Yukiumi Kita

Ab initio quantum Monte Carlo study of the positronic hydrogen cyanide molecule

Quantum Monte Carlo methods are used to investigate the binding of a positron to the hydrogen cyanide (HCN) and lithium hydride (LiH) molecules. Our value of the adiabatic positron affinity (PA) of LiH of 1.010(3) eV is very close to the best theoretical value of 1.005 eV, obtained from variational calculations using explicitly correlated Gaussian basis sets [K. Strasburger, J. Chem. Phys. 114, 00615 (2001)]. We have obtained a reliable estimate of 0.0378(48) eV for the PA of the HCN molecule, which is almost 20 times larger than that obtained at the Hartree-Fock level, and strongly supports the binding of a positron in the electrostatic field of the HCN molecule. Our results show the importance of correlation effects for describing weakly bound positronic molecular complexes.

Ab initio quantum Monte Carlo study of the binding of a positron to alkali-metal hydrides

Quantum Monte Carlo methods are used to investigate the binding of a positron to the alkali-metal hydrides, XH (X = Na and K). We obtain positron affinities for the NaH and KH molecules of 1.422(10) eV and 2.051(39) eV, respectively. These are considerably larger than the previous results of 1.035 eV and 1.273 eV obtained from multireference single- and double-excitation configuration interaction calculations. Together with our previous results for [LiH;e(+)] [Y. Kita et al., J. Chem. Phys. 131, 134310 (2009)], our study confirms the strong correlation between the positron affinity and dipole moment of alkali-metal hydrides.

First-principles calculation and transmission electron microscopy observation for hydrogen adsorption on carbon nanowalls

Transmission electron microscopy (TEM) technique was used to investigate the structure of carbon nanowalls (CNWs). The TEM observation clearly indicated the existence of the bended graphene structure in boundary regions between the crystallites of CNWs. According to this TEM result, the first-principles calculation was employed for the bended coronene molecule as the model of the boundary region between the crystallites of CNWs, to elucidate the mechanism of the hydrogen adsorption to CNWs. The hydrogen adsorption energies onto both on-top and hollow sites become greater as the bending angle increases, because the electronic structure of the carbon atom at the adsorption site changes from sp2 to sp3 hybridization character by natural bond orbital analysis. Our computational result is reasonably consistent with the specific feature of hydrogen adsorption to CNWs, which had been reported in our previous work by ultraviolet photoelectron spectroscopy and temperature program desorption measurements by Kinoshi...

Formation of Schiff-base for photoreaction mechanism of red shift of GFP spectra

We have proposed the formation of Schiff-base between R96 and chromophore (CRO) to elucidate the reaction mechanism for the irreversible red shift of green fluorescent protein (GFP) spectra under the absence of oxygen. The difference between absorption energies of reactant and product for our GFP models with CIS(D)/6-31G* level is 0.21eV, which is in reasonable agreement with the corresponding experimental value of 0.25eV. We have suggested the irreversible photoreaction mechanism, where the CRO excited from ground (S(0)) state to first excited singlet (S(1)) state immediately turns to the first excited triplet (T(1)) state, and the nucleophilic addition reaction occurs on the T(1) state.

Binding of a Positron to Nucleic Base Molecules and Their Pairs

Hole attack: A theoretical one-electron oxidation of nucleic base molecules and their pairs by positron is proposed, based on the calculations for positron-attached neutral forms of species, adenine (A), thymine (T), guanine (G), cytosine (C), and their Watson-Crick base pairs (A-T and G-C). The results reveal that binding of a positron to neutral isolated nucleic base molecules is base-selective.

Theoretical study of the reversible photoconversion mechanism in Dronpa

We propose a new scheme of the photoactivation and the photobleaching for Dronpa with molecular dynamics method and density functional theory. These processes can be explained by considering cis-trans isomerization in neutral state of chromophore. The proton transfer from anionic to neutral chromophore makes cis-trans isomerization possible via hula-twist rotation process, since the space for cis-trans isomerization is opened at around the region near chromophore by moving out of the imidazole ring on H193 from the position below the phenol ring on chromophore. Then the cis-trans isomerization can occur through the hula-twist process. The contributions from the protein environment around CRO, especially S142 and H193, are indispensable for photoconversion of Dronpa.

AB INITIO CALCULATIONS OF INTERMOLECULAR INTERACTION POTENTIALS OF FULLERENE-FRAGMENTS SYSTEMS

We have performed the ab initio molecular orbital calculations for combinations of the fullerene-fragments as the models of the nonbonding interaction of C60 dimer at the preferred configurations in the simple cubic phase. The intermolecular interaction potentials have been calculated using several basis sets with MP2 level of the electron correlation energy and the basis set superposition error correction. The strong dispersion attractions that is dominant in the van der Waals interaction has been found for the combinations of the fullerene-fragments. The equilibrium intermolecular distances obtained are in good agreement with the corresponding experimental value. The repulsive region of the intermolecular interaction calculated by ab initio method is found to be express by an atom–atom interaction potential with an anisotropic exponential type repulsive term, which is less steep than the conventional Lennard–Jones type potential.

Ferromagnetic spin coupling in the chromium dimer cation: Measurements by photodissociation spectroscopy combined with coupled-cluster calculations

The magnetic coupling of the chromium dimer cation, Cr2 (+), has been an outstanding problem for decades. An optical absorption spectrum of Cr2 (+) has been obtained by photodissociation spectroscopy in the photon-energy range from 2.0 to 5.0 eV. Besides, calculations have been performed by the equation-of-motion coupled-cluster singles and doubles method for vertical excitation of the species. Their coincidence supports our assignment that the ground electronic state exhibits a ferromagnetic spin coupling, which is contrary to those of neutral and negatively charged dimers, Cr2 and Cr2 (-), in their lowest spin states.

Quantum Chemical Investigation of the Doppler Broadening of Positron Annihilation Radiation Spectra in Polymers

In order to elucidate the element specificity of the S parameters of polymers containing C, O, or F atoms reported by Sato et al. with the Doppler broadening of positron annihilation radiation measurement [K. Sato, et al., Phys. Rev. B 71, 012201, (2005).], we theoretically analyzed the molecular orbitals of small size of the polymer molecules with an ab initio molecular orbital method. With a linear regression analysis, we found a strong correlation between the average kinetic energies of the valence electrons and the contribution of a free positron to the experimental S parameters for the C-, O-, and F-groups. As with a consideration of the kinetic energies of the valence electrons and the electronegativity of the C, O, and F atoms, we conclude that the element specificity of the experimental S parameters for each of the C-, O-, and F-groups is roughly explained by a difference in the electronegativity of the C, O, and F atoms.

Ab Initio Investigations of Stable Geometries of the Atmospheric Negative Ion NO 3 − (HNO 3 ) 2 and Its Monohydrate

The possible stable geometries of the atmospheric negative core ion \( {\text{NO}}_{3}^{ - }\left({{\text{HNO}}_{3} } \right)_{2} \) and its monohydrate were theoretically investigated with the second order Moller-Plesset perturbation theory (MP2) in consideration of the effect of electron correlation. For both ionic clusters, we obtained the different stable geometries from the previous study by Drenck and coworkers (Int J Mass Spectrom 273:126–131, 2008) [1] with the density functional theory of Becke 3-parameters hybrid functional (B3LYP). The non-planar geometry with two hydrogen-bondings between one oxygen atom on \( {\text{NO}}_{3}^{ - } \) and each hydrogen atom of two HNO3 fragments is found as the most stable structure of the core ion at 0 K. For the monohydrate, the most stable geometry at 0 K is found as the H 2 O-embedded form in which one water molecule is located at the center of the cluster with hydrogen-bondings to \( {\text{NO}}_{3}^{ - } \) and HNO3 fragments. Our results show that the hydrogen bond network of the core ion can be strongly perturbed by a single water molecule. We also discussed the relative abundance of conformers of these ionic clusters under a finite temperature.