Theoretical study on the excess electron binding mechanism in the [CH(3)NO(2).(H(2)O)(n)](-) (n = 1-6) anion clusters

Water Network Shape-Dependence of Local Interactions with the Microhydrated -NO2- and -CO2- Anionic Head Groups by Cold Ion Vibrational Spectroscopy

We report the structural evolutions of water networks and solvatochromic response of the CH₃NO₂^- radical anion in the OH and CH stretching regions by analysis of the vibrational spectra displayed by cryogenically cooled CH₃NO₂^-·(H₂O)_n=1-6 clusters. The OH stretching bands evolve with a surprisingly large discontinuity at n = 6, which features the emergence of an intense, strongly red-shifted band along with a weaker feature that appears in the region assigned to a free OH fundamental. Very similar behavior is displayed by the perdeuterated carboxylate clusters, RCO₂^-·(H₂O)_n=5-7 (R = CD₃CD₂), indicating that this behavior is a general feature in the microhydration of the triatomic anionic domain and not associated with CH oscillators. Electronic structure calculations trace this behavior to the formation of a "book" isomer of the water hexamer that adopts a configuration in which one of the water molecules resides in an acceptor-acceptor-donor (AAD) (A = acceptor, D = donor) H-bonding site. Excitation of the bound OH in the AAD site explores the local network topology best suited to stabilize an incipient -XO₂H-OH^-(H₂O)₂ intracluster proton-transfer reaction. These systems thus provide particularly clear examples where the network shape controls the potential energy landscape that governs water network-mediated, intracluster proton transfer. The CH stretching bands of the CH₃NO₂^-·(H₂O)_n=1-6 clusters also exhibit strong solvatochromic shifts, but in this case, they smoothly blue-shift with increasing hydration with no discontinuity at n = 6. This behavior is analyzed in the context of the solute-ion polarizability response and partial charge transfer to the water networks.

Molecular-level origin of the carboxylate head group response to divalent metal ion complexation at the air-water interface

We exploit gas-phase cluster ion techniques to provide insight into the local interactions underlying divalent metal ion-driven changes in the spectra of carboxylic acids at the air-water interface. This information clarifies the experimental findings that the CO stretching bands of long-chain acids appear at very similar energies when the head group is deprotonated by high subphase pH or exposed to relatively high concentrations of Ca2+ metal ions. To this end, we report the evolution of the vibrational spectra of size-selected [Ca2+·RCO2-]+·(H2O) n=0to12 and RCO2-·(H2O) n=0to14 cluster ions toward the features observed at the air-water interface. Surprisingly, not only does stepwise hydration of the RCO2- anion and the [Ca2+·RCO2-]+ contact ion pair yield solvatochromic responses in opposite directions, but in both cases, the responses of the 2 (symmetric and asymmetric stretching) CO bands to hydration are opposite to each other. The result is that both CO bands evolve toward their interfacial asymptotes from opposite directions. Simulations of the [Ca2+·RCO2-]+·(H2O) n clusters indicate that the metal ion remains directly bound to the head group in a contact ion pair motif as the asymmetric CO stretch converges at the interfacial value by n = 12. This establishes that direct metal complexation or deprotonation can account for the interfacial behavior. We discuss these effects in the context of a model that invokes the water network-dependent local electric field along the C-C bond that connects the head group to the hydrocarbon tail as the key microscopic parameter that is correlated with the observed trends.

Water network-mediated, electron-induced proton transfer in [C5H5N ⋅ (H2O)n](-) clusters

The role of proton-assisted charge accommodation in electron capture by a heterocyclic electron scavenger is investigated through theoretical analysis of the vibrational spectra of cold, gas phase [Py ⋅ (H2O)n=3-5](-) clusters. These radical anions are formed when an excess electron is attached to water clusters containing a single pyridine (Py) molecule in a supersonic jet ion source. Under these conditions, the cluster ion distribution starts promptly at n = 3, and the photoelectron spectra, combined with vibrational predissociation spectra of the Ar-tagged anions, establish that for n > 3, these species are best described as hydrated hydroxide ions with the neutral pyridinium radical, PyH((0)), occupying one of the primary solvation sites of the OH(-). The n = 3 cluster appears to be a special case where charge localization on Py and hydroxide is nearly isoenergetic, and the nature of this species is explored with ab initio molecular dynamics calculations of the trajectories that start from metastable arrangements of the anion based on a diffuse, essentially dipole-bound electron. These calculations indicate that the reaction proceeds via a relatively slow rearrangement of the water network to create a favorable hydration configuration around the water molecule that eventually donates a proton to the Py nitrogen atom to yield the product hydroxide ion. The correlation between the degree of excess charge localization and the evolving shape of the water network revealed by this approach thus provides a microscopic picture of the "solvent coordinate" at the heart of a prototypical proton-coupled electron transfer reaction.