Infrared Spectra of UFn (n = 1-4) in Solid Ne and Ar Matrices: Symmetry-Breaking D2d 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 C2v structure and UF3 has a pyramidal C3v configuration, UF4 adopts a D2d geometry rather than a Td 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 UFn (n = 2-4) and argon atoms induced vibrational redshift. Bonding analyses revealed that U-F bonds in UFn (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.