Proton Dynamics in Hydrogen‐bonded Crystals

Abstract

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 Hbonds [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. The aims of this article are: 1. 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 protontransfer coordinate and a low-frequency vibration. 2. To interpret experimental structural and spectroscopic regularities of crystals with a quasi-symmetric A A H fragment using a model 2D potential energy surface (PES).