Jun-ya Hasegawa

Bifunctional Porphyrin Catalysts for the Synthesis of Cyclic Carbonates from Epoxides and CO2: Structural Optimization and Mechanistic Study

We prepared bifunctional Mg(II) porphyrin catalysts 1 for the solvent-free synthesis of cyclic carbonates from epoxides and CO2. The activities of 1d, 1h, and 1i, which have Br(-), Cl(-), and I(-) counteranions, respectively, increased in the order 1i < 1h < 1d. Catalysts 1d and 1j-m, which bear four tetraalkylammonium bromide groups with different alkyl chain lengths, showed comparable but slightly different activities. Based on the excellent catalyst 1d, we synthesized Mg(II) porphyrin 1o with eight tetraalkylammonium bromide groups, which showed even higher catalytic activity (turnover number, 138,000; turnover frequency, 19,000 h(-1)). The catalytic mechanism was studied by using 1d. The yields were nearly constant at initial CO2 pressures in the 1-6 MPa range, suggesting that CO2 was not involved in the rate-determining step in this pressure range. No reaction proceeded in supercritical CO2, probably because the epoxide (into which the catalyst dissolved) dissolved in and was diluted by the supercritical CO2. Experiments with (18)O-labeled CO2 and D-labeled epoxide suggested that the catalytic cycle involved initial nucleophilic attack of Br(-) on the less hindered side of the epoxide to generate an oxyanion, which underwent CO2 insertion to afford a CO2 adduct; subsequent intramolecular ring closure formed the cyclic carbonate and regenerated the catalyst. Density functional theory calculations gave results consistent with the experimental results, revealing that the quaternary ammonium cation underwent conformational changes that stabilized various anionic species generated during the catalytic cycle. The high activity of 1d and 1o was due to the cooperative action of the Mg(II) and Br(-) and a conformational change (induced-fit) of the quaternary ammonium cation.

Excited and ionized states of free base porphin studied by the symmetry adapted cluster‐configuration interaction (SAC‐CI) method

The SAC(symmetry adapted cluster)/SAC‐CI method is applied to the calculations of the ground, excited, and ionized states of the free base porphin. The electronic spectrum of porphin is well reproduced and new assignments for the B (Soret), N, L, and M bands are proposed. The present result shows that the four‐orbital model is strongly perturbed for the B and N bands by the excitations from the lower 4b1u MO and that the σ electron correlations are important for the description of the excited states. The absorption peaks in the ionization spectrum are assigned and the reorganization effect is found to be large especially for the n and σ electron ionizations.

Theoretical Studies on the Color-Tuning Mechanism in Retinal Proteins

The excited states of the three retinal proteins, bovine rhodopsin (Rh), bacteriorhodopsin (bR), and sensory rhodopsin II (sRII) were studied using the symmetry-adapted cluster-configuration interaction (SAC-CI) and combined quantum mechanical and molecular mechanical (QM/MM) methods. The computed absorption energies are in good agreement with the experimental ones for all three proteins. The spectral tuning mechanism was analyzed in terms of three contributions:  molecular structures of the chromophore in the binding pockets, electrostatic (ES) interaction of the chromophore with the surrounding protein environment, and quantum-mechanical effect between the chromophore and the counterion group. This analysis provided an insight into the mechanism of the large blue-shifts in the absorption peak position of Rh and sRII from that of bR. Protein ES effect is primarily important both in Rh and in sRII, and the structure effect is secondary important in Rh. The quantum-mechanical interaction between the chromophore and the counterion is very important for quantitative reproduction of the excitation energy. These results indicate that the present approach is useful for studying the absorption spectra and the mechanism of the color tuning in the retinal proteins.

Electronic Excitations of the Green Fluorescent Protein Chromophore in Its Protonation States: SAC/SAC-CI Study

Two ground‐state protonation forms causing different absorption peaks of the green fluorescent protein chromophore were investigated by the quantum mechanical SAC/SAC‐CI method with regard to the excitation energy, fluorescence energy, and ground‐state stability. The environmental effect was taken into account by a continuum spherical cavity model. The first excited state, HOMO‐LUMO excitation, has the largest transition moment and thus is thought to be the source of the absorption. The neutral and anionic forms were assigned to the protonation states for the experimental A‐ and B‐forms, respectively. The present results support the previous experimental observations.

Excited states of GFP chromophore and active site studied by the SAC-CI method: Effect of protein-environment and mutations

Excited states of fluorescent proteins were studied using symmetry‐adapted cluster‐configuration interaction (SAC‐CI) method. Protein‐environmental effect on the excitation and fluorescence energies was investigated. In green fluorescent protein (GFP), the overall protein‐environmental effect on the first excitation energy is not significant. However, glutamine (Glu) 94 and arginine (Arg96) have the red‐shift contribution as reported in a previous study (Laino et al., Chem Phys 2004, 298, 17). The excited states of GFP active site (GFP‐W22‐Ser205‐Glu222‐Ser65) were also calculated. Such large‐scale SAC‐CI calculations were performed with an improved code containing a new algorithm for the perturbation selection. The SAC‐CI results indicate that a charge‐transfer state locates at 4.19 eV, which could be related to the channel of the photochemistry as indicated in a previous experimental study. We also studied the excitation and fluorescence energies of blue fluorescent protein, cyan fluorescent protein, and Y66F. The SAC‐CI results are very close to the experimental ones. The protonation state of blue fluorescent protein was determined. Conformation of cyan fluorescent protein indicated by the present calculation agrees to the experimentally observed structure.

Highly Active and Robust Metalloporphyrin Catalysts for the Synthesis of Cyclic Carbonates from a Broad Range of Epoxides and Carbon Dioxide.

Bifunctional metalloporphyrins with quaternary ammonium bromides (nucleophiles) at the meta, para, or ortho positions of meso-phenyl groups were synthesized as catalysts for the formation of cyclic carbonates from epoxides and carbon dioxide under solvent-free conditions. The meta-substituted catalysts exhibited high catalytic performance, whereas the para- and ortho-substituted catalysts showed moderate and low activity, respectively. DFT calculations revealed the origin of the advantage of the meta-substituted catalyst, which could use the flexible quaternary ammonium cation at the meta position to stabilize various anionic species generated during catalysis. A zinc(II) porphyrin with eight nucleophiles at the meta positions showed very high catalytic activity (turnover number (TON)=240 000 at 120 °C, turnover frequency (TOF)=31 500 h(-1) at 170 °C) at an initial CO2 pressure of 1.7 MPa; catalyzed the reaction even at atmospheric CO2 pressure (balloon) at ambient temperature (20 °C); and was applicable to a broad range of substrates, including terminal and internal epoxides.

Quaternary ammonium hydroxide as a metal-free and halogen-free catalyst for the synthesis of cyclic carbonates from epoxides and carbon dioxide

Tetrabutylammonium hydroxide (TBAH) and other quaternary ammonium hydroxides catalyzed the cycloaddition of CO2 to epoxides under solvent-free conditions to give cyclic carbonates. When TBAH was exposed to CO2, TBAH was converted into tetrabutylammonium bicarbonate (TBABC), which was a catalytically active species. A D-labeled epoxide and an optically active epoxide were used to study the reaction mechanism, which invoked three plausible pathways. Among them, path A seemed to be predominant; the bicarbonate ion of TBABC attacks the less hindered C atom of the epoxide to generate a ring-opened alkoxide intermediate, which adds to CO2 to give a carbonate ion, and the subsequent cyclization yields a cyclic carbonate. Density functional theory (DFT) calculations successfully delineated the potential energy profile for each reaction pathway, among which path A was the lowest-energy pathway in accordance with the experimental results. The tetrabutylammonium (TBA) cation carries the positive charges on the H atoms, but not on the central N atom, and the positively charged H atoms close to the central N atom form an anion-binding site capable of stabilizing various anionic transition states and intermediates.

Excited States of the Photosynthetic Reaction Center of Rhodopseudomonas viridis: SAC−CI Study

The excitation spectrum of the photosynthetic reaction center (PSRC) of Rhodopseudomonas (Rps.) viridis is assigned by using the SAC(symmetry adapted cluster)−CI(configuration interaction) method. All the chromophores included in the PSRC, bacteriochlorophyll b dimer (special pair, P), bacteriochlorophyll b in L- and M-branches (BL and BM), bacteriopheophytin b in L- and M-branches (HL and HM), menaquinone (MQ), ubiquinone (UQ), and four different hemes, c-552, c-554, c-556, and c-559 in c-type cytochrome subunit, were calculated within the environment of proteins, waters, and the other chromophores which were dealt with by the point-charge electrostatic model. We have assigned successfully all the peaks in the experimental spectrum in the energy range from 1.2 to 2.5 eV. The assignment was done by comparing the SAC−CI theoretical spectrum with the experimental one in excitation energy, oscillator strength, linear dichroism data (angle of transition moment), and other experimental information available. A...

Entropically Favored Adsorption of Cellulosic Molecules onto Carbon Materials through Hydrophobic Functionalities

Carbon-based materials have attracted interest as high-performance catalysts for the aqueous-phase conversion of cellulose. The adsorption of β-glucans plays a crucial role in the catalytic performance of carbons, although the primary driving force and details of the adsorption process remain unclear. This study demonstrates that adsorption occurs at hydrophobic sites on the carbon surface and that hydrophilic groups are not involved. Analysis of adsorption temperature dependence also reveals that the entropy change associated with adsorption is positive. Our results indicate that adsorption occurs by entropically driven hydrophobic interactions in addition to CH-π hydrogen bonding. These same CH-π hydrogen bonds are also confirmed by DFT calculations. The adsorption of β-glucans on carbons is significantly stronger than the affinity between β-glucans. The adsorption equilibrium constants of β-glucans on carbons increase exponentially with increasing degrees of polymerization, which supports the theory of strong interactions between the carbon and the long β-glucans found in cellulose.

SAC-CI method applied to molecular spectroscopy

Publisher Summary This chapter describes the SAC–CI (Symmetry-Adapted Cluster– Configuration Interaction) method applied to molecular spectroscopy. The SAC–CI method was proposed in 1978 as an accurate electronic-structure theory for the ground, excited, ionized, electron-attached, and high-spin states of atoms and molecules. Since then, it has been successfully applied to various chemistries including more than 150 molecules and established to be a useful method for studying chemistry and physics involving various electronic states. The topics covered in the chapter are electronic excitation spectra, ionization spectra, collision-induced absorption processes, photochemical reactions, inner-shell ionizations, equilibrium geometries of excited states, and ESR hyperfine splitting constants of radicals. The applied systems are organic and inorganic compounds, vander Waals complexes, transition metal complexes, phthalocyanines, and the bacterial photosynthetic reaction center. These results show the reliability and applicability of the SAC–CI method for studying molecular spectroscopy.