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.
Microwave rotational transitions have been observed at low J (0-3) for several isotopic species of the Ne-HCN dimer using the Balle/Flygare Mark II Fourier transform spectrometer with a pulsed nozzle as the source. For 2oNe-HC 14N, the main K =0 transitions give rotational constants B, D J, and H J of 2772.816 and 1.280 MHz and 1.173 kHz. The 14N nuclear quadrupole constant increases linearly with J(J+ 1) at a slope Dx of -12.7 kHz fro~ a value for Xa( 14 N) of -0.957 MHz at J=O. The pseudodiatomic approximation for Band Xact 4 N) leads to a value of 3.89 A for the Ne to HC 14N center-of-mass (c.m.) distance R, and to 46.8° for the "average" bend angle () of HC 14N. Some of the K=O, J= 1 ..... 2, and J=2 ..... 3 transitions exhibit one or two weak satellites -30 MHz away, usually below, but also both above and below. The J= 1 ..... 2 low frequency satellites for 2oNe-HC 14N and 2oNe_ HC 15N, nominally 1 11 ..... 2 12 , are symmetrical doublets with splittings of 305 and 439 kHz, respectively. The 14N hyperfine structure (hfs) is identical for the two 2oNe-HC 14N components as is the Stark effect for 2oNe-HC 15N. The molecular mechanics for clusters (MMC) model was used to calculate potential energy surfaces for Rg-HCN dimers, giving stabilities of 21, 37, 85, and 108 cm -I with He, Ne, Ar, and Kr as the rare gas. A qualitative comparison of the experimental properties for the dimers with Ne, Ar, and Kr as the rare gas is based on the surfaces. The extremely mobile internal dynamics of Ne-HCN are attributed to its potential surface, which is both very shallow and isotropic.
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