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Atomistic simulation of hierarchical nanostructured materials for optical chemical sensing
Published in Alexander Bagaturyants, Vener Mikhail, Multiscale Modeling in Nanophotonics, 2017
Alexander Bagaturyants, Vener Mikhail
The 2D potential energy surface (PES) of the quasi-symmetric OHO fragment was used for treatment of PT dynamics. The PES described semiquantitatively the main experimental regularities for a strong hydrogen bond. Strong coupling of modes (proton motion and O⋯O vibration) and dynamic asymmetry of the PES were taken into account. The respective wave equation was solved numerically with adiabatic separation of the fast (proton) and slow (O⋯O vibration) subsystems. Quantum jumps between vibrational levels of both subsystems under the random force action of the environment were assumed to accomplish PT from one well into another The tunnel transitions between the slow subsystem levels, corresponding to the proton localization in different wells, were most important, their probabilities depending strongly on the O⋯O equilibrium separation. At large O⋯O distances (almost equal to 2.64 A∘) the total tunnel transition probability from definite O⋯O vibrational levels (m) of one well into all possible levels of the other well is shown to increase with m. Such a promotion of the proton tunneling was observed by several authors at the laser selective vibrational excitation of the slow subsystem. For smaller O⋯O separation (about 2.52 A∘) no vibrational assistance of the proton tunneling occurs. The microscopic mechanism of the process for these two cases was interpreted in terms of the respective matrix elements [146].
Dynamics of water in real space and time
Published in Molecular Physics, 2019
Figure 4 shows the Arrhenius plot of the self-diffusion coefficient of IXS together with the results by previous studies using 1H NMR [18], isotopic tracer measurement [19], and quasi-elastic neutron scattering [20]. Our result agrees with others at 318 K, but sharply deviates from others at lower temperatures. In this narrow temperature range our data shows the Arrhenius behaviour, with the activation energy of 10.4 kJ/mol (=107.8 meV) and D0 = 20.1 × 10−8 m2s−1. This activation energy is significantly different from the accepted values of the activation energy of diffusion at room temperature, 17.6 kJ/mol (=182.4 meV), which is almost twice large as the present result. Generally, all elements in the liquid show similar diffusivity at high temperatures [21], so this discrepancy is puzzling. However, it should be noted that by the present IXS approach the self-diffusion of oxygen is measured, whereas that of hydrogen is measured by the conventional techniques such as NMR and the tracer method. At low temperatures proton tunnelling could become significant, but it is not likely to have an effect at room temperature [22]. In the range of r below 1 Å the contribution from the distinct-part is small. Even it is present, it is an increasing function of r and t, with the leading term of r2 because of symmetry. Therefore, it tends to lead to an overestimation of Ds, and is not likely to explain the discrepancy. The correction for energy resolution was made by dividing the intermediate scattering function, F(Q, t), through the Fourier-transform of the resolution function, Sres(E),
Microwave spectrum of the complex of 3,3,3-trifluoro-2-(trifluoromethyl)propanoic acid and formic acid
Published in Molecular Physics, 2019
Javix Thomas, Michael J. Carrillo, Agapito Serrato, Fan Xie, Wolfgang Jäger, Yunjie Xu, Wei Lin
There have been increased research efforts focusing on hydrogen-bonded complexes formed by two carboxylic acids to study their structural and energetic features, in particular those associated with the concerted double proton tunnelling events. The potential energy surfaces of some of these complexes, like formic acid (FA) dimer [1], benzoic acid-FA [2], and propiolic acid-FA [3–5], show the shape of a double-minimum potential. When the ground vibrational states of the concerted double proton tunnelling motions, supported in the two potential wells, are of identical energy, the molecular system exhibits quantum tunnelling. The features of this potential surface are very similar to the surfaces of systems with a classical single proton transfer motion like malonaldehyde [6–9] and tropolone [10,11]. Fourier transform microwave spectroscopy (FTMW) has been utilised as a powerful technique to probe gas phase molecular structure and dynamics of non-covalently bound complexes [12,13], including heterodimers of carboxylic acids in the gas phase. In these MW studies, the tunnelling splittings due to the double proton transfer between the two acids were determined precisely. For example, Howard and co-workers [14] published a beautiful paper on acetic acid-FA and showed that the double proton tunnelling motion is strongly coupled to the methyl internal rotation. On the other hand, when the two ground states are of different energy, i.e. the two wells are non-equivalent, quantum tunnelling will be quenched. Instead, one would expect to detect two distinct conformations of the acid dimer. However, because of the cooling effect of the supersonic expansion utilised in the FTMW spectrometers, sometimes only one conformer is accessible experimentally, for example in the cases of perfluorobutyric acid-FA [15], cyclopropanecarboxylic acid-FA [16], and acrylic acid-FA complexes [17]. It would be interesting to explore more heterodimers of carboxylic acids to gain a better understanding of the hydrogen bonding in these systems and to appreciate the conditions under which the double proton tunnelling motion occurs.