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Pruyne J.G.,Trinity College at Hartford | Lee M.-T.,Paul Scherrer Institute | Lee M.-T.,University of Bern | Fabri C.,Laboratory of Physical Chemistry | And 5 more authors.
Journal of Physical Chemistry C | Year: 2014

The liquid-vapor interface is difficult to access experimentally but is of interest from a theoretical and applied point of view and has particular importance in atmospheric aerosol chemistry. Here we examine the liquid-vapor interface for mixtures of water, sodium chloride, and formic acid, an abundant chemical in the atmosphere. We compare the results of surface tension and X-ray photoelectron spectroscopy (XPS) measurements over a wide range of formic acid concentrations. Surface tension measurements provide a macroscopic characterization of solutions ranging from 0 to 3 M sodium chloride and from 0 to over 0.5 mole fraction formic acid. Sodium chloride was found to be a weak salting out agent for formic acid with surface excess depending only slightly on salt concentration. In situ XPS provides a complementary molecular level description about the liquid-vapor interface. XPS measurements over an experimental probe depth of 51 Å gave the C 1s to O 1s ratio for both total oxygen and oxygen from water. XPS also provides detailed electronic structure information that is inaccessible by surface tension. Density functional theory calculations were performed to understand the observed shift in C 1s binding energies to lower values with increasing formic acid concentration. Part of the experimental -0.2 eV shift can be assigned to the solution composition changing from predominantly monomers of formic acid to a combination of monomers and dimers; however, the lack of an appropriate reference to calibrate the absolute BE scale at high formic acid mole fraction complicates the interpretation. Our data are consistent with surface tension measurements yielding a significantly more surface sensitive measurement than XPS due to the relatively weak propensity of formic acid for the interface. A simple model allowed us to replicate the XPS results under the assumption that the surface excess was contained in the top four angstroms of solution. © 2014 American Chemical Society. Source


Dezhampanah H.,Laboratory of Physical Chemistry | Bordbar A.K.,University of Isfahan | Salimian Z.,University of Isfahan | Safaei E.,Institute for Advanced Studies in Basic Sciences
Journal of Porphyrins and Phthalocyanines | Year: 2010

The association behavior of tetrakis(N, N', N", N'"-tetramethyl tetra-2,3-pyridino)-porphyrazine copper(II) ([Cu(II) 2,3-tmtppa] 4+) was investigated in aqueous solutions at 25 °C and various ionic strengths using optical absorption and resonance light scattering (RLS) spectroscopies. The results show that [Cu(II) 2,3-tmtppa] 4+ does not have any affinity for aggregation due to increasing of salt concentration and exists as monomers even in homogeneous aqueous solutions of high ionic strengths (more than 1 M NaCl). Interaction of [Cu(II) 2,3-tmtppa] 4+ with calf thymus DNA has also been studied in 1 mM aqueous phosphate buffer of pH 7.0, by optical absorption and RLS spectroscopies, and thermal denaturation experiments. The appearance of hypochromicity of less than 10% and bathochromicity shift of δγ ≤ 2 nm in UV-vis spectra of [Cu(II) 2,3-tmtppa] 4+, increasing of thermal melting point of DNA, and no change in RLS spectra of porphyrazine due to interaction with DNA, represent the minor outside groove binding mode without any stack aggregate formation. The thermodynamic of the binding of [Cu(II) 2,3-tmtppa] 4+-DNA also has been studied. The binding constant (K) was obtained by analysis of the optical absorption spectra of the complex at various DNA concentrations using SQUAD software. The value of K was estimated to be 2.34 × 10 5 ± 0.06 M 1 at 25 °C. The thermodynamic parameters were calculated by van't Hoff equation. The enthalpy and entropy changes were 41.83 ± 3.28 kJ/mol and 242.08 ± 9.88 J/mol.K at 25 °C, respectively. The results indicate that the process is entropy-driven and suggest that hydrophobic interactions are the main driving forces for the complex formation. Increasing of the ionic strength due to addition of NaCl destabilized porphyrazine-DNA complexes, indicating the competition of Na + ions with porphyrazine complexes for occupation of minor groove binding sites. Copyright © 2010 World Scientific Publishing Company. Source


Yulikov M.,Laboratory of Physical Chemistry
Electron Paramagnetic Resonance | Year: 2015

Essential details of two techniques for distance measurements between non-identical spin labels are summarized. One technique is based on double electron-electron resonance (DEER) between Gd(III) ions and nitroxide radicals. The other technique is based on indirect measurements of stochastic dipolar interaction between Ln(III) ions and organic radicals via the change of longitudinal relaxation of the latter species. Combination of these techniques with double electron-electron resonance in pairs of identical spin labels (nitroxide-nitroxide or Gd(III)-Gd(III)) allows to suggest a new experimental strategy for multiple distance measurements in orthogonally-labelled samples. General discussion of advantages and disadvantages of the new strategy for studies of biomacromolecules and their complexes is given along with illustrative experimental examples. In particular, performance of Gd(III)-nitroxide DEER is compared to other possible combinations of nonidentical spin label pairs, while relaxation enhancement in pairs Fe(III)-organic radical is compared to the case of Dy(III)-nitroxide pairs. © The Royal Society of Chemistry 2015. Source


Ulvila V.,Laboratory of Physical Chemistry | Phillips C.R.,ETH Zurich | Halonen L.,Laboratory of Physical Chemistry | Vainio M.,Laboratory of Physical Chemistry | Vainio M.,Center for Metrology and Accreditation
Optics Express | Year: 2014

We demonstrate that it is possible to obtain a mid-infrared optical frequency comb (OFC) experimentally by using a continuous-wavepumped optical parametric oscillator (OPO). The comb is generated without any active modulation. It is based on cascading quadratic nonlinearities that arise from intra-cavity phase mismatched second harmonic generation of the signal wave that resonates in the OPO. The generated OFC is transferred from the signal wavelength (near-infrared) to the idler wavelength (mid-infrared) by intracavity difference frequency generation between the OPO pump wave and the signal comb. We have produced a mid-infrared frequency comb which is tunable from 3.0 to 3.4 μm with an average output power of up to 3.1 W. © 2014 Optical Society of America. Source


Bauer J.,Laboratory of Physical Chemistry | Tomisic V.,Laboratory of Physical Chemistry | Vrkljan P.B.A.,XVIII Gymnasium
Journal of Chemical Education | Year: 2012

A laboratory exercise has recently been reported in which the students use the initial rates method based on the clock reaction approach to deduce the rate law and propose a reaction mechanism for the oxidation of iodide by iron(III) ions. The same approach is used in the exercise proposed herein; the students determine the dependence of the reaction rate on ionic strength and temperature in a straightforward manner. The previously proposed mechanism is further supported by the results, the rate-determining step is identified, and the activation parameters E a, Δ†H, and Δ†S are determined for the overall reaction of the first two steps of the mechanism. The significance of such kinetic studies in elaborating reaction mechanisms is clearly demonstrated to the students. © 2012 American Chemical Society and Division of Chemical Education, Inc. Source

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