Neon matrix induced perturbations of weak hydrogen-bonded complexes. Part II. The HF dimer and its isotopomers

Abstract

The infrared spectra of the (HF)₂, (DF)₂ and HF-DF isotopomers have been investigated in neon matrices between 8000 and 20 cm⁻¹. They significantly increase the number of one and two quanta vibrational transitions identified in matrices, especially those involving deuterated species, and also include relative intensity measurements. A straightforward assignment is proposed for five fundamental transitions of each isotopomer: the two H(D)F stretches (ν₁, ν₂) and three out of four intermolecular modes (ν₄, ν₅, ν₆). The non-observation of the ν₃ band, predicted by the calculations at high frequency and with a strong intensity, is unexplained. Two kinds of two quanta transitions have been identified: ν_intra + ν_inter, with ν_intra = ν₁ or ν₂ and ν_inter = ν₄, ν₅, ν₆ and those involving only ν_intra (2ν₁, 2ν₂ and ν₁ + ν₂). From the first set, one deduces the ν_inter frequencies in the first excited states of ν₁ (free HF) and ν₂ (H-bonded HF): they noticeably increase in the ν₂ excited state, which corresponds to a strengthening of the hydrogen bond, but do not significantly vary in the ν₁ excited state. Assuming the transferability of the anharmonicity coefficients x_ij = ν_i+j − (ν_i + ν_j), with i = 1, 2 and j = 4, 5, 6, from the neon matrix to the gas, we propose a new determination of the ν_inter frequencies in the gas phase which cannot be measured directly in the absence of molecular beam experiments in the far infrared. Finally, from the second set, the intramolecular anharmonicity coefficients x₁₁, x₂₂ and x₁₂ are obtained for (HF)₂.