6,982 research outputs found

    Path integral evaluation of the kinetic isotope effects based on the quantum instanton approximation

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    A general method for computing kinetic isotope effects is described. The method uses the quantum-instanton approximation and is based on the thermodynamic integration with respect to the mass of the isotopes and on the path-integral Monte-Carlo evaluation of relevant thermodynamic quantities. The central ingredients of the method are the Monte-Carlo estimators for the logarithmic derivatives of the partition function and the delta-delta correlation function. Several alternative estimators for these quantities are described here and their merits are compared on the benchmark hydrogen-exchange reaction, H+H_2->H_2+H on the Truhlar-Kuppermann potential energy surface. Finally, a qualitative discussion of issues arising in many-dimensional systems is provided.Comment: 11 pages, 2 figures, proceeding

    The Thatcher Experiment: The First Two Years

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    macroeconomics, Thatcher

    Exploiting classical nucleation theory for reverse self-assembly

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    In this paper we introduce a new method to design interparticle interactions to target arbitrary crystal structures via the process of self-assembly. We show that it is possible to exploit the curvature of the crystal nucleation free-energy barrier to sample and select optimal interparticle interactions for self-assembly into a desired structure. We apply this method to find interactions to target two simple crystal structures: a crystal with simple cubic symmetry and a two-dimensional plane with square symmetry embedded in a three-dimensional space. Finally, we discuss the potential and limits of our method and propose a general model by which a functionally infinite number of different interaction geometries may be constructed and to which our reverse self-assembly method could in principle be applied.Comment: 7 pages, 6 figures. Published in the Journal of Chemical Physic

    Path-integral virial estimator for reaction rate calculation based on the quantum instanton approximation

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    The quantum instanton approximation is a type of quantum transition state theory that calculates the chemical reaction rate using the reactive flux correlation function and its low order derivatives at time zero. Here we present several path-integral estimators for the latter quantities, which characterize the initial decay profile of the flux correlation function. As with the internal energy or heat capacity calculation, different estimators yield different variances (and therefore different convergence properties) in a Monte Carlo calculation. Here we obtain a virial-type estimator by using a coordinate scaling procedure rather than integration by parts, which allows more computational benefits. We also consider two different methods for treating the flux operator, i.e., local-path and global-path approaches, in which the latter achieves a smaller variance at the cost of using second-order potential derivatives. Numerical tests are performed for a one-dimensional Eckart barrier and a model proton transfer reaction in a polar solvent, which illustrates the reduced variance of the virial estimator over the corresponding thermodynamic estimator.Comment: 23 pages, 5 figures, 1 tabl

    SELECTIVE IODINATION USING DIARYLIODONIUM SALTS

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    Aryl iodides have become widely recognized as versatile synthetic intermediates, owing to aromatic iodine’s excellent ability to participate in oxidative addition reactions. Iodoarenes readily undergo a variety of synthetic transformations including metal catalyzed cross-coupling reactions, diaryliodonium chemistry, formation of organometallics through reduction or metal-halogen exchange, as well as classical SN2 type chemistry. Because a wide array of transformations are available for aryl iodides, organic molecules containing this moiety often serve as vital precursors to highly desirable pharmaceuticals. This thesis describes a simple and selective two-step approach to the formation of aryl iodides. This method proceeds through an easily purified diaryliodonium salt intermediate, which is subsequently converted to the corresponding aryl iodide. This method is an effective and general process for the selective synthesis of a large variety of monoiodinated arenes that are difficult to access by other approaches. Advisor: Stephen G. DiMagn

    SELECTIVE IODINATION USING DIARYLIODONIUM SALTS

    Get PDF
    Aryl iodides have become widely recognized as versatile synthetic intermediates, owing to aromatic iodine’s excellent ability to participate in oxidative addition reactions. Iodoarenes readily undergo a variety of synthetic transformations including metal catalyzed cross-coupling reactions, diaryliodonium chemistry, formation of organometallics through reduction or metal-halogen exchange, as well as classical SN2 type chemistry. Because a wide array of transformations are available for aryl iodides, organic molecules containing this moiety often serve as vital precursors to highly desirable pharmaceuticals. This thesis describes a simple and selective two-step approach to the formation of aryl iodides. This method proceeds through an easily purified diaryliodonium salt intermediate, which is subsequently converted to the corresponding aryl iodide. This method is an effective and general process for the selective synthesis of a large variety of monoiodinated arenes that are difficult to access by other approaches. Advisor: Stephen G. DiMagn
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