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Ion chemistry is fundamentally important 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.  Our research bridges two traditional but typically separate areas of chemistry: gas-phase ion chemistry and ion chemistry in solution.  The former provides information about the intrinsic structures and reactivities of ions, whereas the latter includes effects of the solvent.  Much can be learned by comparisons of gas-phase and solution-phase studies, including information about how water stabilizes and orients around ions.  To obtain additional information about how water effects ion stability and reactivity, we have developed a novel ion nanocalorimetry method that makes it possible to accurately measure the thermochemistry resulting from ion activation, whether by ion-electron recombination or photoexcitation.  In brief, gaseous hydrated ions are stored in the cell of a Fourier-transform ion cyclotron resonance mass spectrometer where they equilibrate to a set temperature by interacts with the blackbody radiation field generated by a temperature controlled cell.  Activation of these ions, either by capture of thermally generated electrons or by laser photoactivation results in a rapid increase in the nanodrop temperature and subsequent sequential loss of water molecules.  The internal energy deposition 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.  

In our previous report, we described an important application of this method to measuring the absolute half-cell single electron reduction potential of Eu3+.  By measuring electron recombination energies of size-selected clusters as a function of cluster size, an absolute reduction enthalpy for Eu3+ in infinite dilute solution can be obtained.  In combination with absolute entropy measurements in solution, a value for the absolute reduction potential is obtained, from which the absolute standard hydrogen electrode potential was determined.

We have made several important advances since our previous report.  We demonstrated that we could accurately model the width of our product ion distributions using a modified Thomson liquid drop model to obtain water binding energies for large clusters, and a statistical model for the energy release into translations and rotations of the products.  We showed that non-ergodic dissociation can occur for smaller clusters, but that dissociation of the larger clusters is statistical (Donald, W. A.; Williams, E. R. J. Am. Soc. Mass Spectrom. 2010, 21, 615-625).

Some ions do not undergo one-electron reduction in solution, and we find that the same is true for these ions in sufficiently large nanodrops.  Capture of an electron by nanodrops containing some trivalent metal ions results in formation of ion-electron pairs.  We measured electron recombination energies for La3+(H2O)n, n = 42-46 and found that the trend in these values as a function of cluster size is consistent with a structural transition from a surface-located excess electron at smaller size (n < ~56) to a more fully solvated electron at larger sizes (n > ~60).  By extrapolating the recombination energies as a function of cluster size, a value for the bulk hydration enthalpy of the electron (-1.3 eV) is obtained.  This method has the advantage that it does not require estimates of the absolute proton or hydrogen hydration enthalpies (Donald, W. A.; Demireva, M.; Leib, R.; Aiken, M. J.; Williams, E. R. J. Am. Chem. Soc. 2010, 132, 4633-4640).

A key advance has been the ability to experimentally calibrate our nanocalorimetry by using a UV laser to excite hydrated ions with a known amount of energy and measuring the extent of water loss that occurs.  To demonstrate this, we borrowed an excimer laser from a colleague and made measurements as a function of ion charge state, cluster size and photon energy.  These measurements should make it possible to obtain an absolute half-cell reduction potentially entirely from experimental data with no modeling required.   A manuscript describing this work has been submitted.  An interesting application of this method is the ability to indirectly measure ion fluorescence from hydration ions.  If upon photoexcitation, a hydration ion emits a photon, fewer water molecules will be lost than when all the excitation is converted into internal modes.  A key advantage of this indirect fluorescence detection method is that all dissociation products resulting from emission are observed irrespective of the direction in which the photon is emitted, i.e., this is equivalent to 100% photon collection efficiency.  We demonstrated this method with hydrated protonated proflavin ions with up to 50 water molecules attached.  We found that the energy of the emitted photon is similar to that in bulk for ions with more than ~20 water molecules attached and that the bulk quantum yield was obtained for clusters with more than ~30 water molecules.  These are the first measurements of fluorescence from hydrated ions and should make it possible to obtain a greater understanding of the role of water in stabilizing excited states (Donald, W. A.; Leib, R. D.; Demireva, M.; Negru, B.; Neumark, D. M.; Williams, E. R. J. Am. Chem. Soc. 2010, 132, 6904-6905).

We also investigated a cluster-pair correlation method to obtain and absolute proton solvation energy (which is directly related to the absolute standard hydrogen electrode potential), using a significantly larger data set than used previously.  We showed that the value depended on the data set size and the types of ions used.  In addition, we developed a new analysis method that significantly improves the precision of this method.  A manuscript describing this work has been submitted.