Rika Sekine

Validity and Limitations of the Bridged Annulene Model for Porphyrins

According to the bridged annulene model, macrocyclic aromaticity of a porphyrinoid species can be attributed to the annulene-like main macrocyclic conjugation pathway (MMCP). Macrocyclic aromaticity, however, is given theoretically as a sum of contributions from all macrocyclic circuits. We found that the aromaticity due to each macrocyclic circuit is determined formally but broadly by Hückels [4n + 2] rule of aromaticity. Nitrogen atoms in the pyrrolic rings effectively suppress the variation in the number of π electrons staying along each macrocyclic circuit. As a result, all or most macrocyclic circuits in oligopyrrolic macrocycles are made aromatic (or antiaromaitc) in phase with the MMCP. Thus, the MMCP is not a determinant of macrocyclic aromaticity but can be regarded as a good indicator of this quantity. This is why the bridged annulene model appears to hold for many porphyrins.

The origin of global and macrocyclic aromaticity in porphyrinoids

The global and macrocyclic aromaticity of porphyrinoids was characterized using our graph theory of aromaticity. The sequential line plots of topological resonance energy (TRE) against the number of π-electrons (N(π)) for different porphyrinoids are similar with four major extrema to those for five-membered heterocycles. This supports the view that five-membered rings are the main origin of global aromaticity in porphyrinoids. Macrocyclic circuits contribute significantly to macrocyclic π-circulation but modestly to global aromaticity. Macrocyclic aromaticity/antiaromaticity in oligopyrrolic macrocycles can be predicted by formally applying Hückels [4n + 2] rule to an annulene-like main macrocyclic conjugation pathway (MMCP). This bridged annulene model can be justified by examining the contribution of individual macrocyclic circuits to macrocyclic aromaticity. A Hückel-like rule of macrocyclic aromaticity was found for porphyrinoid species.

Atomic layer control in Sr‐Cu‐O artificial lattice growth

Atomic layer growth control of the infinite‐layer compound, SrCuO2, based artificial lattices is demonstrated on SrTiO3 (100). Two different patterns have been observed by reflection high‐energy electron diffraction during the alternating supply of strontium and copper metal under the flow of NO2. The intensity of the incommensurate streak increases with an increasing supply of Cu and decreases to zero with an increasing supply of Sr. Precise control of atomic layer growth is successfully carried out by in situ intensity monitoring of the incommensurate streak.

Aromatic character of polycyclic π systems formed by fusion of two or more rings of the same size.

The sequential line plot of topological resonance energy (TRE) against the number of π electrons (N(π)) for any polycyclic aromatic hydrocarbon (PAH) is very similar with the same number of extrema to that for benzene. Thus, global aromaticity of a PAH molecular ion strongly reflects that of a benzene molecular ion. Likewise, the N(π) dependence of TRE for any polycyclic π system formed by fusion of two or more rings of the same size reflects that for a monocyclic species of the same ring size. In general, TREs for such polycyclic π systems and their molecular ions can be interpreted consistently by reference to those for neutral and charged monocyclic species of the same ring size.

Electronic and Magnetic Characteristics of Polycyclic Aromatic Hydrocarbons with Factorizable Kekulé Structure Counts

The Kekulé structure count (K) for some types of polycyclic aromatic hydrocarbons (PAHs), such as fluoranthene and perylene, can be factorized into the product of those for two or more aromatic subunits. The ring-current map for these PAHs placed in a perpendicular magnetic field exhibits a substantial localization on aromatic subunits. We found that such localization of π circulation is a characteristic of fairly small K-factorizable species in the neutral electronic state. Even in such a case, no single π molecular orbital (πMO) is associated with localized π circulation. Apparent localization of π circulation is caused by the superposition of currents induced by all occupied πMOs. π circulation is less localized in larger K-factorizable species.

Geometric and Electronic Structures of Tc and Mn Clusters by Density Functional Calculations

The geometric structures of Tc2-Tc5 and Mn2-Mn8 clusters were optimized by the Amsterdam density functional method. The trimeric, tetrameric, and pentameric Tc clusters exhibit the equilibrium structures of isosceles triangle, tetrahedron, and square pyramid, respectively. The structures of the Mn clusters up to Mn4 are similar to those of the respective Tc clusters, while the Mn5, Mn6, Mn7, and Mn8 clusters have the distorted trigonal bipyramidal, distorted octahedral, pentagonal bipyramidal, and C2v structures, respectively. The Tc clusters are paramagnetic, while the Mn clusters are basically ferromagnetic.

Valence electronic structure of uranyl nitrate UO2(NO3)2·2H2O

Abstract Experimental and theoretical studies on the electronic structure of a uranyl nitrate hydrate, UO 2 (NO 3 ) 2 ·2H 2 O, have been performed by X-ray photoelectron spectroscopy (XPS) and with relativistic DV-Xα molecular orbital methods. The XPS spectra are measured within five minutes of X-ray irradiation, which causes negligible damage to the sample. Taking into consideration the calculated results, each peak of the experimental spectrum is assigned. The theoretical spectrum of the uranyl nitrate is in good agreement with the present experimental spectrum.

Atomic-number dependence of relativistic effects on chemical bonding using the non-relativistic and relativistic discrete-variational Xα methods

Abstract We have performed molecular orbital calculations to study the relativistic effects on chemical bonding in hexafluoride molecules such as SF6, SeF6, MoF6, TeF6, WF6, and UF6, using the nonrelativistic and relativistic discrete-variational Xα methods. The atomic-number dependence of the relativistic effects was examined by means of bond overlap population analysis. It is found that the relativistic effects become remarkably important for the molecules containing the elements with atomic number larger than 50.

Transport Properties of SrCuO2 Thin Films with Artificially Modulated Structures

Transport properties of SrCuO2 thin films with and without the artificial modulated structures formed by periodical incorporation of extra Sr layers as (SrCuO2)10Sr by MBE are presented. The resistivity of the film is found to decrease by one order of magnitude when they are post-oxidized in situ. For the films with an extra Sr layer (double Sr-layer formation) for every ten units of SrCuO2 cells, an anomaly in resistivity is observed around 100 K which coincides with the reported Tc of the high-Tc superconductor of infinite layers.

Discrete-variational Dirac–Slater calculation on valence band XPS for UC

We have performed relativistic molecular orbital calculations for the valence band structure of uranium monocarbide (UC), using the discrete-variational Dirac–Slater (DV–DS) method. We examine three kinds of model clusters (UC6, CU6, CU6C18) for the bulk electronic structure of UC. The calculated valence band spectrum (CU6C18) agrees with the experimental X-ray spectrum at peak position and intensities as well. The peaks in the experimental spectrum are assigned to the corresponding U and C orbital components.