i>d</i> Structure of UF₄

This study addresses two persistent challenges in uranium fluoride chemistry: resolving decades-long spectral assignment conflicts across UF₂, UF₃, and UF₄ species, and conclusively settling the symmetry controversy of UF₄. By the cryogenic matrix isolation IR spectroscopy technique in combination with relativistic quantum chemical calculations, we experimentally tracked the stepwise formation of UF to UF₆ in neon and argon matrices. Theoretical validation has led to a reassignment of the infrared absorption bands for UF₂, UF₃, and UF₄, defining their molecular geometries. While UF₂ exhibits a V-shaped <i>C</i>_2<i>v</i> structure and UF₃ has a pyramidal <i>C</i>_3<i>v</i> configuration, UF₄ adopts a <i>D</i>_2<i>d</i> geometry rather than a <i>T</i>_<i>d</i> symmetry, arising from the Jahn-Teller distortion, which was verified by complete active space second-order perturbation theory (CASPT2) calculations incorporating spin-orbit coupling, supporting predictions from relativistic density functional theory and BW-MRCCSD calculations by Johnson et al. Moreover, weak van der Waals interactions between UF_<i>n</i> (<i>n</i> = 2-4) and argon atoms induced vibrational redshift. Bonding analyses revealed that U-F bonds in UF_<i>n</i> (<i>n</i> = 1-6) possess dual ionic-covalent character, with ionic contributions of 78-88%. The covalent enhancement in fluorides arises from the overlap of U 5f/6d orbitals with F 2p orbitals and their near-degeneracy. These findings reconcile historical discrepancies, establish definitive benchmarks, and advance uranium fluoride chemistry for nuclear fuel applications.