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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 117 (2002), S. 1962-1966 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The proton tunneling reaction in malonaldehyde at low temperatures is investigated. The principal aim of this study is to find the optimal tunneling path at 0 K in the framework of the semiclassical theory with a global optimization method. An amount of 11366 ab inito points was determined in the reaction swath (i.e., the conformational space enclosed by the minima and the transition state) of malonaldehyde. With a simulated annealing approach, the path with the smallest integral of the imaginary action through the swath from minimum to minimum was determined. Surprisingly the optimal tunneling path was found to be quite far off the large curvature tunneling path [i.e., the straight connection of the two minima large-current tunneling (LCT path)]. At the beginning, it is following the minimum energy path (MEP) (i.e. the path with the lowest energy connecting the two minima and passing through the transition state), and then it is describing a curved path through the reaction swath. This curve was determined several times with different annealing schemes, which ended up with the same result—the tunneling path is proceeding close to the MEP rather than to the LCT path. Along the optimal tunneling path, the ground-state tunneling splitting was calculated with a new semiclassical method introduced in an accompanying study [C. S. Tautermann, A. F. Voegele, T. Loerting, and K. R. Liedl, J. Chem. Phys. 117, 1967 (2002), following paper]. Another focus of investigation was the influence of deformation of the tunneling paths and a general scheme of determining an approximated optimal tunneling path at 0 K is introduced. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 117 (2002), S. 1967-1974 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new method for calculating the ground-state tunneling splitting is presented. It is based on the semiclassical theory including recently derived corrections and it is the first method, which explicitly takes into account the whole conformational space between the minima and the transition state. The density-functional theory is used to determine the qualitative shape of the potential energy surface (PES) and high level ab initio calculations provide information about the stationary points. With a dual level scheme, the low-level energy surface is mapped onto the high-level points to get a good quantitative description of the high-level PES. Therefore, the new method requires no adjustment of additional parameters like scaling of the energy barrier as is necessary in other methods. Once the high-level PES is calculated, the most probable tunneling paths are determined with a global optimization procedure. Along this representative tunneling path, the tunneling splitting is calculated with additional consideration of zero-point vibrational effects. The method is applied to three molecular systems, namely hydrofluoric acid dimer, malonaldehyde, and tropolone. These systems were chosen because their energy barriers differ strongly (1 kcal/mol–7 kcal/mol). The predicted tunneling splittings agree very well with the experimental ones, therefore, we expect our method to be generally applicable, independent of the magnitude of the energy barrier. © 2002 American Institute of Physics.
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  • 3
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present transfer rates for the concerted hydrogen exchange in cyclic water clusters (H2O)n (n=3,4) based on ab initio hypersurfaces. The studied hydrogen exchange involves bond breaking and forming and is in contrast to flipping motions of "free" hydrogen atoms in a "chemical" reaction. The rates are calculated for gas-phase systems using canonical, variational transition state theory. Multidimensional tunneling corrections are included assuming both a small and a large reaction path curvature. Hybrid density functional theory [B3LYP/6-31+G(d)] was used to evaluate the potential energy hypersurface with interpolated corrections of second order perturbation theory [MP2/6-311++G(3pd,3df)] at the three stationary points for both systems. Large curvature tunneling corrections are included in dual-level direct ab initio dynamics for the cyclic tri- and tetramer of water. The ridge of the reaction swath serves as an estimate for the tunneling probability of various straight-line corner cutting paths. Our results suggest that the investigated species interconvert on a time scale of seconds. The ground-state tunneling splitting is proportional to the square root of the transition probability at the energy of the minima, which is available from the calculation of tunneling corrections. The associated tunneling splittings are estimated to be between 10−4 and 10−5 cm−1, which is close to the experimental resolution limit. © 1998 American Institute of Physics.
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  • 4
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: High-density amorphous ice was heated at constant pressures of between 0.52 to 1.9 GPa from 77 K up to 240 K. The formed phases were characterized by x-ray diffractograms of samples recovered under liquid N2 at 1 bar. The (meta)stability domain of ice XII thus revealed extends between (approximate)158–212 K from (approximate)0.7 to (approximate)1.5 GPa. We further discuss whether ice XII has a low-temperature region of stability within the ice VI domain. Our (meta)stability domain of ice XII is in a different region of water's phase diagram than that shown by Koza et al. [Phys. Rev. Lett. 84, 4112 (2000)]. © 2002 American Institute of Physics.
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  • 5
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 435.2005, 7041, E1-, (1 S.) 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Arising from: Y.-Z. Yue & C. A. Angell Nature 427, 717–720 (2004); Yue & Angell <weblink url="http://www.nature.com/nature/journal/v435/n7041/full/nature03708.html">reply.It has been unclear whether amorphous glassy ...
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