Takayoshi Ishimoto

Geometrical structure of benzene and naphthalene: ultrahigh-resolution laser spectroscopy and ab initio calculation.

Geometrical structures of the isolated benzene and naphthalene molecules have been accurately determined by using ultrahigh-resolution laser spectroscopy and ab initio calculation in a complementary manner. The benzene molecule has been identified to be planar and hexagonal (D(6h)) and the structure has been determined with accuracies of 2 × 10(-14) m (0.2 mÅ; 1 Å = 1 × 10(-10) m) for the C-C bond length and 1.0 × 10(-13) m (1.0 mÅ) for the C-H bond length. The naphthalene molecule has been identified to be symmetric with respect to three coordinate axes (D(2h)) and the structure has been determined with comparable accuracies. We discuss the effect of vibrational averaging that is a consequence of zero-point motions on the uncertainty in determining the bond lengths.

Structure and excited-state dynamics of anthracene: Ultrahigh-resolution spectroscopy and theoretical calculation

Rotationally resolved ultrahigh-resolution spectra of the S(1) (1)B(2u)<--S(0) (1)A(g) transition of anthracene-h(10) and anthracene-d(10) have been observed using a single-mode UV laser and a collimated supersonic jet. We have determined rotational constants of the zero-vibrational levels of the S(0) and S(1) states by analyzing the precisely calibrated transition wavenumbers of rotational lines. We measured Zeeman splitting of each rotational line in the external magnetic field, of which the magnitude was small and strongly dependent on the rotational quantum numbers. We have shown that the magnetic moment in the S(1) (1)B(2u) state arises from J-L coupling with the S(2) (1)B(3u) state and that mixing with the triplet state is negligibly small. We concluded that the main radiationless transition in the S(1) state of anthracene is not intersystem crossing to the triplet state but internal conversion to the ground state. We also examined methods of ab initio theoretical calculation to determine which method most closely yielded the same values of rotational constants as the experimentally obtained ones. Moller-Plesset second-order perturbation method with a 6-31G(d,p) basis set yielded approximately the same values for the S(0) (1)A(g) state with an error of less than 0.04%. Geometrical structure in the S(0) (1)A(g) state of the isolated anthracene molecule has been accurately determined by this calculation. However, configurational-interaction with single excitations, time-dependent Hartree-Fock, and time-dependent density-function-theory methods did not yield satisfactory results for the excitation energy of the S(1) (1)B(2u) state. Symmetry-adapted-cluster configuration-interaction calculation was sufficiently good for the excitation energy and rotational constants.

Electron-electron and electron-nucleus correlation effects on exponent values of Gaussian-type functions for quantum protons and deuterons

Electron-electron and electron-nucleus correlation effects on exponent (alpha) values of Gaussian-type functions (GTFs) for quantum protons and deuterons in BH3, CH4, NH3, H2O, and HF molecular systems and their deuterated counterparts were analyzed using the second-order Moller-Plesset (MP2) level of theory of the multicomponent molecular orbital (MCMO-MP2) method. This method can simultaneously determine both nuclear and electronic wave functions. Results showed that the average alpha value (alpha(ave)) of the optimized alpha in single s-type ([1s]) GTF for a proton and a deuteron is similar to that determined using the Hartree-Fock level of the MCMO (MCMO-HF) method. In contrast, due to the electron-nucleus correlation effect, the s- and p-type ([1s1p]) GTFs are delocalized compared with those determined using the MCMO-HF method. For the H-bonded complexes, differences in the interaction energy induced by the H/D isotope effect were clearly evident because the D...Y bond distance for D complex is longer than the H...Y for H complex. Also, the basis set superposition error for the interaction energy in every H complex was similar to that in every D complex. The results here clearly demonstrate that the protonic and deuteronic basis functions based on alpha(ave) values for correlation effects can be applied to the detailed analysis of the quantum effects of protons and the H/D isotope effect in widespread fields that involve H bonds and weak interactions, such as the function of biological molecules, chemical reaction processes, and the design of new materials.

Simultaneous analytical optimization of variational parameters in Gaussian-type functions with full configuration interaction of multicomponent molecular orbital method by elimination of translational and rotational motions: Application to isotopomers of the hydrogen molecule

We have extended the multicomponent molecular orbital (MCMO) method to the full-configuration interaction (full-CI) fully variational molecular orbital method by elimination of translational and rotational motion components from total Hamiltonian. In the MCMO scheme, the quantum effects of protons and deuterons as well as electrons can be directly taken into account. All variational parameters in the full-CI scheme, i.e., exponents and centers (alpha and R) in the Gaussian-type function (GTF) basis set as well as the CI coefficients, are simultaneously optimized by using their analytical gradients. The total energy of the H(2) molecule calculated using the electronic [6s3p2d1f] and nuclear [1s1p1d1f] GTFs is -1.161 726 hartree, which can be compared to the energy of -1.164 025 hartree reported using a 512 term-explicitly correlated GTF calculation. Although the d- and f-type nuclear GTFs contribute to the improvement of energy convergence, the convergence of electron-nucleus correlation energy is slower than that of electron-electron one. The nuclear wave functions are delocalized due to the electron-nucleus correlation effect compared to the result of Hartree-Fock level of MCMO method. In addition, the average internuclear distances of all diatomic molecules are within 0.001 A of the previously reported experimental results. The dipole moment of the HD molecule estimated by our method is 8.4 x 10(-4) D, which is in excellent agreement with the experimental result of (8-10) x 10(-4) D.

A fragment molecular-orbital–multicomponent molecular-orbital method for analyzing H∕D isotope effects in large molecules

We have developed a fragment molecular orbital (FMO)-multi-component MO (MC_MO) method to analyze isotope effect due to differences between the quantum effects of protons and deuterons for large molecules such as proteins and DNA. The FMO-MC_MO method enables the determination of both the electronic and the protonic (deuteronic) wave functions simultaneously, and can directly express isotope effects, including coupling effects between nuclei and electrons. In our calculations of two polyglycines, which serve as prototypes for biological molecules, by this method, we clearly observed the geometrical relaxation induced by the HD isotope effect in the intramolecular hydrogen bonding portions of the molecules. The HD isotope effect on the interfragment interaction energy, including that of the hydrogen bonding parts, was also demonstrated: the hydrogen bond was weakened by replacement of hydrogen with deuterium. We also developed electrostatic potential approximations for use in the FMO-MC_MO calculations, and the accuracy of the energy differences induced by the isotope effect was independent of the approximation level of the FMO-MC_MO. Our results confirmed that the FMO-MC_MO method is a powerful tool for the detailed analysis of changes in hydrogen bonding and interaction energies induced by the HD isotope effect for large biological molecules.

A Review of Molecular-Level Mechanism of Membrane Degradation in the Polymer Electrolyte Fuel Cell

Chemical degradation of perfluorosulfonic acid (PFSA) membrane is one of the most serious problems for stable and long-term operations of the polymer electrolyte fuel cell (PEFC). The chemical degradation is caused by the chemical reaction between the PFSA membrane and chemical species such as free radicals. Although chemical degradation of the PFSA membrane has been studied by various experimental techniques, the mechanism of chemical degradation relies much on speculations from ex-situ observations. Recent activities applying theoretical methods such as density functional theory, in situ experimental observation, and mechanistic study by using simplified model compound systems have led to gradual clarification of the atomistic details of the chemical degradation mechanism. In this review paper, we summarize recent reports on the chemical degradation mechanism of the PFSA membrane from an atomistic point of view.

Analysis of isotope effect of hydrogen-absorbing Pd ultra-fine particle by X-ray powder diffraction and first principle multi-component MO calculation

Abstract The isotope effect for the geometrical and electronical relaxations of the hydrogen/deuterium-absorbing ultra-fine particles of Pd has been investigated using an X-ray powder diffraction, which shows that the bond distances of Pd n H are longer about 0.005 A than those of Pd n D. Also, the first principle multi-component molecular orbital (MC_MO) calculation, which takes account of the quantum effect of proton/deuteron, has been employed for the optimization of Pd n H − and Pd n D − ( n =4,6). The H/D isotope effect of MC_MO calculation is good agreement with those of the X-ray powder diffraction and shows a little relaxation of the electronic charge densities.

The valence band structure of AgxRh1–x alloy nanoparticles

The valence band (VB) structures of face-centered-cubic Ag-Rh alloy nanoparticles (NPs), which are known to have excellent hydrogen-storage properties, were investigated using bulk-sensitive hard x-ray photoelectron spectroscopy. The observed VB spectra profiles of the Ag-Rh alloy NPs do not resemble simple linear combinations of the VB spectra of Ag and Rh NPs. The observed VB hybridization was qualitatively reproduced via a first-principles calculation. The electronic structure of the Ag0.5Rh0.5 alloy NPs near the Fermi edge was strikingly similar to that of Pd NPs, whose superior hydrogen-storage properties are well known.

Chemical Degradation Mechanism of Model Compound, CF3 ( CF2 ) 3O ( CF2 ) 2OCF2SO3H , of PFSA Polymer by Attack of Hydroxyl Radical in PEMFCs

To enhance the durability of perfluorosulfonic acid (PFSA) polymer for proton-exchange membrane fuel cells (PEMFCs), we theoretically analyzed the degradation mechanism of PFSA by the attack of a hydroxyl (OH) radical. We used CF 3 (CF 2 ) 3 O(CF 2 ) 2 OCF 2 SO 3 H as a model compound representing the PFSA side chain because the experimental result suggested that the ether group in the PFSA side chain is vulnerable to the OH radical attack. We performed density functional theory calculation to discuss the degradation reaction mechanism of the ether group in the model compound of the side chain and OH radical. Under high humidity condition, we clearly demonstrated the degradation mechanism and reactivity of C-0 bond cleavage in the ether group by the OH radical. This result shows reasonable agreement with the experimental one. However, the OH radical prefers the reaction of the sulfonic acid group to the ether group under the low humidity condition. We found the different reactivity of the OH radical under the low and high humidity conditions. To improve the durability of PFSA, we proposed four directions: (i) enhancement of deprotonation, (ii) protection of ether group by steric hindrance, (iii) enhancement of C-O bond strength, and (iv) substitution of the ether group by other chemical groups. The latter two directions have been theoretically explored more in detail.

Isotope Effect in Hydrogen/Deuterium-absorbing Pd Nanoparticles Revealed by X-ray Powder Diffraction and by a Multi-component MO Method

The isotope effect in Pd nanoparticles that absorb hydrogen or deuterium (i.e., H/D isotope effect) was studied experimentally and theoretically. First, the geometries (i.e., lattice parameters) of such Pd nanoparticles exposed to hydrogen or deuterium gas were measured by using X-ray powder diffraction to determine the lattice parameters. Then, the geometrical and electronic relaxations of Pd n H - and Pd n D - ( n =1–6) clusters, which modeled Pd nanoparticles exposed to hydrogen or deuterium gas, were calculated by using a multi-component molecular orbital (MC_MO) method, which uses first principles of quantum mechanics to account for the quantum effect of proton/deuteron. Experimental results from the diffraction patterns show that the bond distances of Pd nanoparticles exposed to hydrogen gas (and thus had absorbed hydrogen) were about 0.005 A longer than those of exposed to deuterium gas. These results were confirmed by analytical results from the MC_MO calculation for Pd n H - and Pd n D - clusters...