Ryuzo Nakanishi

Theoretical Study on the Excess Electron Binding Mechanism in the [CH3NO2·(H2O)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.

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

A new type of dipole-bound anion composed of water and nitromethane (CH(3)NO(2)) is formed via the incorporation of CH(3)NO(2) into argon-solvated water hexamer anions, (H(2)O)(6) (-)Ar(m). The reaction proceeds as an Ar-mediated process such that an effective energy dissipation through sequential Ar evaporation gives rise to the formation of [CH(3)NO(2)(H(2)O)(6)](-). Photoelectron spectroscopy is employed to probe the electronic properties of the [CH(3)NO(2)(H(2)O)(6)](-) anion, which reveals that the dipole-bound nature of (H(2)O)(6) (-) remains almost intact in the product anion; the vertical detachment energy of [CH(3)NO(2)(H(2)O)(6)](-) is determined to be 0.65+/-0.02 eV. This spectroscopic finding, together with other suggestive evidences, allows us to refer to [CH(3)NO(2)(H(2)O)(6)](-) as a dual dipole-bound anion described as (H(2)O)(6){e(-)}CH(3)NO(2), where the diffuse excess electron interacts with both the (H(2)O)(6) and CH(3)NO(2) moieties via the electron-dipole interactions. The photodestruction of (H(2)O)(6){e(-)}CH(3)NO(2) at 2134 nm (0.58 eV) occurs with a competition between electron detachment and fragmentation. The latter leads exclusively to the formation of CH(3)NO(2) (-)(H(2)O)(3), indicating that the dual dipole-bound anion serves as a precursor to the hydrated valence anion of CH(3)NO(2).

Photoelectron Spectroscopy and Ab Initio Calculations of CS3– Isomers: Carbon Trisulfide and Carbon Disulfide S-Sulfide Anions

Carbon sulfides are known as a class of binary compounds that can exist in various isomeric and/or polymeric forms. As for a sulfur-rich compound with the composition formula CS3, two possible constitutional isomers have been proposed experimentally or theoretically for the neutral species and its corresponding radical cation and anion. Although the previous studies claim that one isomer has a carbon trisulfide (CS3) C-centered configuration and the other has a carbon disulfide S-sulfide (SCSS) chain configuration, they have not yet been fully identified by a spectroscopic method. In this study, we have prepared the anions of those isomers in the gas phase by employing two types of reactions: dissociative electron attachment to 1,3-dithiole-2-thione for CS3(-) formation and the S(-) + CS2 ion-molecule reaction for SCSS(-). Photoelectron spectroscopic measurements reveal that the reactions result in the production of two anionic species that can be well distinguished by their vertical detachment energy. With the aid of ab initio calculations, they are identified distinctively as the anions of carbon trisulfide and carbon disulfide S-sulfide.

Incorporation of ROH (R = CH3, C2H5, 2-C3H7) into (H2O)6(-): substituent effect on the growth process of the hydrogen-bond network.

The condensation reaction of water cluster anions, (H2O)n(-) + H2O → (H2O)n+1(-), offers a prime opportunity to explore the growth process of the hydrogen-bond network involving molecular uptake and network rearrangement. Here, by exploiting an Ar-mediated approach, we investigate the association reaction of water hexamer anions, (H2O)6(-), with ROH (R = CH3, C2H5, 2-C3H7) by mass spectrometry combined with photoelectron spectroscopy. Quantitative analysis of the product mass spectra reveals that incorporation of ROH (R = CH3, C2H5) into (H2O)6(-) occurs with a cross section of the same size as in the (H2O)6(-) + D2O condensation, but with a slightly smaller cross section for R = 2-C3H7. Coexistence of two types of isomers, high electron-binding (type I) and low electron-binding (type II) forms, is observed in all the product ROH·(H2O)6(-) species by photoelectron spectroscopic measurement. These findings, in conjunction with ab initio study of MeOH·(H2O)6(-) structures, lead us to propose a molecular uptake mechanism at play in the incorporation of ROH into the (H2O)6(-) network. This also provides complementary information on the homogeneous condensation process of pure water cluster anions.