Study on Bonding Characteristics and Infrared Spectra of U₂F₁₀

U₂F₁₀ is an important intermediate in molecular laser isotope separation (MLIS), where the polymerization of UF₅ significantly affects the process efficiency. The molecular structure, bonding characteristics, and vibrational properties of U₂F₁₀ were systematically investigated by scalar relativistic density functional theory (DFT) calculations combined with low-temperature matrix-isolation infrared spectroscopy. Theoretical calculations were performed using the B3LYP-D3(BJ) method in Gaussian 16, with the ECP60MDF pseudopotential and def2-TZVP basis set adopted for uranium and fluorine, respectively. Considering the open-shell nature of U₂F₁₀, multiple spin states were optimized to determine the ground-state configuration, and wavefunction stability analyses were conducted for all optimized structures. Bonding characteristics were elucidated through wavefunction analyses including Mayer bond order, natural population analysis (NPA), electron localization function (ELF), and localized orbital locator (LOL). Experimentally, U₂F₁₀ was generated by pulsed Nd:YAG laser (1 064 nm) ablation of a uranium target in an F₂/Ne gas mixture (0.05%-1.0%) and codeposited in a neon matrix at 3.2 K. A high-resolution Fourier transform infrared spectrometer was used to record the spectra of the sample before and after controlled annealing (5.0-10.0 K). Theoretical calculation results show that U₂F₁₀ adopts a D_2h symmetric structure stabilized by two fluorine bridges (U-F_bri bond length of 2.296 Å) without direct U-U bonding. The terminal U-F_end bonds exhibit strong ionic character (bond order is 1.1; NPA charges: U = +2.94e, F_end ≈ –0.58e), whereas the bridging U-F_bri bonds are weaker (bond order is 0.4) but crucial for dimer stability, with the bridging fluorine atoms accumulating higher electron density (NPA charge: F_bri = –0.66e). Thermodynamic analysis confirms that the dimerization of UF₅ to form U₂F₁₀ is a highly exothermic process (ΔH = –39.5 kcal/mol at 4 K). Meanwhile, the stepwise fluorination of U₂ by F₂ exhibit very strong exothermicity (for example, the formation of U₂F₂ has ΔH = –274 kcal/mol). Four experimentally observed absorption bands at 630.2, 614.4, 581.1, and 554.1 cm⁻¹ in neon matrix—exhibiting synchronous intensity changes during annealing and fluorination—were unambiguously assigned to U₂F₁₀ vibrations, aligning with theoretical modes with minor matrix shifts (Δ_Ne-calcd = 4.6-7.8 cm⁻¹). This study experimentally identifies the gaseous U₂F₁₀ for the first time, resolving its structure, ionic bonding characteristics, and spectral features, thereby filling the critical long-standing knowledge gap. The results establish a molecular-level understanding of the stability of the UF₅ dimer and the reactivity of U₂F₁₀, providing key insights for optimizing the molecule laser isotope separation (MLIS) efficiency and the uranium conversion mechanisms for the nuclear fuel cycle.