Note that for graphanol E rot ≫ E hop , which means that hopping will occur rapidly compared with rotation; this is indeed what we observe from visualization of a c96C MD simulation at 600 K, which is included in the Supporting Information. If the magnitudes of the barriers were reversed, one would expect that rotations would be rapid and that proton hopping would be rare events. Given the slow rotation relative to hopping, we propose the following expectations for the detailed PT mechanism: (1) the low value of E hop means that protons will readily hop back and forth between a pair of O atoms, which we call "rattling";