Formation and photodestruction of dual dipole-bound anion (H2O)6{e−}CH3NO2

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.

Vibrationally induced charge transfer in a bimolecular model complex in vacuo

We report vibrationally induced charge transfer from nitromethane anion to methyliodide in a molecular complex. Excitation of a CH stretching vibrational transition in either of the molecular constituents results in dissociative electron transfer to the CH3I molecule, resulting in I(-) product anions. Solvation of the pre-reactive complex with more than two Ar atoms leads to complete quenching of the reaction and can be used to estimate the barrier for this reaction. We discuss the results in the framework of electronic structure calculations and compare the intra-complex electron transfer with vibrationally mediated electron emission in bare nitromethane anion.

Hydrogen-Bond Network Transformation in Water-Cluster Anions Induced by the Complex Formation with Benzene

We report spectroscopic evidence of isomer interconversion in water cluster anions, (H2O)n(-), which occurs through the interaction with a benzene molecule. An anion complex composed of (H2O)6(-) and benzene, Bz·(H2O)6(-), is formed via the reaction of (H2O)6(-)Arm with benzene. The reaction proceeds as an Ar-mediated association process such that a rapid energy dissipation by sequential Ar evaporation efficiently suppresses the thermionic emission of e(-), H2O, or both, giving rise to the formation of Bz·(H2O)6(-). Photoelectron spectroscopy is employed to probe the electronic properties of the anionic species, which reveals that "type I → type II" isomer interconversion proceeds in the (H2O)6(-) moiety during the formation of Bz·(H2O)6(-). With the aid of ab initio calculations, we conclude that the interconversion is driven by preferential stabilization of the H-bond network of type II arrangement through the formation of a nonconventional O-H···π hydrogen bond between (H2O)6(-) and Bz.

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

The excess electron binding mechanism of the anionic nitromethane-water clusters was theoretically investigated using the potential energy surfaces calculated by high-level electronic structure theories. The mechanism was first studied for the dipole-bound and valence-bound anionic states of the CH(3)NO(2)(-) monomer with the ab initio multireference configuration interaction method to reveal the electron transformation process between these anionic states in detail. As a result, it was found that both the NO(2) tilting angle and NO distances play an essential role in this electron transformation. Following this result, various water solvation structures of the valence-bound CH(3)NO(2)(-) anion were optimized with up to six water solvents using the second-order Møller-Plesset (MP2) method. The calculated results predicted that the vertical detachment energy of the valence-bound CH(3)NO(2)(-) anion increases gradually with the hydration number, as is consistent with recent experimental observations. We also investigated metastable complexes composed of CH(3)NO(2) and (H(2)O)(6)(-) by using the MP2 and long-range corrected density functional theory calculations. Two types of dipole-bound forms were obtained for the [CH(3)NO(2).(H(2)O)(6)] anion complex. In one form the excess electron is internally suspended between the two moieties while in the other form two dipolar moieties are cooperatively arranged to reinforce the electron-dipole interaction.