Radiofrequency and Microwave Spectrum of the Hydrogen Fluoride Dimer; a Nonrigid Molecule

The radiofrequency and microwave spectra of the K=0 states of (HF)2, (DF)2, and HFDF have been studied by the molecular beam electric resonance method. A unique hydrogen tunnelling motion involving the breaking and reforming of the hydrogen bond causes a splitting of rotational energy levels for (HF)2 and (DF)2, but not for HFDF. The electric dipole selection rules and nuclear spin statistics for the tunnelling molecules have been derived from a consideration of an extended permutation-inversion group. Rotational constants, tunneling doublings, electric dipole moments, and deuterium quadrupole coupling constants have been determined from the observed spectra of the K=0 states. (HF)2(DF)2HFDF(B+C)/2 (MHz)6504.8±2.06252.194±0.0026500.1±0.1ν (MHz)19 776±121579.877 ±0.004μa (D)2.987±0.0032.9919±0.00063.029±0.003(eqQ)Da (KHz)···110±8270±30These results are interpreted with a semirigid, nonlinear model of the dimer geometry. The F—F distance is 2.79 ± 0.05 Å, and the end hydrogen fluoride unit is bent from 60 to 70° from the F—F axis. Large amplitude motion of the hydrogen and deuterium atoms occurs. Higher hydrogen fluoride polymers have been studied by mass spectroscopy and electric deflection methods. All were observed to be nonpolar which is interpreted to imply cyclic structures.