Knowledge Bank

Rotational spectroscopy and ab initio calculations have been used to elucidate the gas phase structure and internal rotation dynamics of $H3N-HF-HF$. The structure includes the formation of an HN-HF-HF ring, perturbing both the linear hydrogen bond in the $H3N-HF$ dimer and the angle of the HF-HF dimer. This indicates a weak secondary interaction between the fluorine of the outer HF and an ammonia hydrogen. Both the calculations and the observed spectrum indicate that a small, but significant, barrier exists for the internal rotation of the $NH3$ unit. Transitions have been observed for 7 isotopic species, and for the parent isotopomer, observation of both a- and b-type transitions allows confirmation of the assignments via an internally consistent linking of the energy levels. While $NH3$ and HF form an ionic solid $(NH4+F-)$ in the bulk, the isolated dimer is hydrogen bonded in the gas $phasea$. The addition of one HF represents the first step in the microsolvation of $H3N-HF$ and, as such, this complex is useful for understanding the role of local environment in promoting proton transfer. Additionally, energy calculations have been performed to explore the lowest frequency vibration of $H3N-HF-HF$, a ring-opening motion that increases the NFF angle.