Showing papers by "Seiji Tsuzuki published in 2006"

How Ionic Are Room-Temperature Ionic Liquids? An Indicator of the Physicochemical Properties

Abstract: Room-temperature ionic liquids (RTILs) are liquids consisting entirely of ions, and their important properties, e.g., negligible vapor pressure, are considered to result from the ionic nature. However, we do not know how ionic the RTILs are. The ionic nature of the RTILs is defined in this study as the molar conductivity ratio (Λimp/ΛNMR), calculated from the molar conductivity measured by the electrochemical impedance method (Λimp) and that estimated by use of pulse-field-gradient spin−echo NMR ionic self-diffusion coefficients and the Nernst−Einstein relation (ΛNMR). This ratio is compared with solvatochromic polarity scales: anionic donor ability (Lewis basicity), ET(30), hydrogen bond donor acidity (α), and dipolarity/polarizability (π*), as well as NMR chemical shifts. The Λimp/ΛNMR well illustrates the degree of cation−anion aggregation in the RTILs at equilibrium, which can be explained by the effects of anionic donor and cationic acceptor abilities for the RTILs having different anionic and catio...

Magnitude of the CH/π Interaction in the Gas Phase: Experimental and Theoretical Determination of the Accurate Interaction Energy in Benzene-methane

Abstract: The accurate CH/pi interaction energy of the benzene-methane model system was experimentally and theoretically determined. In the experiment, mass analyzed threshold ionization spectroscopy was applied to the benzene-methane cluster in the gas phase, prepared in a supersonic molecular beam. The binding energy in the neutral ground state of the cluster, which is regarded as the CH/pi interaction energy for this model system, was evaluated from the dissociation threshold measurements of the cluster cation. The experimentally determined binding energy (D(0)) was 1.03-1.13 kcal/mol. The interaction energy of the model system was calculated by ab initio molecular orbital methods. The estimated CCSD(T) interaction energy at the basis set limit (D(e)) was -1.43 kcal/mol. The calculated binding energy (D(0)) after the vibrational zero-point energy correction (1.13 kcal/mol) agrees well with the experimental value. The effects of basis set and electron correlation correction procedure on the calculated CH/pi interaction energy were evaluated. Accuracy of the calculated interaction energies by DFT methods using BLYP, B3LYP, PW91 and PBE functionals was also discussed.

Intermolecular Interaction between Hexafluorobenzene and Benzene: Ab Initio Calculations Including CCSD(T) Level Electron Correlation Correction

Abstract: The intermolecular interaction energy of hexafluorobenzene−benzene has been calculated with the ARS-E model (a model chemistry for the evaluation of the intermolecular interaction energy between aromatic systems using extrapolation), which was formerly called the AIMI model. The CCSD(T) interaction energy at the basis-set limit has been estimated from the MP2 interaction energy at the basis-set limit and the CCSD(T) correction term obtained using a medium-sized basis set. The slipped-parallel (Cs) complex has the largest (most negative) interaction energy (−5.38 kcal/mol). The sandwich (C6v) complex is slightly less stable (−5.07 kcal/mol). The interaction energies of two T-shaped (C2v) complexes are very small (−1.74 and −0.88 kcal/mol). The calculated interaction energy of the slipped-parallel complex is about twice as large as that of the benzene dimer. The dispersion interaction is found to be the major source of attraction in the complex, although electrostatic interaction also contributes to the att...

Magnitude and Directionality of the Interaction Energy of the Aliphatic CH/π Interaction: Significant Difference from Hydrogen Bond

Abstract: The CCSD(T) level interaction energies of CH/π complexes at the basis set limit were estimated. The estimated interaction energies of the benzene complexes with CH4, CH3CH3, CH2CH2, CHCH, CH3NH2, CH3OH, CH3OCH3, CH3F, CH3Cl, CH3ClNH2, CH3ClOH, CH2Cl2, CH2FCl, CH2F2, CHCl3, and CH3F3 are −1.45, −1.82, −2.06, −2.83, −1.94, −1.98, −2.06, −2.31, −2.99, −3.57, −3.71, −4.54, −3.88, −3.22, −5.64, and −4.18 kcal/mol, respectively. Dispersion is the major source of attraction, even if substituents are attached to the carbon atom of the C−H bond. The dispersion interaction between benzene and chlorine atoms, which is not the CH/π interaction, is the cause of the very large interaction energy of the CHCl3 complex. Activated CH/π interaction (acetylene and substituted methanes with two or three electron-withdrawing groups) is not very weak. The nature of the activated CH/π interaction may be similar to the hydrogen bond. On the other hand, the nature of other typical (nonactivated) CH/π interactions is completely dif...

Estimated MP2 and CCSD(T) interaction energies of n-alkane dimers at the basis set limit: Comparison of the methods of Helgaker et al. and Feller

Abstract: The MP2 (the second-order Moller-Plesset calculation) and CCSD(T) (coupled cluster calculation with single and double substitutions with noniterative triple excitations) interaction energies of all-trans n-alkane dimers were calculated using Dunning's [J. Chem. Phys. 90, 1007 (1989)] correlation consistent basis sets. The estimated MP2 interaction energies of methane, ethane, and propane dimers at the basis set limit [EMP2(limit)] by the method of Helgaker et al. [J. Chem. Phys. 106, 9639 (1997)] from the MP2∕aug-cc-pVXZ (X=D and T) level interaction energies are very close to those estimated from the MP2∕aug-cc-pVXZ (X=T and Q) level interaction energies. The estimated EMP2(limit) values of n-butane to n-heptane dimers from the MP2∕cc-pVXZ (X=D and T) level interaction energies are very close to those from the MP2∕aug-cc-pVXZ (X=D and T) ones. The EMP2(limit) values estimated by Feller's [J. Chem. Phys. 96, 6104 (1992)] method from the MP2∕cc-pVXZ (X=D, T, and Q) level interaction energies are close to t...

Origin of the Attraction in Aliphatic C−H/π Interactions: Infrared Spectroscopic and Theoretical Characterization of Gas-Phase Clusters of Aromatics with Methane

Abstract: An attractive intermolecular interaction between an aliphatic C-H bond and a pi-electron system (C-H/pi interaction) was characterized on the basis of infrared spectroscopy and high level ab initio calculations. Infrared spectroscopy was applied to several isolated methane clusters with benzene, toluene, p-xylene, mesitylene, and naphthalene in the gas phase, and the spectral changes of the C-H stretch bands in the methane moiety upon the cluster formation were observed. In the theoretical approach, interaction energies of the clusters were evaluated by high-level ab initio calculations. The forbidden symmetric C-H stretch transition weakly appeared in the IR spectra of the clusters, and it confirmed the small deformation of the methane moiety from the T(d)() symmetry, which was predicted by the ab initio calculations. On the other hand, the degenerated asymmetric C-H stretch band showed complicated splitting, which is qualitatively interpreted by a hindered rotor model. Low-frequency shifts upon the cluster formation were seen in the symmetric C-H stretch frequency, though the magnitude of the shifts was extremely small and no clear correlation with the interaction energy was found. On the other hand, the size of the calculated interaction energy well correlates with the polarizability of aromatics. The S(1)-S(0) electronic transition of the aromatic moiety was also observed, and it showed low-frequency shifts upon cluster formation. These results support the dominance of the dispersion interaction over the electrostatic and charge-transfer terms in the aliphatic C-H/pi interaction.

Intermolecular interactions of nitrobenzene-benzene complex and nitrobenzene dimer: Significant stabilization of slipped-parallel orientation by dispersion interaction

Abstract: The CCSD(T) level interaction energies of eight orientations of nitrobenzene-benzene complexes and nine orientations of nitrobenzene dimers at the basis set limit have been estimated The calculated interaction energy of the most stable slipped-parallel (Cs) nitrobenzene-benzene complex was −451kcal∕mol That of the most stable slipped-parallel (antiparallel) (C2h) nitrobenzene dimer was −681kcal∕mol The interaction energies of these complexes are significantly larger than that of the benzene dimer The T-shaped complexes are substantially less stable Although nitrobenzene has a polar nitro group, electrostatic interaction is always considerably weaker than the dispersion interaction The dispersion interaction in these complexes is larger than that in the benzene dimer, which is the cause of the preference of the slipped-parallel orientation in these complexes

Alcohol and proton transport in perfluorinated ionomer membranes for fuel cells.

Abstract: To clarify the transport mechanisms of alcohols and proton in perfluorosulfonated ionomer (PFSI) membranes for fuel cells, four membranes having different equivalent weight (EW) values were examined. Membranes were immersed in methanol, ethanol, and 2-propanol to prepare a total of 12 samples, and membrane swelling, mass (alcohol and proton) transports, and interactions between alcohols and proton were investigated systematically in the fully penetrated state. The membrane expansion fraction theta and alcohol content lambda increased with decreasing the EW value for all the samples. The self-diffusion coefficients (D's) of the alkyl group and of OH (including protons) were measured separately by the pulsed-gradient spin-echo (PGSE)-NMR method and the D's also increased with decreasing the EW value. These results implied that the alcohols penetrate into the hydrophilic regions of the PFSI membranes and diffuse through the space expanded by the alcohols. The ionic cluster regions formed by the alcohols resemble those induced by water in the water swollen membrane, where protons dissociated from sulfonic acid groups transport through the regions together with water molecules. The D values decreased with increasing the molecular weight of alcohols. This trend was supported by activation energies Ea estimated from the Arrhenius plots of D in the temperature range from 30 to -40 degrees C. The PGSE-NMR measurements also revealed that protons move faster than the alkyl groups in the membranes. The proton transport by the Grotthuss (hopping) mechanism was facilitated by the increase of the alcohol content and the decrease of the molecular weight. This result was also supported by the experimental results of proton conductivity kappa and mobility u(H(+)). Density functional theory (DFT) calculations of the interaction energy DeltaE(int) between proton and alcohol (including OH) showed that the /DeltaE(int)/ increases with increasing the molecular weight of alcohols, which is in a inverse relationship with the kappa and u(H(+)) values. The proton transport depends strongly on the DeltaE(int) in the membranes.

Fluorescence Spectroscopic Properties and Crystal Structure of a Series of Donor−Acceptor Diphenylpolyenes

Abstract: A series of p-nitro-p‘-alkoxy(OR)-substituted (E,E,E)-1,6-diphenyl-1,3,5-hexatrienes (1a, R = Me; 1b, R = Et; 1c, R = n-Pr; 1d, R = n-Bu) were prepared The absorption and fluorescence spectra in solution were almost independent of the alkoxy chain length The absorption maximum showed only a small dependence on the solvent polarity, whereas the fluorescence maximum red-shifted largely as the polarity increased The solid-state absorption and fluorescence spectra were red-shifted relative to those in low polar solvents and were clearly dependent on the alkoxy chain length The fluorescence maxima for the crystals of 1b and 1d were observed at 635−650 nm, which were red-shifted by 40−50 nm relative to those for 1a and 1c The Stokes shifts were all relatively small (3000−3500 cm-1) For all four compounds, the fluorescence decay curves in the solid state were able to be analyzed by single-exponential fitting to give the lifetimes of 11−13 ns This indicates that the emission of 1a−d is not originated fro

Two-dimensional structure control by molecular width variation with metal coordination.

Abstract: The self-assembled monolayer of bipyridine derivative 1, which has two alkyl chains on each end, at the HOPG/1-phenyloctane interface was studied by in situ scanning tunneling microscopy (STM). The detailed mechanism of a spontaneous change in the monolayer packing pattern by Pd coordination was studied. Uncomplexed 1 existed in a bent form in the monolayer, and the alkyl chains were interdigitated, whereas Pd-complexed 1 was in a straight form and the alkyl chains were not interdigitated. An intermediate state of 1 was successfully observed during metal coordination. The structure was the bent form with noninterdigitated alkyl chains. Equilibrium intermolecular distances reported from ab initio calculations indicate that the molecular width of the central aromatic part of uncomplexed 1 (7.5 A) is substantially smaller than that of the peripheral alkyl chain part (9.2 A). The bent form was suitable for covering up the surface to maximize the packing density. However, the molecular width of the aromatic un...

Influence of ligand structures on methanol electro-oxidation by mixed catalysts based on platinum and organic metal complexes for DMFC

Abstract: In order to investigate the influence of mixing organic metal complexes with various metal coordination structure on Pt/C catalyst for methanol oxidation reaction (MOR) in acidic media, N , N ′-mono-8-quinolyl- o -phenylenediamine (mqph), N , N ′-bis(anthranilidene)ethylenediamine (anthen) and N , N ′-bis(salicylidene)ethylenediamine (salen), coordinating to Co(II) and Ni(II), were examined as the model compounds. M(mqph), M(anthen) and M(salen) have MN 3 , MN 4 and MN 2 O 2 moiety as the catalytic active sites, respectively. Pt–M(complex)/C mixed catalysts were prepared by depositing the mixture of Pt tetra-ammine chloride and each organic metal complex with various mixing ratio (w/o) on graphite powder and then heat-treating at 600 °C in Ar atmosphere. The Pt–M(complex)/C samples were put on a glassy carbon disk electrode, and tested for electrochemical MOR in 1 mol dm −3 CH 3 OH + 0.05 mol dm −3 H 2 SO 4 aqueous solutions at 25 °C. The mixed catalysts, especially Pt–Ni(mqph)/C and Pt–Co(mqph)/C, were found to enhance the MOR and exhibited higher catalytic activities than Pt/C, although each organic metal complex solely showed no catalytic activity. The catalytic ability was enhanced by mixing up to 50/50 (Pt–M(complex)) mixing ratio for the Co(mqph) and up to 80/20 mixing ratio for the Ni(mqph). Larger amounts of M(complex) resulted in a decrease in the catalytic activities. These results indicate that the organic metal complexes, especially M(mqph), promote effectively the electrochemical MOR on Pt/C catalyst, the degree of which depends strongly on the ligand structure. X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectra (XAS) for the Pt–M(complex)/C mixed catalysts showed that the metal coordination structures of organic metal complexes are partially retained on the graphite powder even after the severe heat-treatment at 600 °C in Ar atmosphere. Ab initio calculations for the organic metal complexes exhibited that the Ni metal site of the Ni(mqph) interact with OH − group more strongly than those of the other organic metal complexes. This fact suggests that the Pt–M(mqph)/C electro-oxidizes a byproduct CO absorbed on Pt by “bifunctional effect” in a similar way as Pt–Ru alloy catalysts do in promoting the effective electrochemical MOR.