Hiroto Tachikawa

The chemical bonding state of nitrogen in kapton-derived carbon film and its effect on the graphitization process

Abstract The chemical bonding state of the nitrogen which remains in the carbon films derived from a polyimide Kapton film was studied by X-ray photoelectron spectroscope and related to the changes in structure and transport properties. The nitrogen remaining after heat treatment up to 2200 °C was in a tertiary bonding state, replacing carbon atoms in hexagonal layers, which was supported by theoretical calculation based on ab-initio molecular orbital theory. The departure of substituted nitrogen atoms from hexagonal layers showed a good correspondence with structure development and changes in transport properties, the presence of nitrogen depressing the developments in structure and transport properties. After the departure of nitrogen above 2300 °C graphitization of the films was very remarkable.

Ab Initio Molecular Dynamics Study on the Electron Capture Processes of Protonated Methane (CH5

Electron capture dynamics of protonated methane (CH5(+)) have been investigated by means of a direct ab initio molecular dynamics (MD) method. First, the ground and two low-lying state structures of CH5 (+) with eclipsed Cs , staggered Cs and C2v symmetries were examined as initial geometries in the dynamics calculation. Next, the initial structures of CH5 (+) in the Franck-Condon (FC) region were generated by inclusion of zero point energy and then trajectories were run from the selected points on the assumption of vertical electron capture. Two competing reaction channels were observed: CH5 (+) + e (-)--> CH4 + H (I) and CH5 (+) + e (-) --> CH3 + H2 (II). Channel II occurred only from structures very close to the s- Cs geometry for which two protons with longer C-H distances are electronically equivalent in CH5 (+). These protons have the highest spin density as hydrogen atoms following vertical electron capture of CH5 (+) and are lost as H2. On the other hand, channel I was formed from a wide structural region of CH5 (+). The mechanism of the electron capture dynamics of CH5 is discussed on the basis of the theoretical results.

A DIRECT AB-INITIO DYNAMICS STUDY ON A GAS PHASE SN2 REACTION F-+ CH3CL CH3F + CL- : DYNAMICS OF NEAR-COLLINEAR COLLISION

Abstract Direct ab-initio dynamics calculations have been applied to a gas phase SN2 reaction F−+CH3Cl→CH3F+Cl−. An ab-initio potential energy surface including all degrees of freedom was used. Total energies and gradients were calculated at each time step. Results for near-collinear collision are reported. The initial geometrical configurations of (F-+CH3Cl) at time zero were randomly generated in the range 180±3° for the collision angle ∠F–C–Cl and of r(F−–C)=6.0–6.5 A. The vibrational phase of CH3Cl was generated to take a temperature of 10 K. It was found that almost all available energy is partitioned into the relative translational mode between the products (∼43%) and the C–F stretching mode (∼57%) at zero collision energy. The other internal modes of CH3F remain in the ground state. The lifetimes of early- and late-complexes F−⋯CH3Cl and FCH3⋯Cl− were short enough to dissociate directly to products, while the energy was not completely distributed within the lifetime. It is concluded that the SN2 reaction F−+CH3Cl proceeds non-statistically via a direct mechanism in the case of near-collinear collision.

Hydrogen atom formation from the photodissociation of water ice at 193 nm

The TOF spectra of photofragment hydrogen atoms from the 193 nm photodissociation of amorphous ice at 90-140 K have been measured. The spectra consist of both a fast and a slow components that are characterized by average translational energies of 2k(B)T(trans)=0.39+/-0.04 eV (2300+/-200 K) and 0.02 eV (120+/-20 K), respectively. The incident laser power dependency of the hydrogen atom production suggests one-photon process. The electronic excitation energy of a branched cluster, (H(2)O)(6+1), has been theoretically calculated, where (H(2)O)(6+1) is a (H(2)O)(6) cyclic cluster attached by a water molecule with the hydrogen bond. The photoabsorption of this branched cluster is expected to appear at around 200 nm. The source of the hydrogen atoms is attributed to the photodissociation of the ice surface that is attached by water molecules with the hydrogen bond. Atmospheric implications are estimated for the photodissociation of the ice particles (Noctilucent clouds) at 190-230 nm in the region between 80 and 85 km altitude.

Direct ab initio molecular dynamics study on a microsolvated SN2 reaction of OH-(H2O) with CH3Cl.

Reaction dynamics for a microsolvated SN2 reaction OH-(H2O)+CH3Cl have been investigated by means of the direct ab initio molecular dynamics method. The relative center-of-mass collision energies were chosen as 10, 15, and 25 kcal/mol. Three reaction channels were found as products. These are (1) a channel leading to complete dissociation (the products are CH3OH+Cl- +H2O: denoted by channel I), (2) a solvation channel (the products are Cl-(H2O)+CH3OH: channel II), and (3) a complex formation channel (the products are CH3OH...H2O+Cl-: channel III). The branching ratios for the three channels were drastically changed as a function of center-of-mass collision energy. The ratio of complete dissociation channel (channel I) increased with increasing collision energy, whereas that of channel III decreased. The solvation channel (channel II) was minor at all collision energies. The selectivity of the reaction channels and the mechanism are discussed on the basis of the theoretical results.

Electron hydration dynamics in water clusters: A direct ab initio molecular dynamics approach

Electron attachment dynamics of excess electron in water cluster (H2O)n (n = 2 and 3) have been investigated by means of full-dimensional direct ab initio molecular dynamics (MD) method at the MP26-311++G(d,p) level. It was found that the hydrogen bond breaking due to the excess electron is an important process in the first stage of electron capture in water trimer. Time scale of electron localization and hydrogen bond breaking were determined by the direct ab initio MD simulation. The initial process of hydration in water cluster is clearly visualized in the present study. In n = 3, an excess electron is first trapped around the cyclic water trimer with a triangular form, where the excess electron is equivalently distributed on the three water molecules at time zero. After 50 fs, the excess electron is concentrated into two water molecules, while the potential energy of the system decreases by -1.5 kcal/mol from the vertical point. After 100 fs, the excess electron is localized in one of the water molecules and the potential energy decreases by -5.3 kcal/mol, but the triangular form still remained. After that, one of the hydrogen bonds in the triangular form is gradually broken by the excess electron, while the structure becomes linear at 100-300 fs after electron capture. The time scale of hydrogen bond breaking due to the excess electron is calculated to be about 300 fs. Finally, a dipole bound state is formed by the linear form of three water molecules. In the case of n = 2, the dipole bound anion is formed directly. The mechanism of electron hydration dynamics was discussed on the basis of theoretical results.

A DFT study on the structures and electronic states of zinc cluster Znn (n = 2-32)

Ab-initio and density functional theory (DFT) calculations have been carried out for zinc clusters Znn (n = 2–32, n is the number of atoms to form a cluster) to elucidate the structure and electronic charge states of the clusters and the mechanism of clustering. The binding energies of Zn atoms were negligibly small at n = 2–3, whereas the energy increased significantly at n = 4 (the first transition). The second transition occurred at n = 8–16. In the larger clusters (n = 16–32), the binding energy increased slightly with increasing cluster size (n). The cluster size dependence of the binding energy and bond length between zinc atoms agreed well with that of the natural population of electrons in the 4p orbital of the zinc atom. In the larger clusters (n > 20), it was found that the zinc atoms in the surface region of the cluster have a positive charge, whereas those in the interior region have a negative charge with a large population in the 4p orbital. The formation mechanism of zinc clusters was discussed on the basis of the theoretical results.

Reaction dynamics following electron capture of chlorofluorocarbon adsorbed on water cluster: A direct density functional theory molecular dynamics study

The electron capture dynamics of halocarbon and its water complex have been investigated by means of the full dimensional direct density functional theory molecular dynamics method in order to shed light on the mechanism of electron capture of a halocarbon adsorbed on the ice surface. The CF(2)Cl(2) molecule and a cyclic water trimer (H(2)O)(3) were used as halocarbon and water cluster, respectively. The dynamics calculation of CF(2)Cl(2) showed that both C-Cl bonds are largely elongated after the electron capture, while one of the Cl atoms is dissociated from CF(2)Cl(2) (-) as a Cl(-) ion. Almost all total available energy was transferred into the internal modes of the parent CF(2)Cl radical on the product state, while the relative translational energy of Cl(-) was significantly low due to the elongation of two C-Cl bonds. In the case of a halocarbon-water cluster system, the geometry optimization of neutral complex CF(2)Cl(2)(H(2)O)(3) showed that one of the Cl atoms interacts with n orbital of water molecules of trimer and the other Cl atom existed as a dangling Cl atom. After the electron capture, only one C-Cl bond (dangling Cl atom) was rapidly elongated, whereas the other C-Cl bond is silent during the reaction. The dangling Cl atom was directly dissociated from CF(2)Cl(2) (-)(H(2)O)(3) as Cl(-). The fast Cl(-) ion was generated from CF(2)Cl(2) (-)(H(2)O)(3) on the water cluster. The mechanism of the electron capture of halocarbon on water ice was discussed on the basis of the theoretical results.

A theoretical study on the vibrationally state-selected hydrogen transfer reaction: NH+3 (ν) + NH3 → NH+4 + NH2. An ab initio MR-SD-CI and classical trajectory approach

Abstract Ab initio MR-SD-CI and classical trajectory calculations have been performed to elucidate the vibrational mode specificity of the title reaction, whose reactive cross section is enhanced by vibrational excitation of the ν 2 umbrella-bending mode of NH + 3 . Potential energy surfaces (PESs) of the reaction have been obtained for vibrationally groun and excited states (vibrational quantum numbers, ν = 0 and 2, respectively) by assuming a hydrogen-bonded structure with fixed bending angles. The MO calculations show that a hydrogen transfer is composed of two elementary steps: (1) an electron transfer from NH 3 to NH + 3 at the avoided crossing region on the entrance PES, and (2) a proton transfer in the (NH 3 · NH 3 ) + intermediate complex region. The PESs show that the avoided crossing point shifts to larger intermolecular separation due to vibrational excitation. Using the ab initio fitted PESs, the classical trajectory calculations elucidate the reaction dynamics. The maximum value of the impact parameter ( b max ) for the reaction is increased by the vibrational excitation. Based on these theoretical results, a simple reaction model has been proposed, in which the electron capturing volume of NH + 3 increases with increasing vibrational quantum number ν.

ELECTRONIC TO VIBRATIONAL AND ROTATIONAL ENERGY TRANSFER IN S(1D) + CO QUENCHING REACTION : AB INITIO MO AND SURFACE HOPPING TRAJECTORY STUDIES

The collisional energy transfer reaction, S(1D)+CO→S(3P)+CO(v,J), has been studied by means of the ab initio MO (molecular orbital) and surface-hopping trajectory calculations. The potential-energy surfaces (PESs) calculated by the ab initio MO calculations showed that the singlet and triplet PESs [S(1D)+CO and S(3P)+CO] are composed of the attractive and repulsive potential curves, respectively. The strongly bond intermediate corresponding to SCO(1∑) is found on the collision region on the singlet PES. The singlet and triplet PESs are crossed each other at region of around r(S−C)=2.4 A with the seam of the conical intersection. By using the fitted ab initio PESs, three-dimensional surface-hopping trajectory calculations are performed under the Landau–Zener approximation. The calculated rotational and vibrational state distributions of the product CO(v,J) are composed of two components due to the contributions from both direct and complex channels. The calculations show that the quenching probability decr...