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  • 1
    Electronic Resource
    Electronic Resource
    Hydrophobic hydration of inert gases: Thermodynamic properties, inherent structures, and normal-mode analysis (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 3719-3727 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Computer simulations on water and aqueous solutions of noble gases have been carried out in order to study the structures of water around a solute. The hydration energy and free energy evaluated for neon (Ne) and xenon (Xe) solutions in the present study were in good agreement with those by experiments. The detailed hydration structures were investigated by means of the so called inherent structures and normal-mode analyses. It was found that the positive excess free energy in the hydration of Xe arises from a decrease in the number of distinct potential-energy minima in configuration space and that the free energy increase in the Ne solution is due partly to the decrease in the number of the potential minima and partly to the anharmonic modes which are harder than those in pure water. The soft anharmonic modes in the Xe solution were almost equivalent to those in pure water. The introduction of a Xe solute gives rise to a change in water structure to a clathrate-like structure and yields an increase in population of the cyclic pentamer connected by hydrogen bonds, which leads to the exothermic hydration.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460822
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  • 2
    Electronic Resource
    Electronic Resource
    Visible and uv spectra of a polydiacetylene with a side group conjugated to the main chain (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5510-5517 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The polarized reflection spectra of single crystals of PDA-CPDO [poly-1-(N-carbazolyl)penta-1, 3-diyn-5-ol], which are π conjugated between the side groups and the main chain, have been measured in the photon energy region from 1.38 to 32 eV for the first time with a polarized synchrotron radiation source. Absorption spectra have been calculated using the Kramers–Kronig relation. Transitions in the visible absorption spectrum, which have been previously attributed to an interband transition in a similar polydiacetylene, have been found to be highly dichroic with respect to the direction along the polymer backbone. The absorption spectrum in this region reveals two broad (ΔE(approximately-greater-than)0.4 eV) peaks at 1.9 and 2.7 eV with an almost equal absorption coefficient of 7.5×104 cm−1. The spectral features in the range from 3 to 8 eV result from electronic transitions of the carbazolyl side groups. A single broad (ΔE∼10 eV) absorption band observed at 18 eV is due to either transitions of σ electrons to higher σ* or π* states of the carbazolyl group, or to ionization processes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459620
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  • 3
    Electronic Resource
    Electronic Resource
    Molecular dynamics simulations on the hydration of fluoroalcohols (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 5834-5842 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular dynamics (MD) calculations have been carried out for aqueous solutions of isopropyl alcohol (IPA) and its fluorinated compounds, 1,1,1-trifluoro-2-propanol (TFIPA) and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIPA). The simulated systems were canonical ensembles containing 216 molecules in each, one of which was alcohol and the temperature was set to 298.15 K. The MCY (Matsuoka–Clementi–Yoshimine) potential was used for water–water interaction, whereas new potential functions were determined for alcohol–water interactions, on the basis of ab initio molecular orbital calculations on more than 1100 different dimeric configurations for each alcohol–water pair. The static properties of solvent water in the vicinity of each functional group of solute were obtained from MD calculations. It is found that the promotion of water structure and the increase of hydrogen bond between water molecules occurs not only near the fluoroalkyl group but also even near hydroxyl group of fluoroalcohols. Furthermore, the alcohol–water interaction is stronger for fluoroalcohols than for aliphatic alcohols, owing to the electronegativity and the electron withdrawing effect of fluorine atoms. The enthalpies of hydration for fluoroalcohols include the contributions from these features of both water–water and alcohol–water interactions. This is not the case for the hydration of aliphatic alcohols, and is the reason for the lack of regular change of enthalpies of hydration with the degree of the substitution of CH3 by the CF3 group. The hydration of IPA is similar to that of other aliphatic alcohols; hydrophobic hydration near the apolar group and the energetic unstabilization of water near the hydroxyl group are observed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455534
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  • 4
    Electronic Resource
    Electronic Resource
    Computer experiments on aqueous solutions. VII. Potential energy function for urea dimer and molecular dynamics calculation of 8 mol % aqueous solution of urea (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 82 (1985), S. 5184-5191 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular dynamics calculation (MD) has been carried out for an aqueous solution of urea at 298.15 K and with experimental density value at ordinary pressure by the use of constant temperature technique developed previously. The total number of molecules is 216, of which 17 are urea. The mole fraction of urea in the solution is thus 0.078. For water–water and water–urea interactions, the MCY potential and previously determined potential have been used. A new urea–urea pair potential is determined by ab initio LCAO SCF calculations for more than 750 different dimer configurations with an STO-3G basis set and subsequent multiparameter optimization of the MO data to a 12-6-3-1 potential energy function. The MD calculation is extended up to 76 000 time steps and final 44 000 time steps (17.6 ps) are used to calculate both static and dynamic properties. Among other information, the following results are important and interesting: (1) Urea molecules exhibit appreciable self-association, (2) such association is possible with strong hydrogen bondings and this is in sharp contrast with the association of t-butanol through hydrophobic interaction studied previously, (3) there is no evidence that urea destroys structure of water, and (4) the decrease of self-diffusion coefficient of water in urea solution is appreciably smaller than that in the case of t-butanol solution.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.448643
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  • 5
    Electronic Resource
    Electronic Resource
    Dynamics of probe particles during sol–gel transition of PBLG–DMF solution and the resulting gel structure (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 3785-3791 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dynamics of polystyrene microgel (PSMG), i.e., a probe particle of highly cross-linked polystyrene latex spheres with radii 20.7 and 284 nm, during the sol–gel transition of poly(γ-benzyl-L-glutamate) (PBLG) dissolved in N,N-dimethylformamide (DMF) solution has been investigated by measurements with the dynamic light scattering technique. All the intensity time correlation functions (ICF) measured from PSMG dispersed in PBLG matrix approximately decayed single exponentially. First, it was confirmed that the diffusion coefficients of PSMG at the sol state of 298.8 K obeyed the stretched exponential relation as a function of concentration of PBLG–DMF solutions. Second, the initial amplitude and the normalized variance of ICF, and the diffusion coefficient of PSMG were altered considerably before and after sol–gel transition with decreasing temperature. The initial amplitude and the normalized variance had a minimum and a maximum at the gelation threshold temperature, respectively. It was also found that PSMG almost free-diffused even at the gel state, as well as the sol state, regardless of low temperature. These facts may strongly support that the resulting PBLG gels induced through microphase separation evidently had a porous and bicontinuous structure at the "swollen gel state," which corresponds to the results observed by a transmission electron microscope. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1388628
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  • 6
    Electronic Resource
    Electronic Resource
    Stability of polar guest-encaging clathrate hydrates (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 3127-3134 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In order to account for the experimental observation that some amines form clathrate hydrates but that alcohols inhibit hydrate formation, we investigate the stability of clathrate hydrates which encage highly polar guest molecules by examining potential energy local minimum structures and also thermally excited structures. First, we examine the local minimum structure at which the total potential energy has a minimum value for amine and alcohol hydrates and inquire whether, in the absence of thermal fluctuations, the conditions for true-clathrate hydrates are satisfied. The local minimum structures of alcohol hydrates are distinguished from those of stable clathrate hydrates of structures I and II, while amine hydrates hold the host lattice of clathrate hydrates. We argue that the difference between the magnitude of the partial charge on the hydrogen atom of the hydroxyl and amino groups plays a much more significant role in the stability of both kinds of clathrate hydrates than the difference in molecular geometry does. Second, we examine kinetic stability by molecular dynamics simulation. Near room temperature the host lattice structure encaging amine remains intact. This is in contrast to the alcohol hydrate, which begins to melt easily with thermal excitation. Interesting is the finding that very long-time-scale (∼100 ps) fluctuations of the host potential energy is observed, as well as fast oscillations caused by temporary partial defects in the host network. Clathrate hydrates of polar guest molecules may experience an unstable state in which the host hydrogen-bonded network is partially broken and guest–host hydrogen-bond generation occurs but the stable state of their host lattices is again restored.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467626
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  • 7
    Electronic Resource
    Electronic Resource
    Optical dephasing of organic dye molecules doped in cross-linked polyvinyl alcohol derivatives: Incoherent photon echo and hole-burning studies (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 3442-3454 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Optical dephasing of the zero-phonon line (ZPL) of organic dye molecules doped in polyvinyl alcohol (PVA) and several cross-linked PVA derivatives was studied by using the incoherent photon echoes and the photophysical persistent hole-burning. It was found in the incoherent photon echo measurements that optical dephasing time of the ZPL increases with increasing the length of a cross-linker introduced to the PVA backbone. The difference in the temperature dependence of the dephasing time was also observed between a dye doped in PVA with and without the cross-link. When a longer cross-linker was introduced, the phonon sideband component in the incoherent photon echo signal became dominant in contrast to that in PVA with a relatively short cross-linker, while the dephasing time of the ZPL was still longer than that in PVA without the cross-link. Hole-burning studies showed that the phonon sideband spectrum of a dye doped in the cross-linked PVAs is essentially similar to that in PVA. However, the depth of the zero-phonon hole burnt with the same burning power was found to depend on the PVA hosts with the cross-linkers. These our findings suggest that the introduced cross-link does not alter the electron–phonon coupling modes or pseudo-local mode, but it drastically reduces the effect of the two-level tunneling systems in PVA on the optical dephasing of a doped dye. As a result, the dephasing time of the dye in the cross-linked PVAs becomes longer than that in PVA without the cross-link. The decrease of the optical dephasing by introducing the cross-link can be interpreted by assuming a void space in PVA, which is created near to the cross-linker. In addition, absorption and fluorescence spectra in these systems were measured, which imply the occurrence of weak dye–dye interaction when the dye is doped in PVA with a much longer cross-linker.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466387
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  • 8
    Electronic Resource
    Electronic Resource
    Polarization switching in a tensile-strained InGaAs/InGaAsP multiple quantum well distributed feedback laser diode (1998)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 72 (1998), S. 3124-3126 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An inhomogeneously biased distributed feedback (DFB) laser diode (LD) with two electrodes switched its polarization mode by 3 mA change of the bias current, maintaining single longitudinal mode oscillation. In the active layer of the LD, 13 nm thick and 0.6% tensile-strained InGaAs multiple quantum well (MQW) equalized the transverse electric and the transverse magnetic modal optical gain at 1.55 μm. With various grating pitches on the same MQW active layer, polarization switching DFB LDs were realized in the wavelength range as wide as 18 nm. The linewidth characteristics during the polarization switching were confirmed to be narrow due to the small switching current. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.121567
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  • 9
    Electronic Resource
    Electronic Resource
    Band-edge photoluminescence of CuGaSe2 films grown by molecular beam epitaxy (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 4318-4322 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Slightly Cu-rich CuGaSe2 films were grown on [001] oriented GaAs substrates by molecular beam epitaxy. Photoluminescence of the films showed a remarkable emission peaked at 1.71 eV at low temperature, which is attributed to recombination of free excitons and bound excitons. The dissociation energy of free excitons and their localization energy to a center are found to be 16.2 and 3.3 meV, respectively. The band-gap energy Eg is estimated to be 1.7310 eV at low temperature. It is suggested that the temperature variation of Eg is dominated by interaction with phonons of 26 meV which corresponds to the mean energy of the optical phonons in CuGaSe2. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.361800
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  • 10
    Electronic Resource
    Electronic Resource
    Molecular dynamics simulation study of the anomalous thermal conductivity of clathrate hydrates (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 9569-9577 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Equilibrium and nonequilibrium molecular dynamics (MD) simulations have been performed in order to account for anomalous thermal conductivity of clathrate hydrates. Evaluated thermal conductivities of clathrate hydrates are lower than those of ice from both types of MD simulations. The evaluated values agree qualitatively with experimental observation. Its anomalous temperature dependence is also reproduced. It is found that those unique properties of clathrate hydrates arise mainly from encaging of guest species and that guest Xe atom even in large cage couples strongly with the host lattice and vibrations relevant to Xe have a large anharmonic contribution. Those modes with large anharmonicity interact with other modes and scatter phonons. This leads to a small thermal conductivity. Increasing temperature changes the "effective'' potential energy curves of guests in a different way from a the normal crystal. Softening of the modes relevant to guests is suppressed, which is unique in clathrate hydrates reducing anharmonicity of the low frequency modes coupled with guest motions. It is concluded that the most likely mechanism for the anomalies is the resonance scattering model among three accounts proposed so far. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471705
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