Single-site Molecule Research Articles

Current and noise in a model of an alternating current scanning tunneling microscope molecule-metal junction

The transport properties of a simple model for a finite level structure (a molecule or a dot) connected to metal electrodes in an alternating current scanning tunneling microscope (ac-STM) configuration is studied. The finite level structure is assumed to have strong binding properties with the metallic substrate, and the bias between the STM tip and the hybrid metal-molecule interface has both an ac and a dc component. The finite frequency current response and the zero-frequency photoassisted shot noise are computed using the Keldysh technique, and examples for a single-site molecule (a quantum dot) and for a two-site molecule are examined. The model may be useful for the interpretation of recent experiments using an ac-STM for the study of both conducting and insulating surfaces, where the third harmonic component of the current is measured. The zero-frequency photoassisted shot noise serves as a useful diagnosis for analyzing the energy level structure of the molecule. The present work motivates the need for further analysis of current fluctuations in electronic molecular transport.

Monte Carlo simulation for pVT relationship of CO 2+ n-C 4H 10 and CO 2+ i-C 4H 10 systems

Monte Carlo simulation has been applied to calculate the pVT relationship of CO 2+butane ( n-C 4H 10 and i-C 4H 10) systems at 310.93 K and up to 9.5 MPa. CO 2 is treated as a single-site molecule and butanes are treated as four-site molecules. The Lennard–Jones (12–6) potential is used as the site–site potentials and the combining rules proposed by Jorgensen et al. [W.L. Jorgensen, J.D. Madura, C.J. Swenson, J. Am. Chem. Soc. 106 (1984) 6638.] are adopted for unlike site pairs. The calculated results of the pVT relationship show good agreement with the experimental data [T. Tsuji, S. Honda, T. Hiaki, M. Hongo, J. Supercrit. Fluids 13 (1998) 15.] by introducing an intersite interaction parameter between unlike molecules. Furthermore, the radial distribution functions and the number of CO 2 and butanes around butanes are calculated as a fundamental information on the microscopic structures. It is found that the radial distribution functions and the number of CO 2 and n-C 4H 10 around n-C 4H 10 are different from those of CO 2 and i-C 4H 10 around i-C 4H 10.

Monte Carlo simulation of solubilities of naphthalene, phenanthrene, and anthracene in supercritical fluids

Monte Carlo method was applied to calculate the solubilities of naphthalene, phenanthrene, and anthracene in supercritical carbon dioxide, ethane, and ethylene. Supercritical fluids were treated as single-site molecules and aromatic compounds were treated as two-site (two benzene groups) or three-site (three benzene groups) molecules. The Lennard–Jones potential was used as the site–site potential. A modified test particle method was used to calculate the residual chemical potential of aromatic compounds in supercritical fluids based on the NVT canonical ensemble. The calculated results of solubilities show good agreement with the experimental values. Moreover, the solubilities of isomers can be distinguished by the present site model and common potential parameters. The microscopic structures of supercritical carbon dioxide around phenanthrene and anthracene are found to be different by calculation of the radial distribution functions.

シミュレーション 超臨界流体＋高沸点化合物系のモンテカルロシミュレーション

The Monte Carlo method has been applied to calculate the solubilities (gas-solid equilibria) of high-boiling compounds (aromatic compounds, higher alcohols, and higher fatty acids) in supercritical fluids. Supercritical fluids were treated as single site molecules, and high-boiling compounds were treated as multisite molecules. The solubilities of aromatic isomers in supercritical carbon dioxide can be quantitatively distinguished by a group contribution site model without any binary interaction parameters. The structures of supercritical carbon dioxide around aromatic isomers are found to be different because of the screen effect of the substituents. The radial distribution functions of supercritical fluids and mean-square end-to-end separations for chain molecules have been reported as fundamental knowledge of the microstructure of chain molecules in the supercritical fluid phase.

Monte Carlo simulation for solubility and spatial structure of fatty acid and higher alcohol in supercritical carbon dioxide with octane

Monte Carlo simulation has been applied to calculate the static properties such as solubilities and spatial structures of the fatty acids palmitic acid (C 15H 31COOH) and stearic acid (C 17H 35COOH), and of the higher alcohol stearyl alcohol (C 18H 37OH) in supercritical carbon dioxide with octane at 308.2 K. Carbon dioxide and octane were treated as single-site molecules for simplification, while the chain molecules (fatty acids and higher alcohol) were approximated as many-site molecules. The residual chemical potentials of the chain molecules in supercritical carbon dioxide with octane were calculated by the isothermal-isobaric Kirkwood method. It was shown that the solubilities (solid-gas equilibria) of fatty acids and higher alcohol in supercritical carbon dioxide with octane as a cosolvent can be calculated quantitatively by introducing an inter-site interaction parameter between unlike pair sites. Further, the mean-square end-to-end separations and the radial distribution functions of carbon dioxide and octane for chain molecules are reported as fundamental knowledge of the microstructure of chain molecules in the supercritical fluid phase.

Monte Carlo Simulation of Solubilities of Aromatic Compounds in Supercritical Carbon Dioxide by a Group Contribution Site Model

The Monte Carlo method was applied to calculate the solubilities of phenol, xylenol isomers, naphthol isomers, and dimethylnaphthalene isomers in supercritical carbon dioxide. These systems are binary mixtures under gas−solid equilibrium conditions. Carbon dioxide was treated as a single-site molecule, and aromatic compounds were treated as multisite molecules. For the aromatic compounds, a benzene ring, a methyl group, and a hydroxyl group were adopted as sites (groups). The Lennard-Jones (12−6) potential was used as the site−site potential, and the Lorentz−Berthelot mixing rules were adopted for unlike pair sites. A modified test particle method was adopted to calculate the residual chemical potentials of aromatic compounds in supercritical carbon dioxide based on the canonical (NVT) ensemble. The calculated solubilities were in good agreement with the experimental data by using common potential parameters determined. The solubilities of isomers can be quantitatively distinguished by the group contribution site model without any binary interaction parameters. Moreover, the microscopic structures of supercritical carbon dioxide around aromatic isomers were studied. The structures of supercritical carbon dioxide around aromatic isomers were found to be different because of the screen effect of substituents.

Monte Carlo calculation of solubilities of aromatic compounds in supercritical carbon dioxide

Monte Carlo method has been applied to calculate the solubilities (gas-solid equilibria) of naphthalene, dimethylnaphthalene isomers, and xylenol isomers in supercritical carbon dioxide. Carbon dioxide was treated as single-site molecule and naphthalene, dimethylnaphthalenes, and xylenols were treated as two-site (two benzene-ring groups), four-site (two benzene-ring and two methyl group), four-site (one benzene-ring, one hydroxyl, and two methyl groups) molecules, respectively. The Lennard-Jones (12-6) potential was used as the site-site potential and the Lorentz-Berthelot mixing rules were adopted for unlike site pairs. A modified test particle method was used to calculate the residual chemical potentials of aromatic compounds in supercritical carbon dioxide based on the NVT canonical ensemble. The calculated results of solubilities show good agreement with the experimental values. The solubilities of isomers can be distinguished by the site model. The residual chemical potentials of xylenol isomers calculated by the site model are affected by the position of methyl group. This fact suggests the screen action of methyl group against hydroxyl group. The site model is very useful to explain the screen action.

Monte Carlo simulation for chain molecules in supercritical ethane

Monte Carlo simulation was applied to calculate the static properties of chain molecules (C28H58, C30H62, and C32H66) in supercritical ethane at 308.15 K. Chain molecule and ethane were treated as a linear chain with many sites and single site molecules, respectively. The potential functions proposed by Jorgensen et al. were used for the interaction energy between pair sites and the rotational potential energy of valence bonds. The residual chemical potential was calculated by the isothermal–isobaric Kirkwood method in order to calculate the solubilities of chain molecules in supercritical ethane. An intermolecular interaction parameter between ethane and chain molecule was introduced to fit the calculated solubilities to the literature data. Furthermore, the mean-square end-to-end separation, the mean-square radius of gyration, the probability density distribution of rotational angles of the chain molecules, and the radial distribution function were reported as fundamental knowledge of the microstructure of the chain molecule in supercritical ethane.