Fig. 5. PTI·H2O and IF-PTI structures and H2O dynamics from DFT and AIMD calculations.
(A) The equilibrium structure of IF-PTI (configuration X) viewed down the c axis. (B) Calculated XRD pattern for IF-PTI (X) compared with experimental data for IF-PTI and PTI·H2O. (C) A series of DFT results showing how an H2O molecule initially positioned by OH···N and NH···O interactions relative to the starting PTI layer passes through the intralayer C12N12H3 ring void and into the adjacent layer. In the stable orientation, one OH bond is oriented toward the interlayer space. The H2O orientation is reversed by reorganization of the H-bonding interactions. As the molecule traverses the interlayer spacing, it undergoes a further orientation reversal as H bonding to the next layer occurs. The process is then repeated as the molecule traverses further layers and interlayer gallery spacings. (D) The energy profile as the H2O molecule passes through the configurations labeled (i) through (xii). Blue and red symbols indicate upward- versus downward-pointing orientations. The position of the PTI layers is indicated with a shaded pink area; it is not atomically flat due to some extent of buckling. Configurations generated by maintaining the initial orientation as H2O is pushed through the ring become metastable at ~0.05 eV above the ground state. An additional energy cost of ~0.3 eV is required to break the initial H-bonding configuration and reorient the molecule to its more stable configuration (inset showing energy as a function of α as the OH···N angle). (E) Oxygen trajectories for H2O molecules intercalated in PTI layers from AIMD simulations at three temperatures. Molecules that do not experience the interlayer crossing exhibit dynamics that explore the intralayer C12N12H3 ring voids accompanied by a reversal in molecular orientation (blue circles) (Fig. 4C). Along with the H2O dynamics recorded at lower temperature, these nanoconfined motions define the Dloc dynamics described by QENS analysis.