FTIR spectra of the HF₂^- and H₂F ₃^- anions isolated in solid argon and neon

Argon diluted samples of HF were bombarded by low energy electrons (150-400 eV) from a thermionic electron source during condensation at 12 K. Several polyfluoride anions, which have extremely strong hydrogen bonds, were characterized using matrix infrared spectroscopy. Three principal ions were trapped in the argon matrix: HF₂ ( 1377 cm ^-1 ), H ₂F₃^- (1815 cm^-1), and Ar _nH⁺ (904 cm^-1). Electron impact experiments performed with HF/DF mixtures gave counterpart absorptions: DF₂ ^- (965 cm^-1), HDF₃^- (1707 cm ^-1), D₂F₃^- (1391 cm^-1), and Ar_nD⁺ (644 cm^-1). The 1377 cm^-1 matrix band is assigned to v₃ of linear, centrosymmetric HF ₂^-, based on agreement with solid state and matrix ion pair spectra, and ab initio calculations, and a proper H/D ratio (1.427) for a linear centrosymmetric species. However, this assignment challenges the recent 1848 cm^-1 gas phase assignment to v₃ of HF ₂^- using laser spectroscopy; the 1848 cm^-1 absorption exhibits an H/D ratio (1.323) that is too low for the v₃ vibration of centrosymmetric HF₂^-, and it should be reassigned to the v1 + v3 combination band. The 1815 cm^-1 matrix band is assigned to the out-of-phase H-F stretching fundamental of C _2vH₂F₃^-, which is in agreement with solid K⁺H₂F₃^- spectra.