H-bonded crystals

Recent work by Dalai and Bussman-Holder has shown that such a description is not appropriate for H-bonded crystals where a displacive component... 

The H-bond Marechal and Witkowski 143> gave a theoretical approach in order to describe the peculiar vibrational features of H-bonded crystals they derive a Hamiltonian to describe the vibrations of a linear crystal (the X-H modes in X-H- -X), but no numerical results are derived. 

These important, but not completely understood, problems are considered here by using the novel, quantum chemical, approach to the microscopical theory of ferroelectrics and related materials [1], The isomorphous H-bonded crystals M3(H/D)(A04)2 (M = K, Rb A = S, Se) are taken as examples. There are two reasons of such choice. This family is investigated actively at present. Moreover, it is a suitable subject of theoretical examination because of simple chemical constitution of the TKHS-like compounds (zero-dimensional H-bond network). 

Assignment of Vibrational Modes. There is little question that important information concerning vibrational assignments comes from polarized IR studies. For H bonded substances the dichroism of the v, stretching mode usually serves as a reference from which the dichroism of other modes can be predicted. Such arguments have been particularly useful in identifying v, and Vt modes (see Fig. 3-1). Some of the H bonded crystals for which polarized IR studies have been an aid in vibrational assignment are listed in Table 3-XI. 

TABLE 3-XI H Bonding Crystals for which IR Dichroism Studies Have Aided Vibrational Assignments... 

Of course the strong interactions present in H bonded crystals are reflected in other physical properties, such as melting point, dielectric behavior, electrical conductivity, and piezoelectric behavior. The... 

Deuterium Isotope Effect. Ubbelohde and co-workers have done much work on the effect of deuterium substitution on the structures of H bonded crystals (1729, 2067, 1728, 1727, 2071, 522, 739, 2068). The results are reviewed in the summary paper, 2068. In most crystals the D bond is slightly longer than the H bond. Furthermore, in those crystals which are ferroelectric, the deuterated crystal has a higher ferroelectric transition temperature (upper Curie temperature). Some of the results summarized by Ubbelohde and Gallagher are... 

TABLE 9-XXVII Isotope Effect in H Bonded Crystals ... 

TABLE 9-XXIX Conductivities and Activation Energies for Proton Migration in H Bonded Crystals "... 

J. McG. Pollock and A. R. Ubbe-lohde. Trans. Faraday Soc. 52, 1112 (1956). Ele9trical conductivity theory for H bonded crystals. 

S.S.S.R., Ser. Fiz. 17, 592-5 (1953). Raman low frequency shifts, H bonded crystals, tartaric acid, Seignette salt, resorcinol, etc. 

A, 417-27 (1939). x-rays isotope effect on lattice spacing in H 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. 

A qualitative description of the structural and spectroscopic properties of H-bonded crystals requires the use of a simple two-mode linear model (9.1). 

Specific Features of H-bonded Crystals with a Quasi-symmetric O - H - O Fragment... 

At present three different codes are widely used for calculations of the structural and spectroscopic properties of H-bonded crystals, for example see Refs. [82-85]. The Car-Parrinello molecular dynamics (CPMD) program [86] and the Vienna ab initio simulation program (VASP) [87, 88] use a plane wave basis set, while an atom centered set is used with periodic boundary conditions in the CRYSTAL... 

A qualitative description of proton dynamics in H-bonded crystals requires a two-dimensional treatment assuming strong coupling between the proton-transfer coordinate and a low-frequency vibration. Due to environmental effects, this coupling is much stronger in crystals than in isolated H-bonded species. DFT calculations with periodic boundary conditions show the barrier along the proton transfer coordinate increases with increase in the 0---0 distance much faster in crystals than in the gas phase system. 

A. T. Hagler, S. Lifson, and P. Dauber, /. Am. Chem. Soc., 101, 5131 (1979). Consistent Force Field Studies of Intermolecular Forces in H-Bonded Crystals. 3. The C = 0 "H —O Hydrogen Bond and the Analysis of the Energetics and Packing of Carboxylic Acids. 

---