Toshihiko Maeyama

Photodetachment of small water cluster anions in the near-infrared through the visible region

Abstract Water cluster anions were efficiently generated by attachment of slow photoelectrons ejected from laser-excited zirconium metal surface, and photodetachment cross sections for (H2O)n− (n=2, 6, 7, and 11) were measured for a photon energy range of 0.74–2.54 eV. A gradual decrease of the cross section with the photon energy was observed for all the species. It was also found that the larger cluster exhibits the larger cross section. Size-dependent features of the spectra were discussed in relation to the excess electron localization with evolution of the cluster size.

Photodestruction spectroscopy of carbon disulfide cluster anions (CS2)n−, n=1–4: Evidence for the dimer core structure and competitive reactions of the dimer anion

Photodestruction spectra of carbon disulfide cluster anions, (CS2)n−, n=1–4, have been measured with a time-of-flight mass spectrometer coupled with an optical parametric oscillator. The spectra of all the cluster anions of n⩾2 were found to exhibit a similar absorption band peaking at 1.6–1.8 eV, suggesting that a C2S4− core is involved in the cluster anions. Photon energy dependence of competition between electron detachment and dissociation of the dimer anion was also observed. It was found that there is a reaction channel of the dimer anion producing C2S2− and S2, as well as the ordinary dissociation into CS2− and CS2. The most stable form of the dimer anion was investigated by ab initio calculations at the unrestricted Hartree–Fock/6-31+G* level, showing that the stable form involves covalent C–C and S–S bonds. Reaction mechanisms are discussed on the basis of electronic symmetries of the parent and the fragments.

Rotational structure and dissociation of the Rydberg states of CO investigated by ion-dip spectroscopy

In a series of spectroscopic work of the Rydberg states of CO, we present the rotational analysis of the v=0 and 1 levels of the singlet ns, np, nd and nf‐Rydberg states (n=4–7). The spectra were measured by ion‐dip spectroscopy with triple resonance excitation via the 3sσ:B 1Σ+ or the 3pσ:C 1Σ+ state. All the spectra were rotationally well resolved and the term value, quantum defect and the rotational constant were obtained for each state. Through the analysis of the rotational structure, the coupling between the Rydberg electron and the ion core has been investigated. For the np‐Rydberg states, a switching from Hund’s case (b) to (d) was clearly observed with the increase of n. A significant perturbation was observed in the 6pπ 1Π and 7pπ 1Π states and it is suggested that these states are perturbed by the state with the same symmetry. For the nf‐Rydberg states, the observed electronic energy was well analyzed by the long range force model and the precise ionization potential was obtained. The Rydberg↔v...

Infrared spectroscopy of precursor clusters for nucleophilic substitution reactions: fluorobenzene-(CH3OH)n (n = 1 and 2)

Abstract Infrared spectra of fluorobenzene-(CH 3 OH) n ( n = 1 and 2) clusters in their ground state were observed in the OH and CH stretching vibrational region. The sizes and structures of the clusters were examined. As a result, the size dependence of the intracluster nucleophilic substitution reactions accompanied with ionization were confirmed. We also found the isomer dependence of the reactivity in fluorobenzene-(CH 3 OH) 2 . The OH stretching vibrations in the electronic exicted state (S 1 ) were also observed. No significant structural change in the electronic excitation was confirmed.

Electron localization in negatively charged formamide clusters studied by photodetachment spectroscopy.

Size-dependent features of the electron localization in negatively charged formamide clusters (FAn-, n = 5-21) have been studied by photodetachment spectroscopy. In the photoelectron spectra for all the sizes studied, two types of bands due to different isomers of anions were found. The low binding energy band peaking around 1 eV is assigned to the solvated electron state by relative photodetachment cross-section measurements in the near-infrared region. It is suggested that nascent electron trapping is dominated by formation of the solvated electron. The higher energy band originates from the covalent anion state generated after a significant relaxation process, which exhibits a rapid increase of electron binding energy as a function of the cluster size. A unique behavior showing a remarkable band intensity of the higher energy band was found only for n = 9.

First observation of ionic π-hydrogen bonds; vibrational spectroscopy of dihydrated naphthalene anion (Nph-(H2O)2)

Abstract Hydrated structure of Nph − (H 2 O) 2 was investigated using infrared (IR) spectroscopy. Among OH stretching vibrations, three π-hydrogen bonds to anionic naphthalene ring, namely ionic π-hydrogen (iπH) bonds, and an interwater hydrogen bond were observed. It allowed us to elucidate the geometrical structure of Nph − (H 2 O) 2 . The unstable naphthalene bare anion was stabilized through the iπH bonds with the water dimer on its ring, forming the one-sided structure. Besides, the intensity of the aromatic CH stretching modes was found to be enhanced upon the electron attachment without significant lowering of their frequencies.

Size-dependent metamorphosis of electron binding motif in cluster anions of primary amide molecules.

Electron binding motifs in cluster anions of primary amides, (acetamide)(n)(-) and (propionamide)(n)(-), were studied with photoelectron spectroscopy. For both the amides, two band series due to distinct isomeric species in the multipole-bound states were found in the low electron binding energy region (<~0.4 eV) of the photoelectron spectra at the excitation wavelength of 1064 nm. In the case of acetamide, the isomer of higher band peak energies is predominant for 6≤ n ≤ 8, but it vanishes completely for n ≥ 9 to be replaced with the lower energy isomer. The same spectral behavior was seen for propionamide exhibiting an exception at n = 7. The isomers appearing in the lower and higher energy sides were attributed to the straight and folded forms of ladder-like hydrogen bond network structures, respectively, on the basis of density functional calculations. In the folded forms, the excess electron is held in the space between two terminal amide molecules of the ladder-like networks. Referring to calculations of potential energy curves with respect to the folding coordinate of the ladder-like networks, it is inferred that the major isomer alternation between n = 8 and 9 originates from an increase of stiffness of the molecular ladders depending on the cluster sizes. In photoelectron spectra at the 355 nm excitation, the valence anion state having a band peak around 2.5 eV was observed to emerge with threshold sizes of n = 13 and 9 for acetamide and propionamide, respectively. Static and dynamical effects of alkyl groups on the electron binding motifs are discussed in comparison with the previous study on formamide cluster anions.

Interpreting the physical background of empirical solvent polarity via photodetachment spectroscopy of microsolvated aromatic ketyl anions.

The physical background of empirical solvent polarity is explored in regard to trends in solute-solvent intermolecular potential energy functions. Aromatic ketyl anions, benzophenone, and 9-fluorenone radical anions, are chosen for a model solute molecule showing solvatochromic behavior similar to betaine-30 dye, which provides the most established solvent polarity scale, E(T)(30). Common features among the ketyl anions and betaine-30 were examined with quantum chemical calculations for the electronic states and solvation structure. Vertical photodetachment and photoabsorption energies were determined for the ketyl anions microsolvated with a single solvent molecule by measuring photoelectron spectra as well as photodetachment excitation spectra for several aprotic and protic solvents. The spectroscopic data were analyzed through quantum chemical calculations based on density functional theory, and their relationship with the characteristics of intermolecular potential energies was considered. As a result, the typical solvent polarity parameter can be interpreted to reflect essentially the gradient of a potential energy function (namely, the strength of force) between a negative charge and the solvent molecules in the attractive region. A large polarity for protic solvents is attributed to an effective interaction of a proton-like hydrogen atom with the negative charge in a short-range.

Characteristic distributions of negatively charged N-monosubstituted amide clusters generated by electron attachment in supersonic expansions

Negatively charged clusters of N-monosubstituted amide molecules [(XCO-NHY)n−; X, Y = H, CH3, C2H5] were formed by injecting slow electrons into the collision region of supersonic expansions. In addition to the dipole-bound dimer species reported by C. Desfrancois, V. Periquet, S. Carles, J. P. Schermann and L. Adamowicz [Chem. Phys., 1998, 239, 475], larger clusters up to n = 50 were created for the first time. Efficient formation of trimer anions was observed for ethyl substituted amides. With a threshold size of n = 6 or 7, prominent peaks of magic numbers were found for clusters of N-methylformamide and N-methylacetamide, whereas smoothly undulated size-distributions are developed for the ethyl substitutions. A pattern was found in the periodicity of the undulations, of which the span extends as the substituents become bulkier. A hypothetical structure for large clusters in a spiral arrangement is proposed from aspects of electrostatic interactions to make an agreement with the pattern.

Photodetachment spectroscopy of fluorenone radical anions microsolvated with methanol: rationalizing the anomalous solvatochromic behavior due to hydrogen bonding.

The attribution of the extraordinary blue shift for the intramolecular charge-transfer absorption band of fluorenone radical anion solvated in protic media was investigated by means of photodetachment spectroscopy of the gas-phase anions microsolvated with methanol, in conjunction with quantum chemical calculations based on density functional theory. Sequential shifts of the vertical detachment energy as a function of the cluster size are consistent with theoretical predictions, where up to two methanol molecules can directly attach to the carbonyl group. In the photodetachment excitation spectra as alternatives to the photoabsorption spectra, with increasing cluster size, a new absorption band grows in the higher-energy region, which coincides with the blue-shifted band in protic media. Spectral simulations using time-dependent density functional theory with the CAM-B3LYP functional reproduced the feature of the phenomenon. Analyses on the electronic configuration elucidated that the extraordinarily blue shifts originate from energy-level repulsion due to solvation-induced resonant coupling with another electronic state. The orbital transition for the counterpart state corresponds to the first absorption band of the neutral fluorenone molecule, which has small oscillator strength from the ground state. It was found that correction of long-range electron exchange correlation is important for the spectral simulation involving the electronic-state coupling.