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Taylor D.E.,U.S. Army | Runge K.,BWD Associates LLC | Cory M.G.,ENSCO | Burns D.S.,ENSCO | And 3 more authors.
Journal of Physical Chemistry C | Year: 2011

A multiscale method for systematically generating predictive models for probe-surface interactions and its independent experimental verification is described. The interaction of three probe molecules (H 2O, NH 3, and NO) with silica was studied using experiment, theoretical quantum chemistry, and molecular dynamics calculations. Quantum chemical (QC) methods were used to compute binding enthalpies and vibrational (infrared, IR) spectra of molecule-surface pairs for three unique surface silanol sites. The probe-surface IR spectral shifts induced by the interaction of the probe molecules with the surface silanol sites were also computed and compared to experiment. The computed IR results are comparable to those of experiment and (a) verified that the surface that has been created using simulation is indeed similar to the experimental surface and (b) shed insight into the underlying physical process leading to the observed shifts. The theoretically determined enthalpies of adsorption (ΔH ads) compared well with experiment falling within the uncertainty of those measured using inverse gas chromatography. For water, ΔH ads,350K= - 13.5kcal/mol (calculated) versus -13.6 ± 2.8kcal/mol (experimental, 330 K < T expt < 370 K). For ammonia, ΔH ads,353K = -15.2 kcal/mol (calculated) versus -12.7 ± 2.9 kcal/mol (experimental, 323 K < T expt < 383 K). Finally, for nitric oxide, ΔH ads,253K = -4.23 kcal/mol (calculated) versus -4.03 ± 0.35 kcal/mol (experimental, 243 K < T expt < 263 K). © 2011 American Chemical Society. Source


Taylor D.E.,U.S. Army | Runge K.,BWD Associates LLC | Cory M.G.,ENSCO | Burns D.S.,ENSCO | And 4 more authors.
Journal of Physical Chemistry C | Year: 2013

A consistent embedding hierarchy is applied to the calculation of binding enthalpies for organophosphate molecules to a silica surface and compared to experiment. The interaction of four probe molecules, dimethyl methylphosphonate (DMMP), diisopropyl methylphosphonate (DIMP), diisopropyl fluorophosphate (DFP), and sarin, with the silica surface is examined. Quantum chemical methods are employed to compute binding enthalpies and vibrational spectra for all interactions between probe molecules and silanol sites on the silica surface. Comparison with experimentally measured infrared shifts indicates that the theoretically modeled adsorption sites are similar to those found in experiment. The calculated binding enthalpies agree well with experiment for sarin, ΔHads,443K = -22.0 kcal/mol (calculated) vs -18.8 ± 5.5 kcal/mol (measured, 433 K < Texpt < 453 K), and DIMP, ΔHads,463K = -26.9 kcal/mol (calculated) vs -29.3 ± 0.9 kcal/mol (measured, 453 K < Texpt < 473 K). Agreement with experiment is less good for DMMP, ΔHads,463K = -19.7 kcal/mol (calculated) vs -26.1 ± 1.5 kcal/mol (measured, 453 K < T expt < 473 K), and DFP, ΔHads,423K = -20.4 kcal/mol (calculated) vs -27.5 ± 3.1 kcal/mol (measured, 413 K < Texpt < 433 K). © 2013 American Chemical Society. Source


Cory M.G.,ENSCO | Taylor D.E.,U.S. Army | Bunte S.W.,U.S. Army | Runge K.,BWD Associates LLC | And 2 more authors.
Journal of Physical Chemistry A | Year: 2011

Rate constants for the reactions of OH radicals with dimethyl phosphonate [DMHP, (CH3O)2P(O)H] and dimethyl methylphosphonate [DMMP, (CH3O)2P(O)CH3] have been calculated by ab initio structural methods and semiclassical dynamics modeling and compared with experimental measurements over the temperature range 250-350 K. The structure and energetics of reactants and transition structures are determined for all hydrogen atom abstraction pathways that initiate the atmospheric oxidation mechanism. Structures are obtained at the CCSD/6-31++G* level of chemical theory, and the height of the activation barrier is determined by a variant of the G2MP2 method. A Transfer Hamiltonian is used to compute the minimum energy path in the neighborhood of the transition state (TS). This calculation provides information about the curvature of the potential energy surface in the neighborhood of the TS, as well as the internal forces that are needed by the semiclassical flux-flux autocorrelation function (SCFFAF) dynamics model used to compute the temperature-dependent reaction rate constants for the various possible abstraction pathways. The computed temperature-dependent rate curves frequently lie within the experimental error bars. © 2011 American Chemical Society. Source


Burns D.S.,ENSCO | Cory M.G.,ENSCO | Taylor D.E.,U.S. Army | Bunte S.W.,U.S. Army | And 2 more authors.
International Journal of Chemical Kinetics | Year: 2013

To compare the effect of primary and secondary CH bonds on hydrogen atom abstraction by hydroxyl radical, rate constants for the reactions of OH radicals with trimethyl phosphate [TMPO, (CH3O) 3P(O)] and triethyl phosphate [TEPO, (CH3CH 2O)3P(O)] have been calculated using the semiclassical flux-flux autocorrelation function (SCFFAF) method and compared with experimental measurements over the temperature range 250-350 K. SCFFAF specifies that structures be obtained at the CCSD/6 31++G* level of chemical theory and the height of the activation barrier be determined using an energy extrapolation, here a variant of the G2MP2 method. Dynamics are generated in the SCFFAF method from forces computed with a transfer Hamiltonian, which provides information about the curvature of the potential energy surface in the neighborhood of the transition state (TS), as well as the required internal forces. The temperature dependent reaction rate constants are calculated for the various possible abstraction pathways, primary hydrogen atom abstraction in the case of TMPO, and primary and secondary in the case of TEPO. Since two energetically favorable parent structures for each system are included in the model, the activation energy is calculated with respect to the conformer that connects to a given TS and the total rate constant at a given temperature is Boltzmann weighted with respect to the parent conformer. The computed temperature dependent rate curves are consistent with published experimental data in both magnitude and temperature dependence. © 2013 Wiley Periodicals, Inc. Int J Chem Kinet 45: 187-201, 2013 Copyright © 2013 Wiley Periodicals, Inc. Source

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