Photoinduction of Fast, Reversible Translational Motion in a Hydrogen-Bonded Molecular Shuttle
nium and hydroxide ions. We describe the solvent coordinate that stabilizes ions, ⌬E, as an electric field because it arises primarily from long-range electrostatic interactions. Local properties that we examined, such as ion coordination number and the presence of specific hydrogen bonds, fail to account for the bond-destabilizing fluctuation in our simulations. Furthermore, the stabilization of ions indicated in Fig. 2 , B and C, is diminished substantially when we artificially remove outer
... ordination shells of the ions. Thus, ⌬E does not arise from a few nearby water molecules. Instead, it is analogous to the collective coordinates that others have imagined for electron transfer (10) and for acid-base proton transfer (33-36). As in Marcus's theory of electron transfer, it is a rare solvent fluctuation that drives the motion of charges. In detail, however, ⌬E differs from the previously defined coordinates, namely the solvent polarization field and the energy gap between diabatic bonding states. ⌬E involves only the energy required to transfer protons adiabatically. The second component of the reaction coordinate that we identified, the hydrogen bond wire length l, is also analogous to a coordinate in these theories, namely the distance between ions. But in the case of autoionization, the appropriate separation coordinate describes the hydrogen bond wires that link the ions, rather than simply describing the distance between them, emphasizing the importance of connectivity in the hydrogen bond network. We conclude that the dynamics of both electric fields and hydrogen bonding play important roles in the autoionization mechanism. Rare electric field fluctuations drive the dissociation of oxygen-hydrogen bonds. Ions produced in this way usually recombine quickly because the solvation fluctuation vanishes within tens of femtoseconds. But when such a fluctuation is coincident with breaking of the hydrogen bond wire (a process normally occurring about once every picosecond), rapid recombination is then not possible. It is with this coincidence of events that the system crosses a transition state. This scenario implies the existence of many shortlived hydronium and hydroxide ions in water. Decay of this transient population over ϳ100 fs is an interesting and, in principle, observable signature of the dynamics revealed by our simulations. . 12. The potential energy required to separate hydronium and hydroxide ions in a cluster of three water molecules was computed by using the BLYP density functional theory and by using a high-level wave function-based method [T. Van Voorhis and P. L. Geissler, unpublished data]. The cluster geometry for these calculations was taken from a wire of hydrogen-bonded molecules present in a bulk water simulation, and protons were placed in a manner similar to that described in (32). Both methods predict an energy of ϳ60 kcal/mol (relative to the neutral cluster) for ions separated by one intervening water molecule. Both also predict similar monotonic decreases in energy as protons are transferred to create neutral water molecules. We calculated the mean force, F N (r ion ), exerted on a Na ϩ Cl Ϫ (aq) ion pair by its environment in a periodically replicated system with N water molecules, as a function of interionic distance, r ion . The model used in these simulations is described in work by P. L. Geissler, C. Dellago, and D. Chandler [ J. Phys. Chem. B 103, 3706 (1999)]. We considered N ϭ 30, 106, 254, and 862, corresponding to cell lengths of ϳ10, 15, 20, and 30 Å, respectively. The resulting functions, F N (r ion ), differ by Ͻ1 kcal/mol per angstrom. A rotaxane is described in which a macrocycle moves reversibly between two hydrogen-bonding stations after a nanosecond laser pulse. Observation of transient changes in the optical absorption spectrum after photoexcitation allows direct quantitative monitoring of the submolecular translational process. The rate of shuttling was determined and the influence of the surrounding medium was studied: At room temperature in acetonitrile, the photoinduced movement of the macrocycle to the second station takes about 1 microsecond and, after charge recombination (about 100 microseconds), the macrocycle shuttles back to its original position. The process is reversible and cyclable and has properties characteristic of an energy-driven piston.