Figure 5: Analysis of the orientations of the molecular axis of OH−(aq).
(a) Joint probability distributions (symmetrized w.r.t. z=0) as a function of the angle α between the O*–H′ bond axis and the xy plane, and the distance zO* of O* w.r.t. the slit pore midplane for the narrow (N, left panel) and wide (W, right panel) systems. (b,c) For system W, the total distribution function from a/right is respectively dissected in terms of configurations in the active state close to transfer events (δ<0.1) and in those corresponding to the hypercoordinated resting state (δ>0.5) where the centred hydrogen bond characterizes the anionic Zundel complex Z, [HO···H···OH]−; see text for definition of the transfer coordinate δ. Concerning orientation and location of the charge defect in the upper layer zO*>0 of the wide pore W, the so-called ‘exposed’ states E are those where α>45° together with zO*>1.2 Å and thus represent situations where the H site of interfacial OH− species points towards the upper surface, while the O–H axis is roughly perpendicular to that surface. The ‘tilted’ states T close to the upper surface are those where α>0 but excluding the E states, whereas the ‘buried’ states B are those where α>0. The corresponding definitions for the ‘lower’ surface, zO*<0 Å, follow from symmetry (see also Fig. 4).
