Microwave rotational spectra, hyperfine interactions, and structure of the hydrogen fluoride dimers

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Abstract
The hyperfine structure has been resolved for the two J=0→1 transitions of (DF)2, for the low frequency J=0→1 transition of (HF)2, and for the J=0→1 transition of the mixed species HFDF. Also, the J=0→1 transition and hyperfine structure were observed for the other mixed species DFHF, not found previously. The relative intensities of the transitions for HFDF and DFHF, and their zero-point vibrational frequencies, correspond to an effective equilibration temperature between the two species of ∼50 K in the gas expansion. Resolution and assignment of the (DF)2 and (HF)2 hyperfine structure was aided by a method for suppressing the Doppler doubling in a Flygare spectrometer; it improved resolution twofold to a FWHM of 5 kHz at 11–14 GHz. The results bear on several features of the angular structure of this unusual system. At equilibrium, the end H/DF in HFDF and DFHF is bent 60±2° from the F⋅⋅⋅F axis, while the H/DF in the hydrogen bond is bent 7±3° in the opposite direction. The tunneling in (DF)2 gives an average angle between the DF’s and the a axis (39.3° and 38.7° for the two transitions) which is 1.5° larger than the composite (37.5°) of the DF’s in DFHF and HFDF. For the (HF)2 transition, the observed average angle (40°) is also 1.5° larger than the composite (38.5°) of the HF angles in the mixed species. The hydrogen bonding in HFDF and (DF)2 decreases the electric field gradient at the deuterium by ∼15% from its value in free DF.