Infrared Spectra of UF ( = 1-4) in Solid Ne and Ar Matrices: Symmetry-Breaking Structure of UF.

This study addresses two persistent challenges in uranium fluoride chemistry: resolving decades-long spectral assignment conflicts across UF, UF, and UF species, and conclusively settling the symmetry controversy of UF. By the cryogenic matrix isolation IR spectroscopy technique in combination with relativistic quantum chemical calculations, we experimentally tracked the stepwise formation of UF to UF in neon and argon matrices. Theoretical validation has led to a reassignment of the infrared absorption bands for UF, UF, and UF, defining their molecular geometries. While UF exhibits a V-shaped structure and UF has a pyramidal configuration, UF adopts a geometry rather than a 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 ( = 2-4) and argon atoms induced vibrational redshift. Bonding analyses revealed that U-F bonds in UF ( = 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.