| 1. |
- Abrahmsen-Alami, S, et al.
(författare)
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Effect of temperature on NMR self-diffusion in aqueous associative polymer solutions
- 1996
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Ingår i: Journal of Physical Chemistry. - : American Chemical Society (ACS). - 0022-3654 .- 1541-5740. ; 100:11, s. 4598-4605
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Tidskriftsartikel (refereegranskat)abstract
- The temperature dependence of polymer self-diffusion rates in aqueous solutions of C12EO200C12 (AP9, Mw = 9300) and C12EO90C12 (AP4, Mw = 4600) and nonmodified analogues PEO10 and PEO4 (Mw = 10 000 and 3400) has been studied. The effect of the hydrophobic end-groups on the self-diffusion of the associative polymer (AP) was found to be proportional to the polymer content and inversely related to the temperature. The variation of the AP self-diffusion coefficient follows an Arrhenius behavior. The resulting apparent activation energies, Ea, increase with polymer content from 15 to 55 kJ/mol in the range 0.5−50 wt % for AP9, whereas the parent PEO10 shows an almost constant Ea of about 25 kJ/mol in the same concentration range. Activation energies derived from self-diffusion and low shear viscosity measurements were found to be quite similar. The distribution of self-diffusion coefficients often observed in AP systems is discussed in terms of distribution of aggregate sizes at low AP content and homogeneity of the network at higher contents. The residence time of an AP monomer in a hydrophobic domain was estimated as 0.1 ms at 25 °C and decreases with temperature. Also included are turbidity measurements on the AP systems.
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| 2. |
- Abrahmsén-Alami, S, et al.
(författare)
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Water self-diffusion in aqueous associative polymer solutions
- 1996
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Ingår i: JOURNAL OF PHYSICAL CHEMISTRY. - : American Chemical Society (ACS). - 0022-3654 .- 1541-5740. ; 100:16, s. 6691-6697
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Tidskriftsartikel (refereegranskat)abstract
- Water self-diffusion in aqueous model associative polymer (AP) solutions, hydrophobically end-capped poly(ethylene oxide), C12EO200C12 (AP9,) and C12EO90C12 (AP4), has been studied with the NMR-PGSE method and compared to the diffusion in nonmodified poly(ethylene oxide) PEO. It was found that it decreases monotonically with increasing polymer concentration, giving Di/D0 ≈ 0.2 at 50 wt % (D0 being the water self-diffusion coefficient in the absence of polymer), independently of polymer molecular weight and modification. In further evaluation of the data, the cell-diffusion model was used. Such an analysis suggests that up to a polymer content of about 2 wt % AP9, water diffusion is not significantly affected by the polymer. Above this concentration, up to about 10 water molecules per EO group are affected in AP9 and AP4 solutions. On increasing the temperature, water self-diffusion increases, following an Arrhenius-like equation, with Ea equal to that of pure water at low polymer content (10 wt %). The activation energy increases with polymer content, and at 50 wt %, Ea is about 30 kJ/mol, independently of polymer type. A minor difference in Ea between AP4 and AP9 solutions at intermediate polymer content is likely to originate from the ability of AP4 to form well-developed cubic phase structures. An increase in temperature was found to lead to a slight dehydration of the associative polymer EO monomers closest to the hydrophobic core.
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| 3. |
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| 4. |
- Albinsson, Bo, 1963, et al.
(författare)
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EXCITED-STATE PROPERTIES OF THE INDOLE CHROMOPHORE - ELECTRONIC-TRANSITION MOMENT DIRECTIONS FROM LINEAR DICHROISM MEASUREMENTS - EFFECT OF METHYL AND METHOXY SUBSTITUENTS
- 1992
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Ingår i: Journal of Physical Chemistry. - : American Chemical Society (ACS). - 0022-3654 .- 1541-5740. ; 96:15, s. 6204-6212
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Tidskriftsartikel (refereegranskat)abstract
- From measurements of UV and IR linear dichroism on molecules partially oriented in stretched polyethylene host the transition moment directions for the first four pi-pi* transitions of indole and some indole derivatives were determined. Relative to the pseudosymmetry long axis of indole, the transitions were normally found to be polarized at (angles counted away from the ring nitrogen): +42 +/- 50-degrees (1A1 --> 1L(by) at 287 nm), -46 +/- 5-degrees (1A1 --> 1L(a) at 265 nm), 0 +/- 15-degrees (1A1 --> 1B(by) at 220 nm), and for the 1A1 --> 1B(a) transition occurring around 200 nm, at least at +/- 30-degrees away from this axis. In addition, indication for a weak, essentially short axis polarized transition was found at 235 nm, possibly due to the 1A1 --> 1C transition. An ambiguity problem regarding the sign of the angles was resolved by exploiting the change of orientation properties upon introduction of substituents. Orientation parameters (including diagonalizing angle) were determined by consideration of a large number of in-plane as well as out-of-plane polarized vibrational transitions. The question regarding effects on the excited states by the presence of methyl and methoxy substituents, at varied positions in the indole chromophore, was addressed in terms of the perturbations they caused on the transition moments. Whereas none of the four transitions was found to be very sensitive in this respect to methyl or methoxy groups introduced in 2-, 3-, 5-, or 7-position of indole, the directions of the weak 1A1 --> 1L(by) but also the strong 1A1 --> 1B(by) transition were found to become significantly altered by a methoxy group in 4- as well as 6-position. The conclusions are consistent with recent fluorescence anisotropy data and semiempirical calculations.
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| 5. |
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| 6. |
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| 7. |
- Björklund, RB, et al.
(författare)
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Partial Reduction of a Vanadia/Silica-Titania Catalyst with NH3 + NO at 473 K Studied by Electrical Conductance and ESR Measurements
- 1992
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Ingår i: Journal of Physical Chemistry. - : American Chemical Society (ACS). - 0022-3654 .- 1541-5740. ; 96:26, s. 10953-10959
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Tidskriftsartikel (refereegranskat)abstract
- Partial reduction of vanadia supported on silica-titania by NH3 + NO has been studied by electrical conductance and ESR measurements. The relationship between electrical conductance and degree of reduction was determined by oxidative and reductive titrations of V(IV) and V(V) species leached from catalyst samples which had been reduced to different levels. In situ monitoring of the steady-state V(IV) ion concentration by electrical conductance during reduction with NH3 + NO in the concentration range 0-600 ppm for each reactant in a 2 vol % O2/Ar carrier gas was performed. In a large excess of NH3, the V(IV) ion concentration increased sharply with even small additions of NO. In a large excess of NO, the NH3 + NO mixture exhibited first an oxidizing character, and the V(IV) ion concentration increased when P(NH3) > 1/6P(NO). The reaction order with respect to the NO concentration, determined from both the NO conversion and the initial rate of catalyst reduction in excess NH3, was found to be less than unity. Determination of the stoichiometry of the reaction with respect to O2 indicated that the gas-phase O2 concentration required to balance the reducing character of NH3 + NO mixtures on the surface was significantly higher than predicted by the balanced equation describing the reaction. ESR measurements on the catalyst detected V(IV) ions as vanadyl groups having two different coordination spheres. Reduction of the catalyst with NH3 + NO caused an increase in the V(IV) signal and a decrease of the hyperfine structure.
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| 8. |
- Carlsson, Christina, 1968, et al.
(författare)
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OPTICAL AND PHOTOPHYSICAL PROPERTIES OF THE OXAZOLE YELLOW DNA PROBES YO AND YOYO
- 1994
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Ingår i: Journal of Physical Chemistry. - : American Chemical Society (ACS). - 0022-3654 .- 1541-5740. ; 98:40, s. 10313-10321
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Tidskriftsartikel (refereegranskat)abstract
- The photophysical properties of the optical DNA probe YOYO (homodimeric derivative of oxazole yellow) have been characterized in terms of the monomeric part, the YO chromophore. In aqueous solutions YO is virtually nonfluorescent but upon binding to DNA its fluorescence quantum yield is strongly increased. A similar enhancement of the fluorescence is observed for YO in the viscous solvent glycerol. The high fluorescence quantum yield of YO, when bound to DNA or in a viscous solution, is proposed to be a result of decreased rotational mobility around the internuclear bridge between the two aromatic ring systems. This hypothesis is based on similar values of the activation energies for the temperature-dependent nonradiative decay processes (E(A) = 53 kJ/mol) and viscous flow (E(A) = 63 kJ/mol), suggesting related rate-limiting mechanisms. A single electronic transition is found to be responsible for the intense visible absorption band. This conclusion is based on the observation of an essentially wavelength-independent reduced linear dichroism and similarly wavelength independent fluorescence anisotropy, and the fact that the emission spectrum is very nearly a mirror image of the absorption spectrum. The conclusion is further supported by quantum mechanical calculations (CNDO/S). By combination of measurements of fluorescence anisotropy of YO in glycerol and linear dichroism of YO in a stretched poly(vinyl alcohol) film, the transition moment of the strong visible absorption band was found to be nearly long axis polarized, in agreement with the CNDO/S calculations. The low-energy electronic transition and its polarization direction in the YO chromophore remain essentially unperturbed in the YOYO dye, suggesting that the results obtained for the excited state of the YO chromophore are applicable also to YOYO. One difference, though, is that in aqueous solutions the two YO chromophores of YOYO interact with each other, forming an internal dimer, resulting in a distorted absorption spectrum.
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| 9. |
- Carlström, Göran, et al.
(författare)
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Shape fluctuations and water diffusion in microemulsion droplets : A nuclear spin relaxation study
- 1989
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Ingår i: Journal of Physical Chemistry. - : American Chemical Society (ACS). - 0022-3654 .- 1541-5740. ; 93:8, s. 3287-3299
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Tidskriftsartikel (refereegranskat)abstract
- Water 2H and 17O spin relaxation data for the microemulsion phase in the AOT/D 2O/isooctane system are reported. The difference between the transverse (R 2) and the longitudinal (R 1) relaxation rates has been measured as function of droplet size, droplet volume fraction, temperature, and resonance frequency. The 2H longitudinal relaxation rate dispersion has been measured over an extensive frequency range, using the field-cycling technique. The focus in the study is on the contribution from slow molecular processes to the quadrupolar relaxation behavior. For the first time in any system, the theoretically predicted relation between the 17O/ 2H ratios of quadrupolar line splittings and of R 2 - R 1 is verified. The extensive experimental data are used to discriminate among three different dynamic models. It is found that water diffusion within the microemulsion droplet cannot account for the experimental data. Instead, a substantial shape polydispersity seems to be required, where at any instant a large fraction of the droplets are nonspherical. However, it is not necessary that the equilibrium shape is nonspherical.
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| 10. |
- Chen, W., et al.
(författare)
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Redox properties of titanium oxides on Pt3Ti
- 1995
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Ingår i: Journal of Physical Chemistry. - : American Chemical Society (ACS). - 0022-3654 .- 1541-5740. ; 99:34, s. 12892-12895
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Tidskriftsartikel (refereegranskat)abstract
- The morphology and electronic structure of surface-segregated titanium oxides on Pt3Ti(111) are presented. Core level photoemission spectra at grazing emission reveal two states of oxidation: a dominant and reducible four-valent oxide together with a small amount of a three-valent oxide is produced by oxidation in 0 2 at and below 400°C; an irreducible three-valent oxide by oxidation in 02 at and above 450 °C. The ratio between the active four-valent and the inactive three-valent oxides decreases with increasing oxidation temperature. The probability for reduction by CO is almost unity for the Ti 4+ oxide, and the conclusion must be that the four-valent oxide plays an active role for catalytic reactions. Scanning tunneling measurements relate these observations to changes in the dispersion and nucleation of the oxide overlayer. The four-valent oxide grows as islands with remaining areas open for CO adsorption while the three-valent oxide spreads on and blocks the crystal surface. Photoemission spectra relate these dispersion effects to an electronic interaction between the Ti 3+ oxide and adjacent Pt atoms. The above observations are in accordance with the common picture of dispersion effects in titania-supported SMSI catalysts and prove that interfacial energies play a crucial role whether the dominant phase is metallic or an oxide.
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| 11. |
- Dannetun, Per, et al.
(författare)
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Evolution of Charge-Induced Gap States in Short Diphenylpolyenes as Studied by Photoelectron Spectroscopy
- 1994
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455 .- 0022-3654 .- 1541-5740. ; 98:11, s. 2853-2858
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Tidskriftsartikel (refereegranskat)abstract
- The evolution of doping-induced electronic states within the otherwise forbidden energy gap has been studied as a function of the polyene length in a series of diphenylpolyenes. The chemical and electronic structures ha ve been studied using both X-ray and ultraviolet photoelectron spectroscopies. The results are interpreted with the help of quantum chemical calculations, performed using the semiempirical Austin Model 1 and valence effective Hamiltonian methods. The molecules studied area series of diphenylpolyenes, DPx, with x = 1-7 C=C double bonds in the pol yene part of the molecule. Since the frontier or bitals of the diphenylpolyenes are localized on the polyene chain portion of the molecule, there is a high degree of separation of the phenyl and polyene parts of the 11"-systems. Hence, many chemical and electronic properties of diphenylpolyenes are expected to be similar to those of short-chain trans-polyacetylene. For the longer molecules, n = 6 or 7, the present results indicate the presence of doubly charged, interacting soliton-antisoliton pairs, which appear as two new energy levels in the otherwise forbidden energy gap. In diphenyldecaheptaene to stilbene, i.e. 1 ≤ x ≤ 5, however, a singly charged state is formed at intermediate doping levels, after which the soliton-antisoliton pairs appear for the fully doubly charged systems. These results show that, remarkably, even for very short polyene segments, charges transferred are stored in the form of ( confined) solitons.
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| 12. |
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| 13. |
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| 14. |
- Engdahl, Anders, et al.
(författare)
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Hydrogen iodide in argon matrices
- 1986
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Ingår i: Journal of Physical Chemistry. - : American Chemical Society (ACS). - 0022-3654 .- 1541-5740. ; 90:23, s. 6118-6121
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Tidskriftsartikel (refereegranskat)abstract
- The infrared spectra of HI and DI in argon matrices have been studied in the 11-25 K interval. The effects of concentration changes and of impurities have been investigated. Dimer and trimer bands have been assigned, as well as bands due to binary complexes with water, hydrogen bromide, hydrogen chloride, and atomic and molecular iodine.
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| 15. |
- Engdahl, Anders, et al.
(författare)
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Water-olefin complexes. : A matrix isolation study
- 1986
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Ingår i: Journal of Physical Chemistry. - : American Chemical Society (ACS). - 0022-3654 .- 1541-5740. ; 90:21, s. 4982-4987
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Tidskriftsartikel (refereegranskat)abstract
- The infrared spectra of the H2O, HDO, and D2O complexes with ethylene, propene, cis-2-butene, trans-2-butene, 2-methylpropene, 2-methyl-2-butene, 2,3-dimethyl-2-butene, 1,3-butadiene, benzene, and naphthalene have been recorded at temperatures from 11 to 20 K. Interaction energies have been estimated. HDO prefers to form a deuterium bond to the unsaturated molecule; however, small amounts of the hydrogen-bonded forms are present in thermal equilibrium with the deuterium-bonded forms. The energy difference between the hydrogen-bonded and deuterium-bonded forms has been estimated for most of the complexes.
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| 16. |
- Engquist, Isak, et al.
(författare)
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D2O ice on controlled wettability self-assembled alkanethiolate monolayers : Cluster formation and substrate-adsorbate interaction
- 1996
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Ingår i: JOURNAL OF PHYSICAL CHEMISTRY. - : AMER CHEMICAL SOC. - 0022-3654 .- 1541-5740. ; 100:51, s. 20089-20096
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Tidskriftsartikel (refereegranskat)abstract
- Infrared reflection-absorption spectroscopy is used to investigate thin (1-200 Angstrom average thickness) overlayers of D2O ice deposited in ultrahigh vacuum on controlled wettability self-assembled monolayers. The monolayers were derived from mixed solutions of HS(CH2)(15)CH3 and HS(CH2)(16)OH, making it possible to examine the whole range of samples from f(OH) = 0.0 to f(OH) 1.0, where f(OH) denotes the molar fraction of OH-terminated thiols in the preparation solution. This paper focuses on the interaction between the ice and the monolayer. It is shown that water molecules do not penetrate into the monolayer but that two types of interaction with the chain-terminating groups occur: hydrogen bond formation with surface hydroxyls and weak dipole-dipole interaction with surface methyls. For surfaces with f(OH) less than 0.3, the latter interaction causes the free OD mode, normally observed at 2729 cm(-1), to shift to 2704 cm(-1), thereby providing a spectral signature feature whose intensity is directly proportional to the relative area of the ice/monolayer interface. Quantitative analysis of the infrared spectra suggests that ice clusters are essentially flat on surfaces with 0.6 less than f(OH) less than 1.0 and become more droplet-like for decreasing f(OH) below 0.6. On f(OH) = 0.0 surfaces, the microscopic clusters display high contact angles (similar to 120 degrees), and full surface coverage does not occur until the average overlayer thickness is 150-200 Angstrom.
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| 17. |
- Engquist, Isak, et al.
(författare)
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Hydrogen Bond Interaction between Self-Assembled Monolayers and Adsorbed Water Molecules and Its Implications for Cluster Formation
- 1995
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Ingår i: Journal of Physical Chemistry B. - : AMER CHEMICAL SOC. - 1520-6106 .- 1520-5207 .- 0022-3654 .- 1541-5740. ; 99:39, s. 14198-14200
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Tidskriftsartikel (refereegranskat)abstract
- Infrared spectroscopy is used to investigate the adsorption of D2O onto self-assembled monolayers of methyl 16-mercaptohexadecanoate on gold. The D2O molecules are shown to interact with the carbonyl oxygens of the monolayer, forming hydrogen bonds and causing a structural rearrangement of the CO2CH3 terminal group. The number of hydrogen bonds decreases as the amorphous-like, essentially flat (two-dimensional) ice overlayer that forms at 100 K changes into polycrystalline-like ice upon annealing at 140 K. This decrease is a consequence of the formation of three-dimensional ice clusters, which leaves a large fraction of the monolayer surface bare.
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| 18. |
- Engquist, Isak, et al.
(författare)
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Temperature-programmed desorption and infrared studies of D2O ice on self-assembled alkanethiolate monolayers : influence of substrate wettability
- 1995
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Ingår i: JOURNAL OF PHYSICAL CHEMISTRY. - : AMER CHEMICAL SOC. - 0022-3654 .- 1541-5740. ; 99:32, s. 12257-12267
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Tidskriftsartikel (refereegranskat)abstract
- This paper examines the relationship between the thermal desorption of thin overlayers of condensed D2O ice and the wettability properties of the supporting substrate surface. Mixed self-assembled monolayers (SAMs) on gold with controlled chemical composition and wettability (-0.4 less than cos theta less than 1.0, where theta represents the static contact angle with water) derived from HS(CH2)(16)OH and HS(CH2)(15)CH3 were used as model surfaces. The D2O ice overlayers were prepared on these substrates by dosing of 0.1-30 langmuirs of D2O in ultrahigh vacuum at 80-120 K and characterized with temperature-programmed desorption (TPD). Infrared reflection-absorption spectroscopy (IRAS) was also used to characterize the structural progressions within the overlayers during the course of the TPD experiments, as well as at selected temperatures before and after annealing of the overlayer structure. The IRAS data show that amorphous-like ice is formed at sufficiently low temperatures (less than or equal to 100 K) on all mixed SAMs, regardless of their wettability. A structural transition of the D2O ice from amorphous-like to polycrystalline-like is observed above 100 K. The exact onset of the transition is strongly dependent on the wettability and varies from about 110 K on the extreme hydrophobic (CH3) substrate to 145-150 K on the hydrophilic (OH) substrate. On the most hydrophilic substrates, the strong hydrogen bond interaction with surface hydroxyls prevents completion of the structural transition before desorption of the D2O overlayer. This type of pinning of the D2O molecules to the substrate surface is most likely responsible for the sharp increase in desorption energy of similar to 0.2 kcal/mol which is seen at cos theta approximate to 0.6, a value defining the hydrophilicity limit above which, for our set of experimental parameters, the transition is no longer completed. The TPD data also support a model of the D2O overlayer as forming clusters of very different shape depending on substrate wettability-flat, two-dimensional clusters on hydrophilic SAMs and dropletlike, three-dimensional clusters on hydrophobic SAMs.
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| 19. |
- Ferry, Anders, et al.
(författare)
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Studies of ionic interactions in poly(propylene glycol)4000 complexed with triflate salts
- 1996
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Ingår i: The Journal of Physical Chemistry. - : American Chemical Society (ACS). - 0022-3654 .- 1541-5740. ; 100:30, s. 12574-12582
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Tidskriftsartikel (refereegranskat)abstract
- In aprotic electrolytes and, in particular, in most polyether-salt systems the molar conductivity increases dramatically with increase in salt concentration from a minimum at low concentrations to an unexpected high maximum. Such increases in conductivity are larger than would be accounted for by Fuoss-Onsager type transport theories in which complete dissociation is assumed. Examples can be found in poly(propylene glycol) (PPG) complexed with triflate salts. The series MCF3SO3/PPG4000 (M = Li, Na, NH4, ND4) has been studied and illustrates the effect of cation on the value of the maximum molar conductivity and the rate of increase of molar conductivity with increase in concentration. Particular attention has been paid to the previously reported enhancement of the ionic conductivity for ND4CF3SO3/PPG4000 electrolytes over that of NH4CF3SO3/PPG4000. Detailed infrared and Raman studies of the polyether C-O-C vibrations and of the salt vibrations have been carried out as a function of temperature (FT-IR) and salt concentration (FT-IR and FT-Raman). Spectral data are reported which show that the polarizability of the anions is effected differently in chemical coordination processes in these ammonium electrolytes than are the dipole moments. We interpret the results in terms of ionic interactions and attribute the enhanced conductivity for the deuterated electrolyte to an increased ionic mobility due to a looser association of the ND4+ cations to the CF3SO3- anions leading to shorter lifetimes of the various associations of ions. We comment on the ionic interactions that have been previously proposed to explain the observed increase in molar conductivity and propose an additional concept, i.e., that with increase in salt concentration there is an increase in the rate of exchange of cations between ion pairs and higher aggregates and between anion-associated sites and those ether oxygen sites to which the cation is less tightly coordinated.
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| 20. |
- GEJJI, SHRIDHAR P, et al.
(författare)
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AB-INITIO VIBRATIONAL FREQUENCIES OF THE TRIFLIC ACID MOLECULE
- 1993
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Ingår i: J PHYS CHEM-US. - : American Chemical Society (ACS). - 0022-3654. ; 97:27, s. 6986-6989
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Tidskriftsartikel (refereegranskat)abstract
- The optimized geometry, harmonic vibrational frequencies, and infrared intensities of the trifluoromethanesulfonic (triflic) acid, CF3SO2OH, have been determined by ab initio self-consistent Hartree-Fock calculations and second-order Moller-Plesset perturbation theory with 6-31G** and lower basis sets. The optimized geometry of the triflic acid molecule is in good agreement with that reported from electron diffraction experiments. The overall symmetry for the molecule, however, is C1, and not C(s) as suggested from the experiments. The symmetric CF3 and SO2 stretching vibrational modes of the triflic acid are seen to be reversed compared to the assignment from the infrared spectra, reported earlier in the literature. The CF3 stretchings and the O=S=O bending normal modes include strong couplings of different internal coordinates. The vibrational frequencies and the infrared intensities are sensitive to the basis set choice as well as to electron correlation effects.
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| 21. |
- GEJJI, SHRIDHAR P, et al.
(författare)
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ABINITIO VIBRATIONAL FREQUENCIES OF THE TRIFLATE ION, (CF3SO3)-
- 1993
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Ingår i: J PHYS CHEM-US. - : American Chemical Society (ACS). - 0022-3654. ; 97:15, s. 3712-3715
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Tidskriftsartikel (refereegranskat)abstract
- The optimized geometry, harmonic vibrational frequencies and infrared intensities of the trifluoromethanesulfonate (triflate) ion, CF3SO3-, have been determined with ab initio self-consistent Hartree-Fock theory by using 3-21G* and 6-31G* Gaussian basis sets. Second-order Moller-Plesset perturbation calculations were also carried out with 6-31G* basis. A normal mode analysis shows that the vibrations assigned as symmetric and antisymmetric CF3 stretching involve other internal coordinates as well, viz., CF3 bending and CS stretching. The corresponding SO3 stretching modes, on the other hand, are almost entirely described with SO stretching coordinates. The assignments of the symmetric and antisymmetric SO3 and CF3 stretching vibrations from Moller-Plesset theory are seen to be different from those reported in the literature. Recent infrared spectroscopic experiments of the triflate ion coordinated to the zinc or lead ion in poly(ethylene oxide) complexes support the conclusions from second-order perturbation theory. The vibrational frequencies and infrared intensities show a strong dependence on basis set and electron correlation.
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| 22. |
- GEJJI, SHRIDHAR P, et al.
(författare)
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GEOMETRY AND VIBRATIONAL FREQUENCIES OF THE LITHIUM TRIFLATE ION-PAIR - AN AB-INITIO STUDY
- 1993
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Ingår i: J PHYS CHEM-US. - : American Chemical Society (ACS). - 0022-3654. ; 97:44, s. 11402-11407
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Tidskriftsartikel (refereegranskat)abstract
- The optimized geometry, harmonic vibrational frequencies, and infrared absorption intensities of the lithium trifluoromethanesulfonate (triflate) ion pair, CF3SO3-Li have been investigated using the ab initio self-consistent Hartree-Fock and correlated second-order Moller-Plesset perturbation theory with the 6-31G* and lower basis sets. In the optimized structure the lithium cation is bound to two of the oxygens of the SO3 group forming a bidentate complex with C(s) symmetry. A local minimum with a monodentate structure was obtained in the HF/3-21G* calculations. The energy difference between the mono- and bidentate structures of the complex is predicted to be nearly 39 kJ mol-1 in this basis. A splitting of 230 and 158 cm-1 is obtained for the antisymmetric SO3 stretching for the bi- and monodentate coordination of the lithium cation with the free anion, respectively. The infrared spectrum of lithium triflate in poly(propylene oxide) shows a splitting of 43 cm-1. The strong interaction of the metal cation with the anion in the 1:1 complex thus overemphasizes the ''splitting behavior'' observed for lithium triflate dissolved in polymers. In the bidentate (MP2/6-31G*) complex the symmetric SO3 stretching shows a downshift of 38 cm-1, in contrast to an upshift of 47 cm-1 for the monodentate complex. The different signs of these frequency shifts have a purely geometric origin. The dependence of this frequency shift on the position of the Li+ ion is discussed.
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| 23. |
- GEJJI, SHRIDHAR P, et al.
(författare)
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STRUCTURE AND VIBRATIONAL FREQUENCIES OF THE MOLECULAR TRICHLOROMETHANESULFONIC ACID AND ITS ANION FROM AB-INITIO CALCULATIONS
- 1994
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Ingår i: J PHYS CHEM-US. - : American Chemical Society (ACS). - 0022-3654. ; 98:35, s. 8687-8692
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Tidskriftsartikel (refereegranskat)abstract
- The equilibrium structure, vibrational frequencies, and the infrared intensities of the trichloromethanesulfonic acid molecule and its anion have been investigated using the ab initio self-consistent Hartree-Fock method and second-order Moller-Plesset perturbation theory with the 6-31G** and lower basis sets. A normal mode analysis shows that, unlike CCl3 stretchings, the SO3 stretching vibrations in the CCl3SO3- anion are pure normal modes comprised of S-O stretching coordinates only. The molecular point group for the trichloromethanesulfonic acid molecule is C-1 and not C-s. The vibrational frequencies and the infrared intensities are sensitive to the basis set choice as well as to the electron correlation effects.
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| 24. |
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| 25. |
- Halle, Bertil, et al.
(författare)
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Hydration of ionic surfactant micelles from water oxygen-17 magnetic relaxation
- 1981
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Ingår i: Journal of Physical Chemistry. - : American Chemical Society (ACS). - 0022-3654 .- 1541-5740. ; 85:14, s. 2142-2147
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Tidskriftsartikel (refereegranskat)abstract
- Water oxygen-17 relaxation rates have been measured for aqueous solutions of micelles composed of ionic surfactants of varying alkyl chain length, head group, and counterion. The concentration of surfactant and salt was also varied. From these measurements, supplemented with relaxation data for short-chain molecules and with quadrupolar splittings from anisotropic mesophases, we derive structural and dynamic information about the molecular details of the water-micelle interaction. Water molecules at the micelle surface reorient anisotropically, typically 2-3 times slower than in pure water. The average lifetime for water molecules associated with sodium dodecyl sulfate micelles is 6-37 ns. The water-hydrocarbon contact in the micellar solutions is equivalent to less than two fully exposed methylene groups per amphiphile. Small head groups and small counterions produce the largest effects on the 17O relaxation rate, as expected from geometrical and electrostatic considerations.
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| 26. |
- HAMMARSTROM, Leif, 1964-, et al.
(författare)
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MECHANISMS OF TRANSMEMBRANE ELECTRON-TRANSFER - DIFFUSION OF UNCHARGED REDOX FORMS OF VIOLOGEN, 4,4'-BIPYRIDINE, AND NICOTINAMIDE WITH LONG ALKYL CHAINS
- 1993
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Ingår i: JOURNAL OF PHYSICAL CHEMISTRY. - : American Chemical Society (ACS). - 0022-3654 .- 1541-5740. ; 97:39, s. 10083-10091
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Tidskriftsartikel (refereegranskat)abstract
- Transmembrane electron transfer in lecithin (phosphatidylcholine) vesicles was studied by pulse radiolysis. Upon reduction, cetylmethylviologen (N-hexadecyl-N'-methyl-4,4'-bipyridinium CMV), cetylbipyridine (4-(N-hexadecylpyridinium-4-yl)pyridine, CB), and cetylnicotinamide (N-hexadecyl-3-(aminocarbonyl)pyridinium, CNA) transferred electrons from the bulk water phase to Fe(CN)63- in the internal water phase of the vesicles. The transmembrane electron transfer was found in all cases to proceed through diffusion of uncharged forms of the redox mediators (CMV0, CB0, and CNA0, respectively) but the kinetic behavior varied considerably. The mechanisms for CB and CNA were simple, the reaction following first-order kinetics, and the transmembrane diffusion was rate limiting (k = (1.5 +/- 0.3) x 10(3) S-1 for CB and k = 3.2 +/- 0.5 s-1 for CNA). The mechanism for CMV was more complicated, and the reaction followed second-order kinetics. The rate-determining step was proposed to be the disproportionation of two viologen radicals formed by the radiation pulse (2CMV+ double-line arrow pointing left and right CMV0 + CMV2+), followed by rapid transmembrane diffusion of CMVO and its subsequent reoxidation by Fe(CN)63-. In pulse radiolysis, and in phosphorescence quenching experiments with Pt2(P2O5)4H84-, CB0 and CB+ were used as models in order to obtain the rates of transmembrane diffusion of CMV0 and CMV+, respectively. Our results exclude the possibility of electron tunneling between viologens on opposite sides of the membrane, and they provide strong arguments against transmembrane diffusion of viologen radical (CMV+).
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| 27. |
- Hemmingsen, Lars, et al.
(författare)
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Ab initio calculations of electric field gradients in cadmium complexes
- 1996
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Ingår i: Journal of Physical Chemistry. - : American Chemical Society (ACS). - 0022-3654 .- 1541-5740. ; 100:12, s. 4803-4809
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Tidskriftsartikel (refereegranskat)abstract
- Calculations of the electric field gradient (EFG) at the cadmium nucleus have been carried out on Cd2+ in the field of two point charges, CdF2, CdCl2, and CdF2Cl2 2- at the RHF, MPn (n = 2, 3, 4), and CCSD(T) levels of theory, in order to evaluate the effects of electron correlation, relativity, and basis set truncation. The EFG has furthermore been calculated in two large molecules (approximately 300 electrons) with biologically relevant cadmium ligands. Different methods to truncate the system have been investigated. The results are compared to experimental values determined in polycrystalline samples. We suggest a reasonably accurate and economic procedure to calculate the EFG on large cadmium complexes. The basis set on cadmium should be large, at least [19s15p9d4f/11s9p5d2f], while 6-31G(d) can be used on the remaining atoms. Correlation should be treated at least at the MP2 level, which is found to be unexpectedly accurate due to cancellation of higher order terms. In this treatment the core orbitals on the ligands and 1s through 3d orbitals on cadmium can be frozen. Surrounding molecules in the crystals have been modeled by an array of point charges. Using this procedure, the error of the elements of the diagonalized EFG tensor is less than 0.3 au (3 × 1021 V/m2) for the investigated complexes.
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| 28. |
- Holmén, Anders, 1967, et al.
(författare)
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ELECTRONIC-TRANSITION DIPOLE-MOMENTS OF THE 1,N-6-ETHENOADENINE CHROMOPHORE
- 1994
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Ingår i: Journal of Physical Chemistry. - : American Chemical Society (ACS). - 0022-3654 .- 1541-5740. ; 98:51, s. 13460-13469
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Tidskriftsartikel (refereegranskat)abstract
- The electronic spectra of 1,N-6-ethenoadenine (epsilon-adenine, epsilon-A), 3-methyl-epsilon-adenine (3-me-epsilon-A), and epsilon-adenosine (epsilon-Ado) have been examined with respect to the number of electronic transitions, their intensities, and their transition moments, on the basis of measurements of UV and IR linear dichroism (LD) on samples partially oriented in stretched poly(vinyl alcohol) films, fluorescence anisotropy (FA), and magnetic circular dichroism (MCD). The experimental results are compared with semiempirical molecular orbitaI calculations. Four pi-->pi(*) transitions were found to contribute to the UV spectrum of the epsilon-adenine chromophore with the following transition moment directions (relative to the ''adenine'' pseudosymmetry long axis, counted toward N-1): +27 degrees +/- 10 degrees (I at 32 500 cm(-1)), -47 degrees +/- 5 degrees (II at 37 000 cm(-1)), -30 degrees +/- 10 degrees (III at 32 000 cm(-1)), and -11 degrees +/- 10 degrees or -49 degrees +/- 10 degrees (IV at approximate to 46 000 cm(-1)). The orientation parameters in the stretched film experiments were determined from polarized IR measurements combined with UV LD and FA data. In addition, fluorescence emission and excitation spectra and fluorescence lifetimes of E-A in aqueous solutions indicate that epsilon-A does not behave as a single spectroscopic species. In contrast to 3-me-epsilon-A and epsilon-Ado, epsilon-A displays an emission wavelength dependent excitation spectrum and a biexponential fluorescence decay. This observation is attributed to a prototropic tautomeric equilibrium between the 1-H and 3-H forms of epsilon-A.
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| 29. |
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| 30. |
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| 31. |
- Jönsson, Bo, et al.
(författare)
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Titrating polyelectrolytes - Variational calculations and Monte Carlo simulations
- 1996
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Ingår i: Journal of Physical Chemistry. - : American Chemical Society (ACS). - 0022-3654 .- 1541-5740. ; 100:1, s. 409-417
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Tidskriftsartikel (refereegranskat)abstract
- Variational methods are used to calculate structural and thermodynamical properties of a titrating polyelectrolyte in a discrete representation. In the variational treatment, the Coulomb potentials are emulated by harmonic repulsive forces between all monomers; the force constants are used as variational parameters. The accuracy of the variational approach is tested against Monte Carlo data. Excellent agreement is obtained for the end-to-end separation and the apparent dissociation constant for the unscreened Coulomb chain. The short-range screened Coulomb potential is more difficult to handle variationally, and its structural features are less well described, although the thermodynamic properties are predicted with the same accuracy as for the unscreened chain. The number of variational parameters is on the order of N2, where N is the number of monomers, and the computational effort scales like N3. In addition, a simplified variational procedure with only two parameters is pursued, based on a rigid-rod approximation of the polymer. It gives surprisingly good accuracy for certain physical properties.
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| 32. |
- Jönsson, Bo, et al.
(författare)
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Variational approach to the structure and thermodynamics of linear polyelectrolytes with Coulomb and screened Coulomb interactions
- 1995
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Ingår i: Journal of Physical Chemistry. - : American Chemical Society (ACS). - 0022-3654 .- 1541-5740. ; 99:4, s. 1251-1266
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Tidskriftsartikel (refereegranskat)abstract
- A variational approach, based on a discrete representation of the chain, is used to calculate free energy and conformational properties in polyelectrolytes. The true bond and Coulomb potentials are approximated by a trial isotropic harmonic energy containing force constants between all monomer-pairs as variational parameters. By a judicious choice of representation and the use of incremental matrix inversion, an efficient and fast-convergent iterative algorithm is constructed, that optimizes the free energy. The computational demand scales as N3 rather than N4, as expected in a more naive approach. The method has the additional advantage that in contrast to Monte Carlo calculations the entropy is easily computed. An analysis of the high- and low-temperature limits is given. Also, the variational formulation is shown to respect the appropriate virial identities. The accuracy of the approximations introduced is tested against Monte Carlo simulations for problem sizes ranging from N = 20 to 1024. Very good accuracy is obtained for chains with unscreened Coulomb interactions. The addition of salt is described through a screened Coulomb interaction, for which the accuracy in a certain parameter range turns out to be inferior to the unscreened case. The reason is that the harmonic variational Ansatz becomes less efficient with shorter range interactions. As a byproduct a very efficient Monte Carlo algorithm was developed for comparisons, providing high statistics data for very large sizes-2048 monomers. The Monte Carlo results are also used to examine scaling properties, based on low-T approximations to end-end and monomer-monomer separations. It is argued that the former increases faster than linearly with the number of bonds.
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| 33. |
- Kenéz, P. Huang, et al.
(författare)
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Counterion spin relaxation in microemulsion droplets
- 1992
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Ingår i: Journal of Physical Chemistry. - : American Chemical Society (ACS). - 0022-3654 .- 1541-5740. ; 96:23, s. 9524-9531
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Tidskriftsartikel (refereegranskat)abstract
- Counterion 23Na spin relaxation data are reported from the microemulsion phase in the AOT/water/isooctane system as a function of the water/AOT ratio, which determines the size of the aqueous droplets. The 23Na NMR measurements comprise three independent relaxation rates, allowing the individual spectral density values to be determined, as well as the second-order quadrupolar dynamic shift. The effect on the relaxation observables of counterion diffusion within the aqueous droplet core is calculated by solving the diffusion equation in the presence of the electrostatic mean field. This allows the 23Na NMR data to be interpreted in terms of the Na+ lateral diffusion coefficient Ds in the surface region, the residual quadrupole coupling constant χs, and a spectral density contribution Jf due to fast local motions. These quantities have also been determined in a previous 23Na NMR study of the reversed hexagonal phase of the same system, with similar results. Although the present data are basically consistent with the expected microstructure of closed droplets with highly mobile counterfoils, a model of monodisperse spherical droplets cannot account quantitatively for all the data. This discrepancy is tentatively ascribed to the existence of small reversed micelles coexisting with the classical microemulsion droplets.
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| 34. |
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| 35. |
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| 36. |
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| 37. |
- LINDGREN, JAN, et al.
(författare)
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THEORETICAL SIMULATION AND EXPERIMENTAL-DETERMINATION OF OH AND OD STRETCHING BANDS OF ISOTOPICALLY DILUTED HDO MOLECULES IN AQUEOUS-ELECTROLYTE SOLUTIONS
- 1993
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Ingår i: J PHYS CHEM-US. - : American Chemical Society (ACS). - 0022-3654. ; 97:20, s. 5254-5259
-
Tidskriftsartikel (refereegranskat)abstract
- A combined theoretical and experimental study of the hydration of ions in aqueous electrolyte solutions is presented. Theoretical simulations of OH and OD stretching bands of isotopically diluted HDO molecules in a 0.44 m lithium formate solution have been performed. The positions of the atoms of the water molecules and the ions were taken from the trajectory file of a rigid-molecule room-temperature molecular dynamics simulation. V(r(OH)) potential energy functions were constructed as a sum of intra- and intermolecular energies and used in a variational calculation of the vibrational energy levels. Vibrational transition densities of states were calculated for HDO molecules in the first and second hydration shells of the ions. Infrared spectra of isotopically isolated HDO molecules in aqueous solutions of NaHCOO, LiClO4, NaClO4, Ca(ClO4)2, and Sr(ClO4)2 have been registered. The spectra were evaluated using an earlier developed double-difference method, where the number of ion-affected water molecules enters as a parameter in the analysis. In the present work, this number is obtained from the theoretical calculations, both for the Li+ and the HCOO- ion hydration. We find that OH/OD groups of water molecules in the second hydration shell of Li+, hydrogen-bonded to the first hydration shell, are affected by the ion. The earlier observed division of IR stretching frequencies for HDO molecules around cations into two distinct groups can now be explained by the presence or absence of such second-hydration-shell water molecules. For HDO molecules hydrogen-bonded to the HCOO- ions, the OH/OD frequency is lowered, compared to bulk water, for the OH/OD group involved in H-bonding to the ion, whereas the frequency is increased for the other OD/OH group, pointing away from the ion. The frequencies of HDO molecules surrounding the CH end of the formate ion are not influenced by the ion.
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| 38. |
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| 39. |
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| 40. |
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| 41. |
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| 42. |
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| 43. |
- MURRAY, JS, et al.
(författare)
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Electrostatic potentials on the molecular-surfaces of cyclic ureides
- 1991
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Ingår i: JOURNAL OF PHYSICAL CHEMISTRY. - : American Chemical Society (ACS). - 0022-3654 .- 1541-5740. ; 95:2, s. 844-848
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Tidskriftsartikel (refereegranskat)abstract
- Ab initio SCF-MO electrostatic potentials have been computed at the STO-5G/STO-3G level on the molecular surfaces of a group of cyclic ureides, in order to assess their relative reactivities toward nucleophiles, as in hydrolysis. The surfaces were defined by the 0.002 electron/bohr3 contour of the molecular electronic density. The relative hydrolytic stabilities within a series of NO2 and NF2 derivatives are predicted, on the basis of the magnitudes of the potentials above the acyl carbons. The surface electrostatic potentials of the polycarbonyl systems parabanic acid and alloxan are shown to be fully consistent with unusually short intermolecular distances that have been observed in crystallographic studies of these compounds.
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| 44. |
- MURRAY, JS, et al.
(författare)
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RELATIONSHIPS BETWEEN COMPUTED MOLECULAR-PROPERTIES AND SOLUTE SOLVENT INTERACTIONS IN SUPERCRITICAL SOLUTIONS
- 1993
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Ingår i: JOURNAL OF PHYSICAL CHEMISTRY. - UNIV NEW ORLEANS,DEPT CHEM,NEW ORLEANS,LA 70148. : AMER CHEMICAL SOC. - 0022-3654 .- 1541-5740. ; 97:19, s. 5144-5148
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Tidskriftsartikel (refereegranskat)abstract
- We focus specifically on solute-solvent interactions in supercritical solutions, as reflected in the enhancement factor E = y2P/P2sat, where Y2 and P2sat are the solubility and vapor pressure of the solute and P is the pressure of the system. For a series of 12 organic solutes in supercritical CO2, We find that E can be represented well in terms of three key solute molecular properties: (a) surface area, (b) the sum of the variances of the positive and negative electrostatic potentials on the surface, and (c) a ''balance' parameter which indicates the degree to which the solute molecule can interact to a similar extent through both its positive and negative regions. These three solute properties were computed at an ab initio self-consistent field minimum basis set level. On the basis of limited data, the same type of relationship appears to be applicable for a given solute-cosolvent combination. When various solvents or cosolvents are used, the enhancement factor of a weakly polar solute correlates well with the solvent/cosolvent molecular sizes. For polar solutes, however, an explicit dependence upon electrostatic interaction tendencies must be included.
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| 45. |
- MURRAY, JS, et al.
(författare)
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RELATIONSHIPS OF CRITICAL CONSTANTS AND BOILING POINTS TO COMPUTED MOLECULAR-SURFACE PROPERTIES
- 1993
-
Ingår i: JOURNAL OF PHYSICAL CHEMISTRY. - UNIV NEW ORLEANS,DEPT CHEM,NEW ORLEANS,LA 70148. : AMER CHEMICAL SOC. - 0022-3654 .- 1541-5740. ; 97:37, s. 9369-9373
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Tidskriftsartikel (refereegranskat)abstract
- It is shown, for a large number of organic compounds of a variety of types, that their critical constants (T(c), V(c), and P(c)) and normal boiling points can be related to computed properties of the respective molecular surfaces. One of these is the surface area; the other is a measure of the molecule's tendency for electrostatic interactions. These properties are obtained by ab initio SCF computations at the STO-5G* level, using STO-3G* optimized geometries.
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| 46. |
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| 47. |
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| 48. |
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| 49. |
- Nordén, Bengt, 1945
(författare)
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Linear and circular dichroism of polymeric pseudoisocyanine
- 1977
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Ingår i: Journal of Physical Chemistry. - : American Chemical Society (ACS). - 0022-3654 .- 1541-5740. ; 81:2, s. 151-159
-
Tidskriftsartikel (refereegranskat)abstract
- The dye l,lt-diethyl-2,2’-cyaniinoed ide (pseudoisocyanine = PIC) was studied by circular dichroism (CD) andlinear dichroism (LD). PIC forms optically active aggregates in aqueous (+)tartrate solution and as complexeswith DNA. They are characterized by J bands at 572 (tartrate) and 553 nm (DNA), with negative and positiveCD, respectively, which may be explained by helical band arrays of dye molecules (left- and right-handed helices,respectively). At higher degrees of aggregation in (+)tartrate a positive J band may be due to helical stackingof PIC polymers such as in a cholesteric liquid crystal. Intense LD indicates that the solution is not randombut contains large domains with local order. It is shown that earlier conclusions about helicity reversals andmechanical induction of optical activity in aggregated PIC when formed in optically inactive media may havebeen due to instrumental sensitivity to linear dichroism from possibly nonhelical PIC fibers. The PIC aggregationwas also studied by LD at different linear flow gradients in aqueous NaCl solution. From LD relaxation lengthsof 4 ~ 1 0 0 0 0 A0 were estimated for the orientable aggregates. The ratio of LD to random absorption at different degree of aggregation suggests that also unalignable (PION oligomers may exhibit J bands. By exciton theoryN was tentatively estimated to be more than 10. An earlier “card-pack” model, with angle cy close to 90°, betweenthe monomer long axis and the polymer axis, is discarded. The suggested structure of PIC, is a coiled bandwith cy = 30-40’. Influence of flow on aggregation was observed: At low gradients the formation of aggregateswith low rotational diffusion is favored (by applying a constant gradient apparently very elongated aggregatesmay be synthesized in this way). Above a certain value (ca. lo00 s-’) the effect of the gradient was the opposite,leading to the destruction of aggregates. PIC was also studied in oriented polyvinyl alcohol film in which itwas monomeric, and in polystyrene film where a J band was observed at 585 nm. In the Appendix factorsdetermining the instrumental LD response are analyzed.
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| 50. |
- Nordén, Bengt, 1945, et al.
(författare)
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Linear dichroism spectroscopy as a tool for studying molecular orientation in model membrane systems
- 1977
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Ingår i: Journal of Physical Chemistry. - : American Chemical Society (ACS). - 0022-3654 .- 1541-5740. ; 81:22, s. 2086-2093
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Tidskriftsartikel (refereegranskat)abstract
- Ordinary linear dichroism (LD) spectroscopy with rectangular optics is not applicable to thin uniaxial sampleshaving the optical axis perpendicular to the plane of the sample. A method has been developed to study suchsamples at inclined optical incidence and a general relation derived between the LD (corrected for polarizedreflection) and the average orientation of the absorbing transition dipole. For molecules of C2”, D2h, or highersymmetry the usual order parameters can be obtained. The method has been tested on anthracene in apolyethylene standard slab of perfectly axial symmetry. The estimated orientation agreed with previous reportson anthracene in stretched polyethylene sheets. A number of chromophoric molecules solubilized in macroscopicallyaligned lamellar mesophases of the systems sodium octanoate/decanol/water and sodium di-2-ethylhexylsulfosuccinate (Aerosol OT)/water have also been studied. In the first system rodlike molecules(diphenylethyne, anthracene, retinal) are oriented with their long axes parallel to the hydrocarbon chains ofthe amphiphile, while an extremely long molecule (&carotene) as well as water-soluble planar cationic dyesare on the average oriented parallel to the lamellar planes. In the Aerosol OT system all molecules studiedwere oriented parallel to the lamellar planes.
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