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Ion Nanocalorimetry: Measuring Absolute Reduction Potentials, and Investigating Effects of Water on Electron Solvation and Ion Fluorescence

  • Author(s): Donald, William Alexander
  • Advisor(s): Williams, Evan R
  • et al.
Abstract

This dissertation reports on the development of a new gas-phase ion nanocalorimetry technique, in which electrochemistry is performed using large "aqueous" nanodrops in vacuo to obtain absolute half-cell potentials in bulk solution. Absolute recombination energies (REs) of nanometer-sized water droplets containing a divalent or trivalent metal ion are obtained from the number of water molecules lost upon electron capture (EC). REs are obtained from the experimentally measured average number of water molecules lost from the cluster, and from both the sum of the threshold water molecule binding energies and the sum of energy that is partitioned into the translational, rotational and vibrational modes of the products for each water molecule lost. The energy removed by the lost water molecules is obtained from established theoretical models. The width of the product ion distribution in these experiments is predominantly attributable to the distribution of energy that partitions into the translational and rotational modes of the water molecules that are lost. These results are consistent with a singular value for the recombination energy. Ion nanocalorimetry has been used to obtain a value for the absolute standard hydrogen electrode potential from three different nanocalorimetry based methods that all agree within 5% of each other (+4.05, +4.11, and +4.21 V). Our extrapolation method, in which REs of size-selected and thermalized Eu3+(H2O)n, n = 55 to 140, are extrapolated to infinite size to obtain the absolute reduction potential of Eu3+(aq) and a value for the absolute SHE potential (+4.11 V), should be the most accurate because a solvation model is not used and therefore, errors associated with solvation models are eliminated.

Water clusters containing ions for which one-electron reduction potentials in aqueous solution are not readily measurable, such as alkaline earth divalent metal ions and most of the trivalent lanthanide ions, form solvent separated metal ion and electron ion pairs upon EC, as long as there are a sufficient number of water molecules to stabilize the ion pair. The dependence of the RE values for Ca(H2O))n2+ on cluster size suggest that the electron is delocalized on the surface of the cluster for n = 32-47, but a transition to a more highly solvated electron is indicated for n = 47-62 by the constant RE values for these ions. For La3+(H2O)n (n = 42 to 160), the trend in recombination energies as a function of hydration extent is consistent with a structural transition from a surface-located excess electron at smaller sizes (n ≤ ~56) to a more fully solvated electron at larger sizes (n ≥ ~60). The recombination enthalpies for n > 60 are extrapolated as a function of the geometrical dependence on cluster size to infinite size to obtain the bulk hydration enthalpy of the electron (-1.3 eV), which is within the wide range of values obtained from previous methods (-1.0 to -1.8 eV). The ion nanocalorimetry method has the advantage that it does not require estimates for the absolute solvation energy of the proton or the H atom.

Whereas EC by hydrated metal ions resulted in only the full internal conversion of the RE into the reduced precursor, some ions can fluoresce upon electronic excitation. We report a new highly sensitive method for detecting the fluorescence of isolated, partially hydrated ions for the first time. Fluorescence is indirectly detected based on the distribution of water molecules lost upon absorption of a UV photon. Photodissociation of hydrated protonated proflavine (n = 13-50) undergoes three photophysical processes upon absorption of a 248 nm photon and excitation to a high energy singlet excited state: full internal conversion and fluorescence to the ground electronic singlet state, and formation of a long-lived triplet state, which slowly undergoes non-radiative intersystem crossing to the ground singlet state. The high sensitivity of this method should make it possible to perform Förster resonance energy transfer experiments with gas-phase biomolecules in a microsolvated environment to investigate how a controlled number of water molecules effects biomolecular structure and dynamics.

Although the precision in the nanocalorimetry method is excellent, the absolute uncertainty obtained is more difficult to assess because the energy removed by the lost water molecules has not been experimentally measured for large hydrated metal ions. Laser induced photodissociation experiments, in which M2+(H2O)n are dissociated by absorption of UV laser light at 193 (6.4 eV) and 248 nm (5.0 eV), are used to directly relate the average number of water molecules lost to the energy that is deposited into the cluster, which can be used to directly convert the average water molecules lost in EC experiments to experimentally measured RE values. These results demonstrate that absolute solution phase reduction potentials can be obtained entirely from experimental data, with no modeling, and should provide the most direct route to establishing an absolute electrochemical scale with high accuracy.

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