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Ion chemistry is of fundamental importance to many areas of science, ranging from catalysis, electrochemical processes (e.g., batteries, fuel cells, electrolysis), atmospheric synthetic, and biological chemistry, astrophysics, biology and physics.  This research bridges two traditional areas in chemistry: investigations of gas-phase structure and reactivity that provide information about the intrinsic properties of ions with traditional solution-phase studies aimed at understanding reactivity in the condensed phase.  A key goal is to obtain thermochemical information about ions in bulk solution using tandem mass spectrometry that is not possible to measure directly by conventional methods.  To accomplish this goal, our group is developing a method called ion nanocalorimetry in which hydrated ions are used to precisely measure ion activation energies.  In brief, when hydrated ions are activated, they sequentially lose water molecules.  Under conditions where these ions are stored at low pressure and at low temperature, the internal energy deposition upon activation can be obtained from the number of water molecules lost.  By accounting for the energy required to break the interactions between the departing water molecules and the rest of the cluster, along with the energy that partitions into the translational, rotational, and vibrational modes of the products, the energy deposited into the ion can be obtained.

We have been using this ion nanocalorimetry method to investigate electrochemistry that occurs in these clusters as a function of cluster size, charge state, and metal ion identity.  Hydrated ions are stored at ultra-high vacuum in the ion cell of a Fourier-transform ion cyclotron resonance mass spectrometer which is typically cooled to ~130 K using a regulated flow of liquid nitrogen to increase the lifetime of these ions.  Clusters are mass selected and allowed to interact with electrons that are thermally generated using a heated cathode that is located 20 cm from the cell.  Electron capture results in loss of a large number of water molecules from these ions.  For example, Ca(H₂O)₃₂²⁺ loses 10 or 11 water molecules and Ru(NH₃)₆(H₂O)₅₅³⁺ loses 17-19 water molecules upon EC.  From the sum of the threshold water molecule binding energies and the amount of energy that is partitioned into translational, rotational, and vibrational modes of the products for each lost water molecule, the internal energy deposited into the ion can be obtained.  Because the EC energy deposition is much faster than the timescale of the experiments, the energy resulting from solvent reorganization to accommodate the change in the oxidation state of the cluster is reflected in the number of water molecules that are lost from the reduced cluster.  Thus, the recombination energy values obtained from this nanocalorimetry method correspond to adiabatic ionization energies of the reduced precursor.

One example of a complete disconnection between the properties of an isolated atom or molecule and the corresponding properties in solution are ionization energies.  The ionization energies of isolated ions and molecules are typically measured on an absolute scale whereas in solution, oxidation or reduction potentials are measured relative to those of other redox active species.  These relative potentials establish a “ladder” of thermochemical values that can be used to determine properties of batteries, fuel cells, etc.  This relative ladder is anchored to the potential of the hydrogen electrode, which is arbitrarily assigned a value of exactly zero volts.  Although it is known from gas-phase studies that attaching solvent molecules to an ion lowers the ionization energy, there is not an established direct connection between any gas-phase measurements and solution-phase electrochemical measurements.  Similarly, although binding energies of individual water molecules to ions in small isolated clusters can be measured with high precision, bulk hydration energies of ions are measured relative to those of other ions to establish a thermochemical ladder that is referenced to the proton hydration energy, which is arbitrarily assigned a value of exactly zero.

We have recently demonstrated a new nanocalorimetry method to obtain an absolute standard hydrogen electrode potential and a real proton solvation energy.  In this method, the reduction energies of Eu(H₂O)_n³⁺ are measured as a function of n from 55 – 140.  These values decrease with increasing cluster size.  By extrapolating these values as a function of the geometric dependence of the cluster reduction energy to infinite size, a value for the reduction enthalpy in bulk aqueous solution is obtained.  From this value, an absolute standard hydrogen electrode potential of +4.11 V and a real proton solvation energy of -269.0 kcal/mol are obtained.  Information about the structure of the larger clusters is obtained from infrared photodissociation spectroscopy experiments, which indicate that the water at the surface of these nanodrops is similar to that at the bulk air-water interface.  These results indicate that the surface potential of the droplet, as a result of water organization at the surface, is the same as that at the bulk air-water interface.  This method for obtaining absolute potentials of redox couples has the advantage that no explicit solvation model is required, which eliminates uncertainties associated with these models.  A paper describing this work has been recently published (Donald, W. A.; Leib, R. D.; Demireva, M.; O’Brien, J. T.; Prell, J. S.; Williams, E. R. J. Am. Chem. Soc. 2009, 131, 13328-13337.)

In contrast to the results for Eu(H₂O)_n³⁺ where the metal ion is reduced, electron capture by M(H₂O)₁₀₃³⁺, M = La, Ce, Pr, Tb, Ho, Tm, and Lu, results in a reduced droplet in which an ion-electron pair is formed.  By extrapolating the measured recombination energies as a function of cluster size to infinite cluster size, a value for the solvation enthalpy of an electron inside the nanodrop of -1.3 eV is obtained.  This method has the advantages that it dos not require estimates of the real proton or hydrogen hydration enthalpies.  A manuscript describing these results has been recently submitted to the J. Am. Chem. Soc.

Future experiments will be aimed at investigating the hydrogen loss that can occur from smaller clusters upon electron capture and at calibrating the ion nanocalorimetry experiments using laser generated photons of a known energy.