Proton Dynamics in Hydrogen-bonded Crystals

In this chapter we will consider molecular crystals with normal hydrogen bonds in which the donor A H interacts with an acceptor B. The so-called bifurcated and trifurcated H-bonds [1] as well as the new multiform unconventional H-bonds [2] are beyond the scope of the present chapter. We will focus on the proton dynamics in molecular crystals with strong and moderate H-bonds [3] in the ground electronic state. Attention will be focused on the interpretation of the structural and spectroscopic manifestations of the dynamics of the bridging proton as established in X-ray, neutron diffraction, infrared, and inelastic neutron scattering (INS) studies of H-bonded crystals. 

Various theoretical approaches have been developed for the description of the structure, spectral properties, and proton tunneling in H-bonded systems [4-7]. Computations for particular H-bonded species in the gas phase have been performed [8]. Due to strong environmental effects the applicability of gas-phase calculations to the proton dynamics in H-bonded crystals is questionable. Many theoretical approaches are based on oversimplified models (harmonic potentials and one-dimensional treatment of proton tunneling) and they usually contain parameters obtained from the experiment to be interpreted. This is why a consistent view on hydrogen bonding phenomenon in molecular crystals is still far from being achieved. 

To show that a uniform and noncontradictory description of the specific properties of molecular crystals with quasi-linear H-bonds can be obtained in terms of a two-dimensional (2D) treatment assuming strong coupling between the proton-transfer coordinate and a low-frequency vibration. 

To interpret experimental structural and spectroscopic regularities of crystals with a quasi-symmetric A H A fragment using a model 2D potential energy surface (PES). 

Hydrogen-Transfer Reactions. Edited by J. T. Hynes, J. P. Klinman, H. H. Limbach, and R. L. Schowen Copyright 2007 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-30777-7 

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I 9 Proton Dynamics in Hydrogen-bonded Crystals Energy, cm ... 

S. Takeda, H. Chihara, T. Inabe, T. Mitani, and Y. Mamyama, NMR study of proton dynamics in the NHO hydrogen bonds in the thermochromic crystals of IV-salicylideneanilines, Chem. Phys. Lett 189(1), 13-17(1992). 

Neutron scattering experiments shed a new light on proton dynamics in the extended arrays of hydrogen bonded centrosymmetric dimer entities of the KHCO3 crystal. Proton dynamics are decoupled from the lattice. Measurements of effective oscillator masses (namely, 1 amu) contribute to full determination of normal coordinates. 

F. Fillaux, B. Marchon, A. Novak J. Tomkinson (1989). Chem. Phys., 130, 257-270. Proton dynamics in the hydrogen bond, inelastic neutron scattering by single crystals of CSH2PO4 at 20 K. 

Tunnelling spectroscopy is unique to observing quantum nonlinear dynamics in crystals. Evidence for proton transfer along hydrogen bonds is another outstanding contribution of INS. It is another manifestation of the decoupling of proton dynamics from the crystal lattice. The quantum nature of proton transfer dynamics even at room temperature is quite unforeseen and contrasts with mechanisms based on semiclassical diffusion jumps. 

Baranov, A. 1., Meiinov, B. V., Tregubchenko, A. V., Khiznichenko, V. R, Shuvalov, L. A., and Schagina, N. M., 1989, Fast proton transport in crystals with a dynamically disordered hydrogen bond network. Solid State Ionics 36 279-282. 

Besides the peaks of the local proton modes typical for hydrogen bond, a sharp peak at 28 meV was observed in KDP [34] and attracted much attention [34,38,39]. This peak exists in DKDP at somewhat higher frequency its intensity decreases in both crystals and its width decreases upon the transition from the FE to the PE phase, without any softening of its frequency [38]. Hence, it is concluded that this mode is connected with the phase transition dynamics, i.e., coupled to the polarization fluctuations. This mode is not the tunneling mode or any local mode of proton or deuteron, but rather some collective optical mode of the lattice that involves substantial proton or deuteron displacement. It has been suggested [38] that this mode corresponds to the mode that has a peak at about 200 cm (25 meV) in Raman scattering and infrared reflectivity spectra, and that it is coupled to the soft mode and usually... 

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