Jeremy T. O'Brien

Absolute Standard Hydrogen Electrode Potential Measured by Reduction of Aqueous Nanodrops in the Gas Phase

In solution, half-cell potentials are measured relative to those of other half cells, thereby establishing a ladder of thermochemical values that are referenced to the standard hydrogen electrode (SHE), which is arbitrarily assigned a value of exactly 0 V. Although there has been considerable interest in, and efforts toward, establishing an absolute electrochemical half-cell potential in solution, there is no general consensus regarding the best approach to obtain this value. Here, ion-electron recombination energies resulting from electron capture by gas-phase nanodrops containing individual [M(NH3)6]3+, M = Ru, Co, Os, Cr, and Ir, and Cu2+ ions are obtained from the number of water molecules that are lost from the reduced precursors. These experimental data combined with nanodrop solvation energies estimated from Born theory and solution-phase entropies estimated from limited experimental data provide absolute reduction energies for these redox couples in bulk aqueous solution. A key advantage of this approach is that solvent effects well past two solvent shells, that are difficult to model accurately, are included in these experimental measurements. By evaluating these data relative to known solution-phase reduction potentials, an absolute value for the SHE of 4.2 +/- 0.4 V versus a free electron is obtained. Although not achieved here, the uncertainty of this method could potentially be reduced to below 0.1 V, making this an attractive method for establishing an absolute electrochemical scale that bridges solution and gas-phase redox chemistry.

Interactions of mono- and divalent metal ions with aspartic and glutamic acid investigated with IR photodissociation spectroscopy and theory.

The interaction of metal ions with aspartic (Asp) and glutamic (Glu) acid and the role of gas-phase acidity on zwitterionic stability were investigated using infrared photodissociation spectroscopy in the spectral range 950-1900 cm (-1) and by hybrid density functional theory. Lithium ions interact with both carbonyl oxygen atoms and the amine nitrogen for both amino acids, whereas cesium interacts with both of the oxygen atoms of the C-terminus and the carbonyl oxygen of the side chain for Asp. For Glu, this structure is competitive, but a structure in which the cesium ion interacts with just the carbonyl oxygen atoms is favored and the calculated spectrum for this structure is more consistent with the experimentally measured spectrum. In complexes with either of these metal ions, both amino acids are non-zwitterionic. In contrast, Glu*Ca (2+) and Glu*Ba (2+) both adopt structures in which Glu is zwitterionic and the metal ion interacts with both oxygens of the C-terminal carboxylate and the carbonyl oxygen in the side chain. Assignment of the zwitterionic form of Glu is strengthened by comparisons to the spectrum of the protonated form, which indicate spectral features associated with a protonated amino nitrogen. Comparisons with results for glutamine, which adopts nearly the same structures with these metal ions, indicate that the lower Delta H acid of Asp and Glu relative to other amino acids does not result in greater relative stability of the zwitterionic form, a result that is directly attributed to effects of the metal ions which disrupt the strong interaction between the carboxylic acid groups in the isolated, deprotonated forms of these amino acids.

Structures of Protonated Dipeptides: The Role of Arginine in Stabilizing Salt Bridges

Structures of protonated dipeptides containing N-terminal Gly, Val, Pro, Lys, His, or Arg and C-terminal Arg are investigated with infrared multiple photon dissociation (IRMPD) spectroscopy between 900 and 1850 cm(-1) and theory. The IRMPD spectra clearly indicate that, when Gly, Val, Pro, Lys, or His are N-terminal to Arg, these protonated dipeptides adopt gas-phase structures with a single formal charge site (SCS), whereas ArgArg x H(+) has a salt-bridge (SB) structure in which the C-terminus is deprotonated and two basic sites are protonated. There are only subtle differences in the IRMPD spectra for dipeptides containing Gly, Val, Pro, and Lys. A sharp, intense peak at 1080 cm(-1) is observed for HisArg x H(+) that is attributed to the neutral histidine side chain, an assignment that is confirmed by comparison to the IRMPD spectrum of (HisArg x H(2))(2+). Lowest-energy B3LYP/6-31+G(d,p) structures and energies for the SCS and SB forms of these protonated dipeptides indicate that stability of the SB form relative to the SCS form generally increases with increasing gas-phase basicity of the N-terminal amino acid, but only ArgArg x H(+) is calculated to have a SB ground state at 298 K, in agreement with the results from IRMPD spectroscopy. This is the first direct experimental evidence for a salt-bridge structure in a gaseous protonated peptide, and ArgArg x H(+) is the smallest protonated peptide for which a SB structure has been reported. These results suggest that SB structures should be common for protonated peptides containing at least two arginine residues and may also occur for large protonated peptides or proteins with at least one arginine residue and other basic residues, such as lysine or histidine.

Structural and electric field effects of ions in aqueous nanodrops.

Ensemble infrared photodissociation (IRPD) spectra in the hydrogen stretch region (∼2950-3800 cm(-1)) are reported for M(H(2)O)(35-37), with M = I(-), Cl(-), HCO(3)(-), OH(-), tetrabutyl-, tetrapropyl-, and tetramethylammonium, Cs(+), Na(+), Li(+), H(+), Ba(2+), Ca(2+), Co(2+), Mg(2+), La(3+), and Tm(3+), at 133 K. A single, broad feature is observed in the bonded-OH region of the spectra that indicates that the water network in these clusters is bulk-like and likely resembles liquid water more strongly than ice. The free-OH region for all of these clusters is dominated by peaks corresponding to water molecules that accept two and donate one hydrogen bond (AAD water molecules), indicating that AAD water molecules are more abundant at the surface of these ions than AD water molecules. A-only water molecules are present in significant abundance only for the trivalent metal cations. The frequency of the AAD free-OH stretch band shifts nearly linearly with the charge state of the ion, consistent with a Stark shift attributable to the ions electric field. From these data, a frequency range of 3704.9-3709.7 cm(-1) is extrapolated for the free-OH of AAD water molecules at the (uncharged) bulk liquid water surface, consistent with sum-frequency generation spectroscopy experiments. Differences in both the bonded- and the free-OH regions of the spectra for these ions are attributable to ion-induced patterning of the water network that extends to the surface of the clusters, which includes water molecules in the third and fourth solvation shells; that is, these ions pattern water molecules at long distance to various extents. These spectra are simulated using two different electrostatic models previously used to calculate OH-stretch spectra of bulk water and aqueous solutions and parametrized for bonded-OH frequencies. These models qualitatively reproduce a number of features in the experimental spectra, although it is evident that more sophisticated treatment of water molecule and ion polarizability and vibrational coupling is necessary for more quantitative comparisons.

Zn2+ Has a Primary Hydration Sphere of Five: IR Action Spectroscopy and Theoretical Studies of Hydrated Zn2+ Complexes in the Gas Phase

Complexes of Zn(2+)(H(2)O)(n), where n = 6-12, are examined using infrared photodissociation (IRPD) spectroscopy, blackbody infrared radiative dissociation (BIRD), and theory. Geometry optimizations and frequency calculations are performed at the B3LYP/6-311+G(d,p) level along with single point energy calculations for relative energetics at the B3LYP, B3P86, and MP2(full) levels with a 6-311+G(2d,2p) basis set. The IRPD spectrum of Zn(2+)(H(2)O)(8) is most consistent with the calculated spectrum of the five-coordinate MP2(full) ground-state (GS) species. Results from larger complexes also point toward a coordination number of five, although contributions from six-coordinate species cannot be ruled out. For n = 6 and 7, comparisons of the individual IRPD spectra with calculated spectra are less conclusive. However, in combination with the BIRD and laser photodissociation kinetics as well as a comparison to hydrated Cu(2+) and Ca(2+), the presence of five-coordinate species with some contribution from six-coordinate species seems likely. Additionally, the BIRD rate constants show that Zn(2+)(H(2)O)(6) and Zn(2+)(H(2)O)(7) complexes are less stable than Zn(2+)(H(2)O)(8). This trend is consistent with previous work that demonstrates the enthalpic favorability of the charge separation process forming singly charged hydrated metal hydroxide and protonated water complexes versus loss of a water molecule for complexes of n ≤ 7. Overall, these results are most consistent with the lowest-energy structures calculated at the MP2(full) level of theory and disagree with those calculated at B3LYP and B3P86 levels.

Hydration of gaseous copper dications probed by IR action spectroscopy.

Clusters of Cu (2+)(H 2O) n , n = 6-12, formed by electrospray ionization, are investigated using infrared photodissociation spectroscopy, blackbody infrared radiative dissociation (BIRD), and density functional theory of select clusters. At 298 K, the BIRD rate constants increase with increasing cluster size for n >or= 8, but the trend reverses for the smaller clusters where Cu (2+)(H 2O) 6 is less stable than Cu (2+)(H 2O) 8. This trend in stability is consistent with a change in fragmentation pathway from loss of a water molecule for clusters with n >or= 9 to loss of hydrated protonated water clusters and the formation of the corresponding singly charged hydrated metal hydroxide for n <or= 7. The lowest-energy structures of Cu (2+)(H 2O) n , n = 6-8 and 10, identified at the B3LYP/LACV3P**++ level of theory, all have coordination numbers (CN) of 4, although structures with a CN = 5 are within about 10 kJ/mol for all clusters except Cu (2+)(H 2O) 8. IR action spectra indicate the presence of hydrogen bonding for all clusters, and results for Cu (2+)(H 2O) n , n = 6-8, are consistent with a CN = 4, although minor contributions from structures with higher CN cannot be ruled out. Bands in the action spectra of Cu (2+)(H 2O) n , n = 10-12, show the presence of water molecules that accept two hydrogen bonds and donate one hydrogen bond as well as single hydrogen bond acceptors clearly indicating the onset for formation of a third solvent shell at a relatively small cluster size.

Directly Relating Gas-Phase Cluster Measurements to Solution-Phase Hydrolysis, the Absolute Standard Hydrogen Electrode Potential, and the Absolute Proton Solvation Energy

Solution-phase, half-cell potentials are measured relative to other half-cell potentials, resulting in a thermochemical ladder that is anchored to the standard hydrogen electrode (SHE), which is assigned an arbitrary value of 0 V. A new method for measuring the absolute SHE potential is demonstrated in which gaseous nanodrops containing divalent alkaline-earth or transition-metal ions are reduced by thermally generated electrons. Energies for the reactions 1) M(H(2)O)(24)(2+)(g) + e(-)(g)-->M(H(2)O)(24)(+)(g) and 2) M(H(2)O)(24)(2+)(g) + e(-)(g)-->MOH(H(2)O)(23)(+)(g) + H(g) and the hydrogen atom affinities of MOH(H(2)O)(23)(+)(g) are obtained from the number of water molecules lost through each pathway. From these measurements on clusters containing nine different metal ions and known thermochemical values that include solution hydrolysis energies, an average absolute SHE potential of +4.29 V vs. e(-)(g) (standard deviation of 0.02 V) and a real proton solvation free energy of -265 kcal mol(-1) are obtained. With this method, the absolute SHE potential can be obtained from a one-electron reduction of nanodrops containing divalent ions that are not observed to undergo one-electron reduction in aqueous solution.

Nanocalorimetry in mass spectrometry: A route to understanding ion and electron solvation

A gaseous nanocalorimetry approach is used to investigate effects of hydration and ion identity on the energy resulting from ion–electron recombination. Capture of a thermally generated electron by a hydrated multivalent ion results in either loss of a H atom accompanied by water loss or exclusively loss of water. The energy resulting from electron capture by the precursor is obtained from the extent of water loss. Results for large-size-selected clusters of Co(NH3)6(H2O)n3+ and Cu(H2O)n2+ indicate that the ion in the cluster is reduced on electron capture. The trend in the data for Co(NH3)6(H2O)n3+ over the largest sizes (n ≥ 50) can be fit to that predicted by the Born solvation model. This agreement indicates that the decrease in water loss for these larger clusters is predominantly due to ion solvation that can be accounted for by using a model with bulk properties. In contrast, results for Ca(H2O)n2+ indicate that an ion–electron pair is formed when clusters with more than ≈20 water molecules are reduced. For clusters with n = ≈20–47, these results suggest that the electron is located near the surface, but a structural transition to a more highly solvated electron is indicated for n = 47–62 by the constant recombination energy. These results suggest that an estimate of the adiabatic electron affinity of water could be obtained from measurements of even larger clusters in which an electron is fully solvated.

Changes in binding motif of protonated heterodimers containing valine and amines investigated using IRMPD spectroscopy between 800 and 3700 cm(-1) and theory.

Proton-bound dimers consisting of valine and basic primary and secondary amines of varying gas-phase basicity (GB) were investigated using infrared multiple photon dissociation (IRMPD) spectroscopy between 800 and 3700 cm(-1), collisionally activated dissociation, and theory. The low-energy dissociation of these dimers results in a sharp transition from formation of primarily protonated valine to protonated base for dimers with ethylamine and propylamine, respectively, from which a GB of approximately 880 kJ/mol is deduced for valine, a value that is slightly higher than previously reported. The IRMPD spectra clearly indicate that, for bases with GB values within 20 kJ/mol of that of valine, the base coordinates to the N-terminus of a nonzwitterionic form of valine. In contrast, calculations indicate that valine is zwitterionic for complexes where the base is less basic. For bases with GB values at least 20 kJ/mol greater than that of valine, the spectra indicate a transition in structure, and for diethylamine (DeltaGB = 40 kJ/mol), the dominant structure is one in which the base coordinates to the carbonyl oxygen of a nonzwitterionic form of valine and the carboxylic acid donates an intramolecular hydrogen bond to the N-terminus. These results are consistent with the destabilization of the N-terminally coordinated structure due to the increasing difference in proton affinities of the constituent molecules and the increasing importance of a stabililizing hydrogen bond formed in the C-terminally coordinated structure. Even when the GB of the base is 40 kJ/mol higher than that of valine, the form of the amino acid is nonzwitterionic, indicating that careful application of the kinetic method should provide reliable information about the basicity of valine and other aliphatic amino acids.

Effects of Electron Kinetic Energy and Ion-Electron Inelastic Collisions in Electron Capture Dissociation Measured using Ion Nanocalorimetry

Ion nanocalorimetry is used to measure the effects of electron kinetic energy in electron capture dissociation (ECD). With ion nanocalorimetry, the internal energy deposited into a hydrated cluster upon activation can be determined from the number of water molecules that evaporate. Varying the heated cathode potential from −1.3 to −2.0 V during ECD has no effect on the average number of water molecules lost from the reduced clusters of either [Ca(H2O)15]2+ or [Ca(H2O)32]2+, even when these data are extrapolated to a cathode potential of zero volts. These results indicate that the initial electron kinetic energy does not go into internal energy in these ions upon ECD. No effects of ion heating from inelastic ion-electron collisions are observed for electron irradiation times up to 200 ms, although some heating occurs for [Ca(H2O)17]2+ at longer irradiation times. In contrast, this effect is negligible for [Ca(H2O)32]2+, a cluster size typically used in nanocalorimetry experiments, indicating that energy transfer from inelastic ion-electron collisions is negligible compared with effects of radiative absorption and emission for these larger clusters. These results have significance toward establishing the accuracy with which electrochemical redox potentials, measured on an absolute basis in the gas phase using ion nanocalorimetry, can be related to relative potentials measured in solution.