Ryoichi Fukuda

High-pressure Diels?Alder cycloaddition reaction of fullerenes is an important synthetic method for the thermally stable cycloadducts. The effects of high pressure on the potential energy surfaces of Diels-Alder cycloaddition of cyclopentadiene and C₆₀ were studied with a recently developed approach, the polarizable continuum model for extreme pressure (XP-PCM). It is revealed that the high pressure reduces the activation energies and increases reaction energies drastically, making the DA reaction more favorable. The pressure effects on the reaction energetics can be divided into the cavitation and electronic contributions. For the activation energy, the cavitation contribution is significant in comparison with the electronic contribution. To assist future experiments, the activation volume and reaction volume were computed on the basis of the relationship between activation energy or reaction energy with the pressure as a consequence of the fitting linear correlation between activation energy or reaction energy with the pressure.

The performances, in the description of excited state potential energy surfaces, of several density functional approximations representative of the currently most applied exchange correlation functionals' families have been tested with respect to post Hartree-Fock references (here Symmetry Adapted Cluster-Configuration Interaction results). An experimentally well-characterized intermolecular proton transfer reaction has been considered as test case. The computed potential energy profiles were analyzed both in the gas phase and in toluene solution, here represented as a polarizable continuum model. The presence of intermolecular (dark) and intramolecular (bright) charge transfer excited states, whose polarity strongly differs along the reaction pathway, makes clear that only subtle compensation between spurious electronic effects -related to the incorrect asymptotic behavior of the functional- and solvent stabilization of polar states leads to the overall correct description of this excited state reaction when using global hybrids with low percentage of Hartree-Fock exchange.

Bimetallic alloy nanoparticles are promising candidates for replacing platinum group metals utilized in the catalytic removal of nitrogen oxides in exhaust gas. In this study, we investigated the electronic, interfacial, and surface structures of copper/ruthenium alloy nanoparticles by quantum chemical computations using 135-atomic cluster models. We employed Ru-core/Cu-shell models in which the Ru-core takes both fcc (face-centered cubic) and hcp (hexagonal closed-packed) structures. The fcc-core model has a coherent Cu/Ru interface, while the hcp-core model involves an incoherent interface. This incoherence results in discontinuity in the lattice structure and the valence electronic structure, and generates step-like structures on the surface of the particle. Such a step-like site enhances the catalytic activities for nitric oxide dissociation. The orbital energies suggest that the alloying can control the oxidation tendency of clusters. Charge-transfer occurs between the Cu shell and Ru core; the surface layer of the clusters has a positive charge, although the surface atoms are not directly binding to the core Ru atoms. The interfacial structure of core?shell interphase is a crucial factor to be considered in designing the properties of alloy nanoparticles.

N-Hydroxypyridine-2(1H)-thione (N-HPT) is an important photochemical generator of hydroxyl radicals; however, it has been pointed out that N-HPT is not a specific precursor of hydroxyl radical. Photoionization of N-HPT competes with photochemical N-O bond cleavage in neutral aqueous solution. The possibility of a competitive reaction could be critical for studies using N-HPT as the radical precursor; therefore, the detailed behaviors of electronic excitation and ionization of N-HPT and its deprotonated anion, which is the dominant tautomer under neutral pH conditions, are studied using quantum chemical methods with the symmetry-adapted cluster-configuration interaction (SAC-CI) method and the polarizable continuum model (PCM). The detailed assignment of the UV-vis spectra of N-HPT is provided, and the origin of the observed negative solvatochromism is found to be the charge transfer excitation between the sulfur and the pyridine ring. The photochemical N-O bond cleavage occurs via the conical intersections between the lowest π→π^* and π→σ^* states and between the π→σ^* and ground state, when N-HPT dissociates into PyS· and ·OH radicals. The calculated ionization potentials of N-HPT and the deprotonated N-HPT anion are 5.75 and 4.67 eV in PCM water. This demonstrates that the charge transfer excitation energy between N-HPT and liquid water becomes significantly lower for the deprotonated anion in comparison with the neutral molecule. Even under mild photochemical conditions, photoinduced ionization of N-HPT may occur in neutral aqueous solution.

The recent advances in the development and application of density-based indexes for the description of the nature and the quantification of the extent of charge transfer associated with a given electronic transition are here reviewed. Starting from the basic definition of the indexes, a brief overview of their potential as indicators of potentially problematic cases in the description of charge transfer excitations using Time Dependent Density Functional Theory (TD-DFT) will be first given together with their possible application for comparing TD-DFT results to post Hartree-Fock (post-HF) calculations. After this methodological part, several examples of the application of density-based indexes to describe, from a quantitative and qualitative point of view, the charge transfer character (for instance in push-pull systems) or to map excited state reaction pathways (for instance in the case of Excited State Proton Transfer reactions) will be given to exemplify the insights that these indexes may bring to the description and design of new compounds of potential technological relevance.

The geometries and electronic structures of selenolate-protected Au nanoclusters, Au₂₄(SeR)₂₀ and Au₂₀(SeR)₁₆, and their thiolate analogues are theoretically investigated with DFT and SCS-MP2 methods, to elucidate the electronic structure of their unusual Au₈ core and the reason why they have the abnormal entangled geometry of "staple-like" chain ligands. The Au₈ core is understood to be an [Au₄]²⁺ dimer in which the [Au₄]²⁺ species has a tetrahedral geometry with a closed-shell singlet electronic structure. The SCS-MP2 method successfully reproduced the distance between two [Au₄]²⁺ species but the DFT with various functionals failed, suggesting that the dispersion interaction is crucial between these two [Au₄]²⁺ species. The SCS-MP2-calculated formation energies of these nanocluster compounds indicate that the thiolate staple-like chain ligands are more stable than the selenolate ones but the Au₈ core more strongly coordinates with the selenolate staple-like chain ligands than with the thiolate ones. Though Au₂₀(SeR)₁₆ has not been reported yet, its formation energy is calculated to be large, suggesting that this compound can be synthesized as a stable species. The aurophilic interaction between the staple-like chain ligands and between the Au8 core and the staple-like chain ligand plays an important role for the total stability. This is one of the important reasons why the unusual entangled ligands are involved in these compounds.

The proton induced Ru-C bond variation which was previously found to be relevant in the water oxidation has been investigated using cyclometalated ruthenium complexes with three phenanthroline (phen) isomers. The designed complexes, [Ru(bpy)₂(1,5-phen)]⁺ ([2]⁺), [Ru(bpy)₂(1,6-phen)]⁺ ([3]₊), and [Ru(bpy)₂(1,7-phen)]⁺ ([4]⁺) were newly synthesized and their structural and electronic properties were analyzed by various spectroscopy and theoretical protocol. Protonation of [4]⁺ triggered profound electronic structural change to form remote N-heterocyclic carbene (rNHC), while protonation of [2]⁺ and [3]⁺ did not affect their structures. It was found that changes in the electronic structure of phen beyond classical resonance forms control the rNHC behavior. The present study provides new insights into the ligand design of related Ru catalysts.

Attenuated total reflection far-ultraviolet (ATR-FUV) spectra in the 145-260 nm region were measured for surfaces (thickness: 50-200 nm) of various kinds of Nylons in cast films to explore their electronic transitions in the FUV region. ATR-FUV spectra show two major bands near 150 and 200 nm in the surface condensed phase of Nylons. Transmittance (Tr) spectra were also observed in particular for the analysis of valence excitations. Time-dependent density functional theory (TD-DFT/ CAM-B3LYP) calculations were carried out using the model systems to provide the definitive assignments of their absorption spectra and to elucidate their peak shifts in several Nylons, in particular, focusing on their crystal alignment structures and intermolecular hydrogen bondings. Two major bands of Nylon films near 150 and 200 nm are characterized as σ-Rydberg 3p and π-π^* transitions of Nylons, respectively. These assignments are also coherent with those of liquid n-alkanes (n=5-14) and liquid amides observed previously. The Rydberg transitions are delocalized over the hydrocarbon chains, while the π-π^* transitions are relatively localized at the amide group. Differences in the peak positions and intensity were found in both ATR- and Tr-FUV spectra for different Nylons. A redshift of the π-π^* amide band in the FUV spectra of Nylon 6 and Nylon 6/6 models in α form is attributed to the crystal structure pattern and the intermolecular hydrogen bondings, which result in the different delocalization character of the π-π^* transitions and transition dipole coupling.

In this work, we investigated the properties of the triplet excited states of heterocyclic compounds including their geometries, electronic properties, and phosphorescence energies by using both the direct symmetry-adapted cluster-configuration interaction (SAC-CI) method and the TD-DFT approach with the PBE0 exchange-correlation functional (TD-PBE0). The target states are the ππ^* and nπ^* triplet states of furan, pyrrole, pyridine, p-benzoquinone, uracil, adenine, 9,10-anthraquinone, coumarin, and 1,8-naphthalimide as well as the Rydberg states. The present benchmark demonstrates that these two methods provide reasonably accurate geometries for the excited states of these heterocyclic compounds. The calculated Stokes shifts, which reflect geometry changes, were consistent for both these methods. The trends of agreement with experimental or reference values obtained for a panel of exchange-correlation functionals used to compute the absolute emission energies from the triplet states, differ from those found for the singlet excited states. Some of the low-lying triplet excited states were examined in detail for the first time, including vibrational analysis.

H₂ dissociation by Ag clusters supported on the θ-Al₂O₃ (110) surface has been investigated using density functional theory calculations. The crucial role of the dual perimeter site of Ag cluster and the surface oxygen (O) site of the alumina support is demonstrated with three theoretical models: anchored cluster, isolated cluster, and anchored cluster on hydroxylated alumina. The heterolytic cleavage of H₂ at the silver-alumina interface, yielding Ag-H^δ- and O-H^δ+, is thermodynamically and kinetically preferred compared with H₂ cleavage at two Ag atomic sites on top of the Al₂O₃-supported Ag cluster and the homolytic cleavage of H₂ on the isolated Ag cluster. The hydroxylation at the O site of the alumina reduces the H₂ dissociation activity, which indicates that the interfacial bare O site is indispensible. It is concluded that the interfacial cooperative mechanism between the Ag cluster and Lewis acid-base pair site (bare Al-O site) is essentially relevant for the H₂ activation over Ag-loaded Al₂O₃ catalysts.

Attenuated total reflection far-ultraviolet (ATR-FUV) spectra in the 140-260 nm region were measured for several types of liquid amides (formamide, FA; N-N-methylformamide, NMF; N-methylacetamide, NMA; N,N-dimethylformamide, NdMF; and N,N-dimethylacetamide, NdMA) to investigate their electronic transitions in the FUV region. The spectra were compared with the corresponding gas-phase spectra to examine the shift in the major absorption band in the 180-200 nm region going from the gas phase to the liquid phase, and it was found that the peak shift was dependent on the particular amide. FA and NMF, which exhibit intermolecular C=O···H-N hydrogen bonding, show a large shift of ~0.60 eV to lower energy; however, NMA, which also exhibits hydrogen bonding, shows only a small shift. In NdMF and NdMA, C=O groups seem to be coupled, which results in a small peak shift. Two types of quantum chemical calculations, time-dependent density functional theory (TD-DFT) and symmetry-adapted-cluster configuration interaction (SAC-CI) method, were performed to elucidate the origin of the shifts and the band assignments. The shift estimated by the monomer and dimer models with TD-DFT reproduced well the observed shift from the gas phase to the liquid phase. This suggests that the intermolecular hydrogen-bonding interaction significantly affects the magnitude of the shift. The many-body effects were also considered using the larger cluster models (trimer to pentamer). The energy shift calculated using SAC-CI with the monomer and the state-specific polarizable continuum model was also accurate, indicating that the nonlinear polarization effect appears to be important. As for the band assignments, it was found that though the major band can be mainly attributed to the π-π* transition, several types of Rydberg transitions also exist in its vicinity and mixing of orbitals with the same symmetry occurs. The number and type of Rydberg transitions in the spectra depend upon the type of amide molecules. The valence-Rydberg coupling of the π-π* transition is more significant than n-π* transition, which also holds in the pure liquid phase.

The low-lying electronic excited states of biphenyl were studied with the symmetry-adapted cluster-configuration interaction (SAC-CI), complete active space self-consistent field (CASSCF), complete active space perturbation theory of the second-order (CASPT2), and the time-dependent density functional theory (TDDFT). The molecular geometries in the ground and excited states were optimized with using the SAC-CI and TDDFT for singlet and triplet states. The energies of vertical excitations, emissions, and adiabatic transitions were calculated. The TDDFT calculations significantly underestimated the excitation energy of the 1¹B₁ state, while the SAC-CI and CASPT2 provided the essentially similar results. The present SAC-CI and CASPT2 calculations concluded that the lowest singlet state of isolated biphenyl is the 1¹B₃ state that takes a planar geometry and the second lowest state is the 1¹B₂ state with a twisted geometry. The present results were consistent with the previous experimental findings. The 1¹B₁ state that has a charge-separated biracial character in the vertical excitation relaxed into a planar quinoide structure in which bond alternations were emphasized. The other states took a benzenoid structure. The ultraviolet (UV) absorption and circular dichroism (CD) spectra below 7 eV were calculated with the SAC-CI method. The valence-Rydberg mixings were found to be significant in the second and higher series of excited states.

The structures of low-lying singlet excited states of nine π-conjugated heteroaromatic compounds have been investigated by the symmetry-adapted cluster-configuration interaction (SAC-CI) method and the time-dependent density functional theory (TDDFT) using the PBE0 functional (TD-PBE0). In particular, the geometry relaxation in some ππ* and nπ* excited states of furan, pyrrole, pyridine, p-benzoquinone, uracil, adenine, 9,10-anthraquinone, coumarin, and 1,8-naphthalimide as well as the corresponding vertical transitions, including Rydberg excited states, have been analyzed in details. The basis set and functional dependence of the results was also examined. The SAC-CI and TD-PBE0 calculations showedreasonable agreement in both transition energies and excited-state equilibrium structures for these heteroaromatic compounds.

The mechanism of the aerobic oxidation of methanol to formic acid catalyzed by Au₈^- has been systematically investigated using density functional theory with the M06 functional. The reaction pathways were examined by taking into account the full structural relaxation of the Au₈^-. Stepwise and concerted reaction mechanisms are proposed. The stepwise mechanism is initiated by the hydrogen abstraction of a methoxy species by a superoxo-like anion on the gold cluster, resulting in the formation of formaldehyde. Subsequently, the formaldehyde is activated by the hydroxyl group of a hydroperoxyl-like complex, leading to the formation of a hemiacetal intermediate. The formation of formic acid in the final step is achieved by hydrogen abstraction of the hemiacetal intermediate by atomic oxygen attached to the gold cluster. Our calculations indicate that the first step of the stepwise mechanism, that is, hydrogen abstraction of the methoxy species, is the rate-determining step. Another possible reaction pathway involving a single-step hydrogen abstraction, a concerted mechanism, is also discussed. This mechanism may also be responsible for the reasonable catalytic activity of aerobic oxidation of methanol on Au₈^- because of the low activation energy barrier.

A series of organic sensitizers with the direct electron injection mechanism and high molar extinction coefficient comprising double donors, π-spacer, and anchoring acceptor groups (D-D-π-A type) were synthesized and characterized by experimental and theoretical methods for dye-sensitized solar cells. (E)-2-cyano-3-(5"-(4-((4-(3,6-di- tert-butylcarbazol-9-yl)phe-nyl)(dodecyl)amino)phenyl) -[2,2':5',2"-terthiophen]-5-yl) acrylic acid (UB3) showed the performance with a maximal IPCE value of 83%, Jsc value of 10.89 mA cm^-2, V_oc value of 0.70 V, and FF value of 0.67, that correspond to an overall conversion efficiency of 5.12% under AM 1.5G illumination. The molecular geometry, electronic structure, and excited states were investigated with the density functional theory, time-dependent density functional theory, and symmetry-adapted cluster-configuration interaction method. The double donor moieties are contributed not only to enhance the electron-donating ability, but also inhibit aggregation between dye molecules and prevent iodide/triiodide in the electrolyte from recombining with injected electrons in TiO₂. Detailed assignments of the UV-visible spectra below ionization threshold were given. The low-lying light harvesting state has intramolecular charge transfer character with high molar extinction coefficient because of the long π-spacer. Our experimental and theoretical findings supported the potential of direct electron injection from dye to TiO₂ by one-step with electronic excitation for the present D-D-π-A sensitizers. The direct electron injection, inhibited aggregation, and high molar extinction coefficient may be the origin of the observed high efficiency. This type of D-D-π-A structure with direct electron injection would simplify the strategy for designing organic sensitizers.

The electronic excited states and optical absorption spectrum of C₆₀ fullerene below 6.2 eV (200 nm) were calculated using the ab initio many-body wavefunction theory of cluster expansion method: the symmetry-adapted cluster-configuration interaction method. Not only optically allowed states but also optically forbidden states were calculated for studying the observed weak absorptions in the visible region. The lowest calculated singlet excited state was the 1¹G_g state. The electron correlation effects are important in determining the energy levels of the four low-lying states that have the character of degenerated HOMO-LUMO transition. The lowest optically allowed 1¹T_1u state was calculated at 3.67 eV; this is significantly higher than the energy values found in previous density functional calculations. The observed weak absorption around 3.08 eV appears to correspond to the optically forbidden 1¹T_2u state with intensity borrowing via vibronic couplings.

Photoisomeric transformations in ruthenium polypyridyl complexes have been rarely reported. Herein we report the geometrical transformation of cyclometalated trans-[Ru(tpy)(PAD)(OH₂)]⁺ ([1]⁺) to the cis-[Ru(tpy)(PAD)(OH₂)]⁺ ([1a]⁺) (tpy = 2,2';6',2"-terpyridine, PAD = 2-(pyrid-2'-yl)acridine) isomer upon irradiation of visible light (λ ≥ 420 nm). Due to a proton-induced tautomeric equilibrium between the Ru-C bond and Ru=C coordination, the π* energy levels of PADH are lower than those of tpy by 12.61 and 12.24 kcal mol^-1, respectively, in [1]⁺ and [1a]⁺. Isomers [1]⁺ and [1a]⁺ both act as catalytic oxygen-evolving complexes (OECs) chemically as well as electrochemically.

The aerobic oxidation of methanol to formic acid catalyzed by Au₂₀ has been investigated quantum chemically using density functional theory with the M06 functional. Possible reaction pathways are examined taking account of full structure relaxation of the Au₂₀ cluster. The proposed reaction mechanism consists of three elementary steps: (1) formation of formaldehyde from methoxy species activated by a superoxo-like anion on the gold cluster; (2) nucleophilic addition by the hydroxyl group of a hydroperoxyl-like complex to formaldehyde resulting in a hemiacetal intermediate; and (3) formation of formic acid by hydrogen transfer from the hemiacetal intermediate to atomic oxygen attached to the gold cluster. A comparison of the computed energetics of various elementary steps indicates that C-H bond dissociation of the methoxy species leading to formation of formaldehyde is the rate-determining step. A possible reaction pathway involving single-step hydrogen abstraction, a concerted mechanism, is also discussed. The stabilities of reactants, intermediates and transition state structures are governed by the coordination number of the gold atoms, charge distribution, cooperative effect and structural distortion, which are the key parameters for understanding the relationship between the structure of the gold cluster and catalytic activity in the aerobic oxidation of alcohols.

In this series of studies, we systematically apply the analytical energy gradients of the direct symmetry-adapted cluster-configuration interaction singles and doubles nonvariational method to calculate the equilibrium geometries and vibrational frequencies of excited and ionized states of molecules. The harmonic vibrational frequencies were calculated using the second derivatives numerically computed from the analytical first derivatives and the anharmonicity was evaluated from the three-dimensional potential energy surfaces around the local minima. In this paper, the method is applied to the low-lying valence singlet and triplet excited states of HAX-type molecules, HCF, HCCl, HSiF, HSiCl, HNO, HPO, and their deuterium isotopomers. The vibrational level emission spectra of HSiF and DSiF and absorption spectra of HSiCl and DSiCl were also simulated within the Franck-Condon approximation and agree well with the experimental spectra. The results show that the present method is useful and reliable for calculating these quantities and spectra. The change in geometry in the excited states was qualitatively interpreted in the light of the electrostatic force theory. The effect of perturbation selection with the localized molecular orbitals on the geometrical parameters and harmonic vibrational frequencies is also discussed.

In this paper, we present the theory and implementation of a nonequilibrium solvation model for the symmetry-adapted cluster (SAC) and symmetry-adapted cluster-configuration interaction (SAC-CI) method in the polarizable continuum model. For nonequilibrium solvation, we adopted the Pekar partition scheme in which solvent charges are divided into dynamical and inertial components. With this nonequilibrium solvation scheme, a vertical transition from an initial state to a final state may be described as follows: the initial state is described by equilibrium solvation, while in the final state, the inertial component remains in the solvation for the initial state; the dynamical component will be calculated self-consistently for the final state. The present method was applied to the vertical photoemission and absorption of s-trans acrolein and methylenecyclopropene. The effect of nonequilibrium solvation was significant for a polar solvent.

In this paper we present the theory and implementation of the symmetry-adapted cluster (SAC) and symmetry-adapted cluster-configuration interaction (SAC-CI) method, including the solvent effect, using the polarizable continuum model (PCM). The PCM and SAC/SAC-CI were consistently combined in terms of the energy functional formalism. The excitation energies were calculated by means of the state-specific approach, the advantage of which over the linear-response approach has been shown. The single-point energy calculation and its analytical energy derivatives are presented and implemented, where the free-energy and its derivatives are evaluated because of the presence of solute-solvent interactions. We have applied this method to s-trans-acrolein and metylenecyclopropene of their electronic excitation in solution. The molecular geometries in the ground and excited states were optimized in vacuum and in solution, and both the vertical and adiabatic excitations were studied. The PCM-SAC/SAC-CI reproduced the known trend of the solvent effect on the vertical excitation energies but the shift values were underestimated. The excited state geometry in planar and nonplanar conformations was investigated. The importance of using state-specific methods was shown for the solvent effect on the optimized geometry in the excited state. The mechanism of the solvent effect is discussed in terms of the Mulliken charges and electronic dipole moment.

The valence ionization spectra up to 20 eV of group six metal carbonyls, chromium hexacarbonyl, molybdenum hexacarbonyl, and tungsten hexacarbonyl were studied by the symmetry-adapted cluster-configuration interaction (SAC-CI) method. The assignments of the spectra are given based on reliable SAC-CI calculations. The relativistic effects including the spin-orbit effects are important for the ionization spectrum of W(CO)₆. The relation between the metal-CO distance and ionization energies was examined. The lowest ionization energies of the three metal carbonyls are approximately the same because of the energy dependence of the metal-CO length and relativistic effects. In Cr(CO)₆, the Cr—CO interaction significantly increases the lowest ionization energy in comparison with Mo(CO)₆ and W(CO)₆ because of the relatively short metal-CO bond length. The relativistic effect reduces the lowest ionization energy of W(CO)₆ because the effective core potential of 5d electrons is more efficiently screened as a result of the relativistic contraction of the inner electrons.

Symmetry-resolved ion-yield spectroscopy of vibrationally excited N₂O in the vicinity of the O 1s ionization threshold has been carried out using an angle-resolved ion detection technique. The spectral intensity of the O 1s → nsσ Rydberg series, which has σ* valence character, is significantly reduced by the excitation of the bending vibration in the electronic ground state. Using an ab initio analysis of the electronic part of the second moment <r²>, this suppression is interpreted as being due to a decrease in the mixing of the valence character in the nsσ Rydberg states with decreasing bond angle.