High-resolution infrared spectroscopy of DF trimer: A cyclic ground state structure and DF stretch induced intramolecular vibrational coupling

We present high resolution, infrared laser absorption spectra of (DF) ₃ in a slit supersonic jet expansion. In contrast to previous structureless near IR spectra of (HF)₃, the (DF)₃ data reveal clear rotational structure characteristic of a cyclic, 6-membered "ring," and therefore provide the first accurate experimental evidence for the equilibrium geometry of any hydrogen fluoride oligomer beyond the well-studied dimer. Furthermore, the spectra display homogeneous rotational fine structure 2-3 orders of magnitude in excess of what could be anticipated from a single vibrational band. Analysis of this fine structure elucidates a novel IVR mechanism which involves single hydrogen bond cleavage, and consequent opening of the (DF)₃ ring on the 40 ps time scale.