Effects of Non‐covalent Interactions on Molecular and Polymer Individuality in Crystals Studied byTHzSpectroscopy and Solid‐State Density Functional Theory

Abstract

This chapter is concerned with the extent to which the conventional knowledge of molecular phonon modes in the terahertz (THz) region is valid and applicable. It introduces one density functional theory (DFT) model that has been used to simulate THz modes of all the crystalline systems of interest and an independently developed mode decomposition method to characterize the nature of any simulated mode of interest. The chapter discusses the nature of molecular phonon modes arising from distinct types of non-covalent interactions and its implications to the interpretation of molecular phonon modes. Following the works in the small molecular systems, it also discusses a practical application of THz spectroscopy and DFT simulations in the structural determination of a polymer material. Lastly, the chapter presents an experimental method that provides qualitative verification of the mode assignments to an extent as whether a given THz mode is dominated by intermolecular or intramolecular vibrations.