Infrared Spectra of UF_n (n = 1-4) in Solid Ne and Ar Matrices: Symmetry-Breaking D_2d 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 C_2v structure and UF₃ has a pyramidal C_3v configuration, UF₄ adopts a D_2d geometry rather than a T_d 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_n (n = 2-4) and argon atoms induced vibrational redshift. Bonding analyses revealed that U-F bonds in UF_n (n = 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.