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  • 1. Goos, Elke
    et al.
    Hippler, Horst
    Kachiani, Chatuna
    Svedung, Harald
    Collisional energy transfer in CH3 radical decomposition—experiment versus theory2002In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 4, p. 4372-4378Article in journal (Refereed)
    Abstract [en]

    Experimentally determined incubation times in the thermal decomposition of methyl radicals were used to obtain collisional energy transfer probability information by adopting a discrete vibrational energy level masterequation scheme with specific rate constants from the statistical adiabatic channel model. The agreement with information from classical molecular dynamic, MD, simulations of CH3–Ar collisions was shown to be remarkably good. Results from MD simulations also support the assumption of thermally equilibrated rotations used here and in earlier work. The sensitivity of the pressure fall-off behaviour of the decomposition channels to remaining uncertainties in the energy transfer profiles is shown to be significant, in this case, as a consequence of the large number of collisions needed to reach activation. Nevertheless, we find classical molecular dynamics simulation to be useful and a good starting point in obtaining the collisional energy transfer kernel to be used in master-equation calculations treating the most obvious quantum effects through the use of discrete energy levels at low energies.

  • 2.
    Kostela, Johan
    Uppsala Universitet.
    Electrochemical Behavior of Redox Molecules in Michelles and in a Bicontinous Cubic Phase2003Licentiate thesis, monograph (Other academic)
  • 3.
    Kostela, Johan
    et al.
    Uppsala Universitet.
    Elmgren, Maja
    Uppsala Universitet.
    Almgren, Mats
    Uppsala Universitet.
    Electrochemical properties and diffusion of a redox active surfactant incorporated in bicontinuous cubic and lamellar phase2005In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 1873-3859, Vol. 50, no 16-17, p. 3333-Article in journal (Refereed)
    Abstract [en]

    The objective of this study was to investigate the electrochemical behaviour of the divalent redox active surfactant, N-cetyl-N′-methylviologen (CMV), in bicontinuous cubic and lamellar phases. The liquid crystalline phases were prepared from the system glycerolmonooleate (GMO)–water (and brine)–cationic surfactant. A comparison of the phase behaviour of GMO with the monovalent cetyltrimethylammonium bromide (CTAB) and the divalent CMV surfactant showed that the surfactants gave about the same effect at the same surface charge density. The electrochemical measurements were made with a mixture of CTAB and CMV as the surfactant. Cyclic voltammetry was used to study the electrochemistry of CMV incorporated in the cubic and lamellar phases that were spread on a gold electrode. The E0-values in the cubic samples were more negative (−0.55 V versus SCE) than in the lamellar samples (−0.53 V versus SCE). This can be explained by the higher charge density in the lamellar phase. The diffusion coefficients were also measured in the cubic phase. The mass transport is slowed down about fifty times in the cubic phase compared to in the pure electrolyte. The concentration dependence on the diffusion coefficient was also investigated. No electron hopping could be observed, which suggest that diffusional movement of the redox probe is the main source of charge transport. By placing the samples on a conducting glass slide, spectroelectrochemical investigations were performed. In the lamellar phase strong dimerization was detected at high concentration of viologen, but much less in the cubic phase.

  • 4.
    Kostela, Johan
    et al.
    Uppsala universitet.
    Elmgren, Maja
    Uppsala universitet.
    Hansson, Per
    Uppsala universitet.
    Almgren, Mats
    Uppsala universitet.
    Electrochemical properties of an amphiphilic viologen in differently charged micelles2002In: Journal of Electroanalytical Chemistry, ISSN 0022-0728, E-ISSN 1873-2569, Vol. 536, no 1-2, p. 97-107Article in journal (Refereed)
    Abstract [en]

    The electrochemical properties of N-tetradecyl-N′-methylviologen (TMV) in differently charged micelles were studied with a glassy carbon electrode using electrochemical techniques. The redox potential varied depending on the charge of the surrounding surfactants. When the viologen was situated in cationic micelles the redox potential for the 2+/1+ reaction was more positive than when situated in negatively charged micelles. The non-ionic micelles destabilised the 2+-state most showing the highest redox potentials. From studies of several different cationic micelles it was concluded that the most important parameter for the redox potential was the surface charge density. A calculation based on a simple model confirmed this. Other interactions also influenced the stability of the redox states. Adsorption, desorption and reorganisation of the surfactants at the electrode surface caused capacitive currents. To control the nonfaradaic current, differential pulse voltammetry (DPV) was used in addition to cyclic voltammetry.

  • 5.
    Kostela, Johan
    et al.
    Uppsala Universitet.
    Elmgren, Maja
    Uppsala Universitet.
    Kadi, Mari
    Uppsala Universitet.
    Almgren, Mats
    Redox activity and diffusion of hydrophilic, hydrophobic, and amphiphilic redox active molecules in a bicontinuous cubic phase2005In: The Journal of Physical Chemistry. B, ISSN 1520-6106, Vol. 109, no 11, p. 5073-Article in journal (Refereed)
  • 6. Nordholm, Strure
    et al.
    Börjesson, Lars Erik Börje
    Ming, Liu
    Svedung, Harald
    Progress on the modeling of the collisional energy transfer mechanism in unimolecular reactions1997In: Berichte Der Bunsen-Gesellschaft: Physical Chemistry, Chemical Physics, ISSN 0005-9021, Vol. 101, p. 574-580Article in journal (Refereed)
    Abstract [en]

    The RRKM theory of unimolecular reaction rates is a statistical mechanical theory based on an assumption of microcanonical equilibrium in the reactant phase space. The energy transfer in reactant medium collisions was originally described by a canonical strong collision assumption, i.e., an assumption of full thermal equilibration in each collision. In our work we first introduce a microcanonical strong collision assumption which gives the RRKM theory a consistent form. We then introduce parametrizations of the degree of weakness (nonergodicity) of the collisions. A concept of collision efficiency is defined. The weakness of the collision is expressed in terms of reduced subsets of active reactant and medium degrees of freedom. The corresponding partially ergodic collision theory (PECT) yields physical functional forms of the collisional energy transfer kernel P(E,E). In order to resolve the energy and temperature dependence and the dependence on interaction strength a multiple encounter theory is introduced (PEMET). Initially each encounter may be described by a semiempirical PECT model. Eventually the encounters may be resolved by quantum dynamical calculations of the semiclassical or CAQE (classical approach quantum encounter) type. Simple statistical collision models only distinguish between hits and misses . In reality the energy transfer efficiency exhibits characteristic fall off with increasing impact parameter b. This b-dependence can be explicitly accounted for in the master equation for the reaction rate coefficient.

  • 7. Nordholm, Sture
    et al.
    Svedung, Harald
    Nyeland, Carl
    Simple analysis of intermolecular potentials: The mBq pair potential for collisions and energy transfer2001In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 3, no 12, p. 2209-2215Article in journal (Refereed)
    Abstract [en]

    We have investigated and compared four different theoretical and empirical procedures for the generation of simple but reliable pairwise atomÈatom potentials to be used as components in the construction of intermolecular potentials. A method is proposed wherein the parameters of a modified Buckingham potential with atomic monopoles (mBq potential) is obtained by Ðtting to MP2 energies for interacting molecules confined to approach in selected symmetry directions. The results for 31 atomic pairs are compared with earlier results obtained by electron gas density functional theory, best available quantum chemical calculation or spectroscopic Ðt. The evidence is that the mBq potentials can be of signiÐcant practical value in a wide range of applications.

  • 8. Svedung, Harald
    Dynamical theory of collisional energy transfer between reactant and medium molecules2001Doctoral thesis, comprehensive summary (Other academic)
  • 9. Svedung, Harald
    et al.
    Börjesson, Lars Erik Börje
    Markovic, Nikola
    Nordholm, Sture
    The mechanism of energy transfer in H2O-H2O collisions - a molecular dynamics simulation1998In: Chemical Physics, ISSN 0301-0104, E-ISSN 1873-4421, Vol. 236, no 1-3, p. 189-205Article in journal (Refereed)
    Abstract [en]

    Earlier work on the activation-deactivation mechanism of gas phase unimolecular reactions is extended to the study of the detailed energy transfer mechanism in collisions of water molecules. Molecular dynamics simulations of binary collisions between a reactant water molecule at high internal energy with medium molecules at various selected initial temperatures are compared with results from approximate statistical theory. Energy transfer is related to i. interaction strength, ii. hard atom–atom encounters, iii. multiple minima in the center of mass separation, iv. collision lifetime and v. anharmonicity of the intramolecular potential function. The observed trends are interpreted within the framework of the partially ergodic multiple encounter theory PEMET. of collisional energy transfer. By comparison with typical stable molecule collisions the water–water collisions are more efficient as a reflection of the strong hydrogen bonding interactions. A good agreement between PEMET and molecular dynamics simulations over a wide range of interaction strengths and initial reactant energies is shown, indicating the possibility of a priori use of the PEMET model.

  • 10. Svedung, Harald
    et al.
    Krems, Roman
    Markovic, Nikola
    Nordholm, Sture
    Pair-potential model for simulation of collisional energy transfer: quantum effects and hardness dependence2001In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 3, no 12, p. 2216-2222Article in journal (Refereed)
    Abstract [en]

    A study of the sensitivity of energy transfer efficiency in molecular collisions is reported with special focus on the hardness of repulsion in atomÈatom contact. An improved pair potential is proposed with independent parameters for atomic size, attraction and hardness determined speciÐcally for a chosen atom pair by supermolecular quantum chemistry or equivalent experimental interaction data. Colinear and full 3D collisions of atomÈdiatomic molecule collisions are simulated using classical or quantum (in the colinear case) mechanics to illustrate : (i) remarkable agreement between classical and quantum dynamics for VÈT energy transfer ; (ii) greater sensitivity to hardness than to attraction and (iii) suitability of MP2 energies in symmetry constrained axial directions as a data set for the determination of pair-potential parameters.

  • 11. Svedung, Harald
    et al.
    Markovic, Nikola
    Nordholm, Sture
    Collisional deactivation of CF3I - a molecular dynamics simulation1999In: Chemical Physics, ISSN 0301-0104, E-ISSN 1873-4421, Vol. 248, no 2-3, p. 195-211Article in journal (Refereed)
    Abstract [en]

    The detailed mechanisms of ro-vibrational energy transfer in collisions between CF3I and argon or propane are investigated. Molecular dynamics simulations of collisions between a reactant CF3I molecule at energies from 50 to 200 kJrmol with medium argon or propane at selected initial temperatures are interpreted in terms of ergodic collision limits. The intramolecular potential used for CF3I is a Morse-stretchrharmonic-bend type function with parameters fitted to equilibrium structure, normal mode frequencies and dissociation energies. Simple generic Buckingham type pair-potentials are used for intermolecular atom–atom interactions. Energy transfer is related to i. geometry of collision, ii. impact parameter, iii. number of atom–atom encounters, iv. average dynamical hardness of interaction at atom–atom collisions, v. number of minima in the center of mass separation and vi. lifetime of the collisional complex. The energy transfer in our molecular dynamics calculations is compared with experimental results for the same colliders. The observed trends are interpreted in terms of detailed collisional mechanisms. Our results highlight the importance of rotational excitation and the repulsive part of the intermolecular potential.

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