Isolation of the simplest hydrated acid

Dissociation of an acid molecule in aqueous media is one of the most fundamental solvation processes but its details remain poorly understood at the distinct molecular level. Conducting high-pressure treatments of an open-cage fullerene C(70) derivative with hydrogen fluoride (HF) in the presence of H(2)O, we achieved an unprecedented encapsulation of H(2)O·HF and H(2)O. Restoration of the opening yielded the endohedral C(70)s, that is, (H(2)O·HF)@C(70), H(2)O@C(70), and HF@C(70) in macroscopic scales. Putting an H(2)O·HF complex into the fullerene cage was a crucial step, and it would proceed by the synergistic effects of “pushing from outside” and “pulling from inside.” The structure of the H(2)O·HF was unambiguously determined by single crystal x-ray diffraction analysis. The nuclear magnetic resonance measurements revealed the formation of a hydrogen bond between the H(2)O and HF molecules without proton transfer even at 140°C.