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  • 1
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    Rate constants of chemical reactions from semiclassical transition state theory in full and one dimension (2016)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2016-07-01
    Description: Semiclassical Transition State Theory (SCTST), a method for calculating rate constants of chemical reactions, offers gains in computational efficiency relative to more accurate quantum scattering methods. In full-dimensional (FD) SCTST, reaction probabilities are calculated from third and fourth potential derivatives along all vibrational degrees of freedom. However, the computational cost of FD SCTST scales unfavorably with system size, which prohibits its application to larger systems. In this study, the accuracy and efficiency of 1-D SCTST, in which only third and fourth derivatives along the reaction mode are used, are investigated in comparison to those of FD SCTST. Potential derivatives are obtained from numerical ab initio Hessian matrix calculations at the MP2/cc-pVTZ level of theory, and Richardson extrapolation is applied to improve the accuracy of these derivatives. Reaction barriers are calculated at the CCSD(T)/cc-pVTZ level. Results from FD SCTST agree with results from previous theoretical and experimental studies when Richardson extrapolation is applied. Results from our implementation of 1-D SCTST, which uses only 4 single-point MP2/cc-pVTZ energy calculations in addition to those for conventional TST, agree with FD results to within a factor of 5 at 250 K. This degree of agreement and the efficiency of the 1-D method suggest its potential as a means of approximating rate constants for systems too large for existing quantum scattering methods.
    Print ISSN: 0021-9606
    Electronic ISSN: 1089-7690
    Topics: Chemistry and Pharmacology , Physics
    Published by American Institute of Physics (AIP)
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  • 2
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    An investigation of one- versus two-dimensional semiclassical transition state theory for H atom abstraction and exchange reactions (2016)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2016-02-26
    Description: We investigate which terms in Reduced-Dimensionality Semiclassical Transition State Theory (RD SCTST) contribute most significantly in rate constant calculations of hydrogen extraction and exchange reactions of hydrocarbons. We also investigate the importance of deep tunneling corrections to the theory. In addition, we introduce a novel formulation of the theory in Jacobi coordinates. For the reactions of H atoms with methane, ethane, and cyclopropane, we find that a one-dimensional (1-D) version of the theory without deep tunneling corrections compares well with 2-D SCTST results and accurate quantum scattering results. For the “heavy-light-heavy” H atom exchange reaction between CH 3 and CH 4 , deep tunneling corrections are needed to yield 1-D results that compare well with 2-D results. The finding that accurate rate constants can be obtained from derivatives of the potential along only one dimension further validates RD SCTST as a computationally efficient yet accurate rate constant theory.
    Print ISSN: 0021-9606
    Electronic ISSN: 1089-7690
    Topics: Chemistry and Pharmacology , Physics
    Published by American Institute of Physics (AIP)
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  • 3
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    [Perspective] Quantum dynamics in the smallest water droplet (2016)
    American Association for the Advancement of Science (AAAS)
    In: Science
    Details
    Publication Date: 2016-03-18
    Description: Water plays a central role in scientific disciplines ranging from geology to astronomy to biology. Yet it is an extraordinarily dif cult liquid to understand because of its complex, ever-changing patterns of hydrogen bonds. Studies of small water clusters have provided important insights into the concerted hydrogen-bond motions that can occur in water. These studies are also crucial for developing an accurate potential function for simulating the properties of liquid water and ice (1). On page 1310 of this issue, Richardson et al. (2) provide evidence for a concerted type of motion in which two hydrogen bonds in a water cluster are broken simultaneously (see the figure). The results have implications for many areas of scientific study, including the chemistry of polar solvents, the conformations of proteins, and the dissolution of ions in minerals. Author: David C. Clary
    Keywords: Chemistry
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Geosciences , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 4
    Electronic Resource
    Electronic Resource
    Quantum mechanical study of the vibrational relaxation of O2+ colliding with Kr (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1972-1978 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Coupled states calculations on the vibrational relaxation of O2+(v=1) colliding with Kr are reported. In the first stage, calculations have been done on single potential energy surfaces and different surfaces have been used. Then treating O2+ as a molecule in a Π ground electronic state, we have performed quantum scattering calculations on the vibrational relaxation on two 1 2A″ and 1 2A′ surfaces. A significant effect due to the inclusion of the second potential surface is reported. A comparison of the calculated rate constants with available experimental data is made. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479465
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  • 5
    Electronic Resource
    Electronic Resource
    Isotopic branching in (He, HD+) collisions: A time-dependent quantum mechanical study in three dimensions (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 10910-10918 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A time-dependent quantum mechanical approach has been used to investigate the reaction He+HD+(v=0–4,j=0–3)→HeH++D; HeD++H in three dimensions for total angular momentum J=0. The vib-rotation (v,j) state-selected reaction probability (Pv,jR) is shown to increase with v over the collision energy (Etrans) range (0.95–2.25 eV) investigated for both the exchange channels, in accord with the experimental results. The isotopic branching ratio Γ=PR(HeH+)/PR(HeD+) generally remains less than unity for different v states at different Etrans in agreement with experiment. But at Etrans=1.0 eV, for v=4, Γ obtained from our calculations for j=0 of HD+ is ∼0.8, in excellent agreement with the earlier quasiclassical trajectory calculations, but a factor of 2 less than that obtained from experiment. This difference could arise from the inclusion of nonzero j states in the experimental study, as Pv,jR is found to be j dependent for both the channels. While Pv,jR (HeH+) decreases initially with increase in j from 0 to 2 and then increases when j is increased further to 3, Pv,jR (HeD+) reveals an unusual j dependence; it is larger for even j states of HD+ than for odd j. As a result, Γ is strongly dependent on j, in contrast to the marginal dependence shown by the earlier quasiclassical trajectory calculations. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480454
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  • 6
    Electronic Resource
    Electronic Resource
    Sticking of hydrogen chloride and chlorine hydroxide to ice: a computational study (1992)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 96 (1992), S. 7079-7088 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100196a044
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  • 7
    Electronic Resource
    Electronic Resource
    Quaternion formulation of diffusion quantum Monte Carlo for the rotation of rigid molecules in clusters (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 5193-5202 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A quaternion formulation is used to derive an algorithm for performing calculations on molecular clusters using the quantum diffusion Monte Carlo method. It is assumed that the monomers in the cluster rotate and translate as rigid bodies. The algorithm is tested on the water dimer and the benzene–water cluster. Comparison with dissociation energies and rotational constants obtained with other methods illustrates the accuracy of the algorithm. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1288788
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  • 8
    Electronic Resource
    Electronic Resource
    Diffusion Monte Carlo simulations of the dipole-bound state of the water dimer anion (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 10559-10565 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: It is shown how the diffusion quantum Monte Carlo method for solving the Schrödinger equation can be adapted to calculate the lowest energy state of a dipole-bound cluster anion. Specifically, the method can be used to calculate the rotationally adiabatic potential curve that arises due to the interaction of an electron with a dipolar molecular cluster. The theory allows the motion of each monomer molecule to be coupled explicitly with the angular motion of the electron. Application is made to the anion of the water dimer. The method gives a clear picture of the reorientation of the monomer water molecules that produces an overall dipole moment for the water dimer that is favorable for forming a dipole-bound state with the electron. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480409
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  • 9
    Electronic Resource
    Electronic Resource
    Quantum scattering calculations on the SN2 reaction Cl−+CH3Br→ClCH3+Br− (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 9483-9491 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The gas-phase SN2 reaction Cl−+CH3Br(v,k)→ClCH3(v′k′)+Br− has been studied using reduced dimensionality time independent quantum scattering theory. The C–Br and C–Cl stretching degrees of freedom (quantum numbers v and v′) and the azimuthal angle (rotation of the CH3 group; quantum numbers k and k′) are treated explicitly. An infinite order sudden approximation and Radau coordinates for the stretching modes are used. The scattering problem is formulated in hyperspherical coordinates. A potential energy surface of Wang, Zhu, and Hase is used. It is found that this surface can reproduce the experimentally observed independence of the rate constant on the internal temperature of CH3Br only if it is scaled to enable the transition state geometry to agree with high level ab initio data. The reaction cross sections show the propensity rule Δk=0 for the azimuthal rotation. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.478913
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  • 10
    Electronic Resource
    Electronic Resource
    Quantum dynamics of the Walden inversion reaction Cl−+CH3Cl→ClCH3+Cl− (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 575-583 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Quantum scattering calculations on the SN2 reaction Cl−+CH3Cl→ClCH3+Cl− are reported. The rotating bond approximation (RBA) has been adapted so that three degrees of freedom including the C–Cl stretching vibration and the CH3 umbrella mode are treated explicitly. The calculations have been done with minor modifications of a potential due to Vande Linde and Hase. It is found that initial excitation of the C–Cl vibration has a large effect on the reaction probabilities, while excitation of the CH3 umbrella vibration is less significant. The reaction is dominated by scattering resonances with lifetimes ranging from 0.1 to 10 ps. It is found that the length of the C–Cl bond at the transition state of the reaction has a particularly pronounced effect on the reaction probabilities. The magnitude of the quantum reaction probabilities compares quite well with those calculated using the quasiclassical trajectory method. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473397
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