Tetsuji Iyama

Density Functional Theory Study of Boron- and Nitrogen-Atom-Doped Graphene Chips

The structures and electronic states of boron- and nitrogen-substituted graphene chips (B-, N-, and BN-doped graphene chips) have been investigated by means of the density functional theory (DFT) method in order to shed light on the mechanism of change in the electronic properties of graphene chips caused by heteroatoms. The atomic charge of nitrogen atoms in N-graphene was a negative value, whereas that of boron atoms in B-graphene was positive. In the case of the BN-doped graphene chip, a charge polarization such as Bδ+–Nδ- was found. It was also found that the B–N bond pair is preferentially formed because of the large heat of formation of the B–N bond. The BN-doped graphene chips showed a large red shift of the band gap compared with that of normal graphene. The electric states of BN-graphenes were discussed on the basis of theoretical results.

Electron detachment dynamics of the microsolvated benzophenone radical anion: A full dimensional direct ab-initio trajectory approach

Electron detachment dynamics of the benzophenone anion–water complex Bp−(H2O) have been investigated by means of full dimensional direct ab-initio trajectory calculation. The structural relaxation process of the neutral complex Bp(H2O) following the vertical electron detachment of Bp−(H2O), which is expressed by [Bp(H2O)]ver → [Bp(H2O)]solv, was calculated by the direct ab-initio trajectory calculations, where [Bp(H2O)]ver and [Bp(H2O)]solv mean the Bp(H2O) neutral complex having a structure at vertical electron detachment point from the Bp−(H2O) anion complex and a relaxation structure of Bp(H2O), respectively. From the dynamics calculations, it was found that the solvation structure of Bp(H2O) is drastically changed to [Bp(H2O)]solv after the electron detachment of Bp−(H2O). According to the structural relaxation including the solvent re-orientation around the benzophenone, the excitation energies for both nπ* and ππ* transitions were blue-shifted as a function of time. The mechanism of the electron detachment process was discussed on the basis of theoretical results.

Interaction of Hydroxyl OH Radical with Graphene Surface: A Density Functional Theory Study

The interaction of a hydroxyl OH radical with a graphene surface has been investigated by the density functional theory (DFT) method in order to elucidate the radical scavenge mechanism of the graphene surface. The DFT calculation showed that the OH radical binds directly to the carbon atom of the graphene surface and a strong C–O bond is formed. The binding energies were dependent on the cluster size and were distributed in the 4.1–9.5 kcal/mol range at the B3LYP/6-31G(d) level of theory. The potential energy curve plotted as a function of the distance of OH from the surface carbon showed that the OH radical can bind to the carbon atom with a low activation barrier: the barrier heights for n = 7 and 14 were calculated to be 3.9 and 1.9 kcal/mol, respectively. Also, it was found that the structural change from sp2 to sp3-like hybridization occurs by the approach of the OH radical.

Density Functional Theory Method for Study of the Mechanism of C–H Bond Formation on Finite-Sized Graphene Surface

The structures and electronic states of hydrogenated graphenes with a finite size have been investigated by a density functional theory (DFT) method. Five graphenes of various sizes (n=2, 4, 7, 14, and 19, where n indicates the number of benzene rings in graphene) were examined as models of a hydrogenated graphene system. The harmonic vibrational frequency corresponding to a C–H stretching mode showed a linear relationship between the frequency and the C–H bond length. The C–H bond formed by the addition of hydrogen atoms was completely polarized as Cδ-–Hδ+ in an equilibrium structure. It was found that the activation barrier is formed by hybridization from sp2 to sp3 in the transition region. The mechanism of C–H bond formation on a graphene surface was discussed on the basis of theoretical results.

Density Functional Theory and Direct Molecular Dynamics Study of the Hydrogen Atom on a Finite-Sized Graphene

Geometrical and electronic structures of a hydrogen-added graphene have been investigated by means of density functional theory (DFT) and direct molecular dynamics (MD) methods. A graphene composed of 19 benzene rings was examined as a model of finite-sized graphene. The hyperfine coupling constants (hfccs) of hydrogen atoms on graphene were calculated to be ~63 G in the bulk region and 20–5 G in the edge region of graphene, indicating that the hfcc of hydrogen atoms is strongly dependent on the bonding site. The excitation energies of hydrogen added graphene were calculated to be 1.28, 1.54, and 1.60 eV, which were lower than that of graphene without hydrogen atoms (2.24 eV). The first electronic transition was assigned to an excess electron transfer band from a defect hydrogen site to the bulk region of graphene. The electronic states and thermal behavior of hydrogen atoms interacting with graphene are discussed on the basis of theoretical results.

Effects of Fluorine Atom Substitution of Graphene Edge Site on the Diffusion of Lithium Ion

Direct molecular orbital–molecular dynamics (MO–MD) method has been applied to diffusion processes of the Li+ ion on a modified graphene surface. A graphene sheet composed of carbon, fluorine and hydrogen atoms were used as a model graphene (Li+C150H3F27, denoted by FH-graphene). The edges of graphene were terminated by fluorine (F-edge region) and hydrogen atom (H-edge region). Simulation temperatures were chosen in the range 250–350 K. It was found that the lithium ion diffuses freely on the surface, but the ion does not approach the F-edge region of the surface. This is due to the repulsive interaction with a positive charged carbon atom where C–F bond is polarized as Cδ+–Fδ-. On the other hand, the Li+ ion approached to the H-edge region and it exited from the H-edge region. The diffusion mechanism of Li+ was discussed on the basis of theoretical results.

Maximum capacity of the hydrogen storage in water clusters

Hydrogen molecules inserted into water clusters have been investigated by means of both density functional theory and ab initio calculations in order to determine a limit to the hydrogen storage capacity of water clusters. Three water clusters, (H2O) m (m = 20, 24 and 28), were examined as the water cages, while the guest hydrogen molecules up to nine were tested in the calculations. The maximum capacities of hydrogen storage for m = 20, 24 and 28 were determined to be n = 3, 6 and 8, respectively, at the MP2/6-311G(d, p)//B3LYP/6-311G(d, p) level of theory. In order to elucidate thermal behaviour of the hydrogen hydrate, direct molecular orbital–molecular dynamics calculations were carried out for (H2) n (H2O) m (n = 6 and 8, m = 28). It was found that the hydrogen molecule escapes from the hexagonal site without large deformation to the water lattice. The electronic states of the hydrogen molecules in the water clusters were discussed on the basis of theoretical results.

Density Functional Theory (DFT) Study on the Addition of Hydroxyl Radical (OH) to C20

The radical addition to the smallest fullerene C20 has been investigated by means of density functional theory (DFT) method in order to elucidate the radical scavenge mechanism of fullerene. The OH radical was examined as an organic radical because the radical has a high reactivity. The DFT calculation showed that the OH radical binds directly to the carbon atom of C20 and a strong C-O bond is formed. The binding energies of the first addition of OH radical were calculated to be 85.2 kcal/mol at the B3LYP/6-311G(d,p) level of theory. In the second radical addition, the binding energy of OH to C20(OH) was 91.5 kcal/mol. The unpaired electron was distributed widely over the C20 surface in the C20(OH) complex.

Effects of Point Charges on the Excitation Energies of Protonated Schiff Base of Retinal

Effects of point charges on the excitation energies of protonated Schiff base of retinal (PSBR) in gas phase have been investigated by means of time-dependent density functional theory (TD-DFT) method in order to shed light on the electrostatic effects on the absorption spectra of PSBR. The positive and negative charges around PSBR were tested for 11-cis of PSBR. The TD-DFT calculations indicated that the first and second excitation energies are blue-shifted by the existence of the negative charge around the N-H site of PSBR, whereas those are red-shifted by positive charge. On the other hand, the spectral feature in β -ionone ring was much different from that of the N-H site: the first and second excitation energies are red-shifted by the negative charge. The spectral feature is drastically changed at the polyene part of PSBR. It was found that the excitation energies of PSBR are affected strongly by the electrostatic environment. The effects of electrostatic interaction on the absorption spectra of PSBR are discussed on the basis of theoretical results.

Molecular Design of High Performance Molecular Devices Based on Direct Ab-initio Molecular Dynamics Method: Diffusion of Lithium Ion on Fluorinated Amorphous Carbon

Hybrid density functional theory (DFT) calculations have been carried out for the lithium adsorbed on a fluorinated graphene surface (F-graphene, C96F24) to elucidate the effect of fluorination of amorphous carbon on the diffusion mechanism of lithium ion. Also, direct molecular orbital–molecular dynamics (MO–MD) calculation [H. Tachikawa and A. Shimizu: J. Phys. Chem. B 109 (2005) 13255] was applied to diffusion processes of the Li+ ion on F-graphene. The B3LYP/LANL2MB calculation showed that the Li+ ion is most stabilized around central position of F-graphene, and the energy was gradually instabilized for the edge region. The direct MO–MD calculations showed that the Li+ ion diffuses on the bulk surface region of F-graphite at 300 K. The nature of the interaction between Li+ and F-graphene was discussed on the basis of theoretical results.