Akira Imamura

Calculations of the excitation energies of all-trans and 11,12s-dicis retinals using localized molecular orbitals obtained by the elongation method

In the present article, the excitation energies of the all-trans and the 11,12s-dicis retinals were calculated by using the elongation method. The geometries of these molecules were optimized with the 4-31G basis set by using the GAUSSIAN 92 program. The wave functions for the calculation of the excitation energies were obtained with CNDO/S approximation by the elongation method, which enables us to analyze electronic structures of aperiodic polymers in terms of the exciton-type local excitation and the charge transfer-type excitation. The excitation energies were calculated by using the single excitation configuration interaction (SECI) on the basis of localized molecular orbitals (LMOs). The LMOs were obtained in the process of the elongation method. The configuration interaction (CI) matrices were diagonalized by Davidson’s method. The calculated results were in good agreement with the experimental data for absorption spectra. In order to consider the isomerization path from 11,12s-dicis to all-trans r...

A general quantum chemical approach to study the locally perturbed periodic systems: A new development of the ab initio crystal elongation method

We have recently proposed the elongation method which is a novel molecular orbital method at the Hartree–Fock level to calculate the electronic structures of large periodic or aperiodic polymers efficiently. This method has the idea of the successive connection of any fragments to obtain the electronic properties of large molecules with any units. In this approach, the stationary conditions of the electronic states against the size extension have been formulated. Studies for molecular systems have suggested that the elongation technique with the stationary conditions may be applicable to periodic systems described by the crystal orbital. A one‐dimensional polymer, a two‐dimensional surface, and a three‐dimensional crystal with a local disordering part can be treated systematically by introducing the elongation technique into a large extended supercell model. In the present study, we develop a new quantum chemical approach for the study of locally perturbed periodic systems by the ab initio crystal orbital...

The contributions of chalcogen to the peierls instability in model crystals of charge-transfer complexes

Abstract We have already proposed a convenient method to estimate the magnitude of the Peierls instability in molecular crystals. In the present paper to examine the reliability of our treatment, we apply it to the two types of model crystals composed of naphthalene cations and chalcogen substituted ones (planar-1,4,5,8-tetrathianaphthalene [TTN] + ). The results show that the chalcogen atoms in TTN molecules play an important role in the electronic conductivity. It was demonstrated that this treatment could reproduce ab initio results and become a useful tool to investigate the relationship between the crystal structure of charge-transfer complexes and their electronic conductivity.

An elongation method to calculate the electronic structure of non-periodical and periodical polymers

Abstract To calculate the electronic structure of non-periodical polymers, we developed a novel molecular orbital method, which we call the elongation method. When this new method is incorporated into existing molecular orbital methods, such as ab intio molecular orbital method, the computation time decreases substantially, without sacrificing reliability. This new method has two steps, localization and elongation, and thus simulates the process of polymerization reaction. First, we briefly describe the localization and elongation steps. Then, we describe the results from applying the ab initio elongation molecular orbital method (a combination of the two methods) to three poly-hydrogen bonding systems, (H 2 O) n , (HF) n , and (HCONH 2 ) n . For all three systems, there was excellent agreement in obtained total energy and electron density compared with those calculated using the conventional ab initio molecular orbital method. Finally, this elongation method was incorporated into the PM3 method and used to calculate the electronic structure of the physiologically active oligopeptide, galanine with 30 amino acid residues. Again, the obtained total energy and the electron density agreed with those calculated by the conventional PM3 method. These results confirm that the elongation method is reliable and can be applied to a variety of huge systems such as proteins and nucleic acids by using various types of molecular orbital theory.

Molecular orbital study on the ferroelectricity of ODD nylons

Abstract Recently, nylon-11 and nylon-7 have been found to be ferroelectric and this ferroelectricity was related to hydrogen bonds between polymer chains. When the crystal structure of nylon-11 is complete and has little disordered structures, the ferroelectricity is difficult to be exhibited because of the fairly strong network of the hydrogen-bonding system. The ferroelectricity is obtained for nylon with the slightly disordered crystal structure. This was confirmed by measuring the vibrational frequency for the N-H bond stretching. For the nylon with the disordered crystal structure, the vibrational frequency was found to shift to the higher wavenumber, indicating the loosening of the hydrogen-bond in question. In the present paper, we calculate the vibrational frequency for the N-H bond stretching by using ab initio molecular orbital method in order to shed light on the relation between the degree of the disorder of the crystal and the magnitude of the shift of the vibrational frequency. In connectio...

5-Methyl-1-phenylpyrrolo[3,4-d]borepin: a polarized aromatic molecule

Spectroscopic data together with molecular orbital calculations show that 5-methyl-1-phenylpyrrolo[3,4-d]borepin is a polar aromatic system, whose stability confirms our guide for construction of stable heteroaromatic compounds.