Mixing of intermolecular and intramolecular vibrations in optical phonon modes: terahertz spectroscopy and solid‐state density functional theory

Advances in terahertz ( THz ) spectroscopy and solid‐state density functional theory ( DFT ) have enabled a better understanding of low‐frequency molecular phonon modes in the region that includes intermolecular interactions. Normal modes in this region, in particular, may have contributions under the harmonic approximation from both intermolecular and intramolecular nuclear motions. A newly developed mode analysis allows us to characterize molecular optical phonon modes in terms of intermolecular and intramolecular vibrational mixing for the C 60 , anthracene, adenine, α ‐glycine, and l ‐alanine crystals. Through systematic investigation, two forms of the vibrational mixing have been identified. One is strong mixing within similar characteristic frequency regions, while the other is weak mixing of distinct characteristic frequency regions separated by a large gap. The former is predictable from classical mechanics and appears in molecular systems having structural flexibility. The latter is nonclassical and has been illustrated in amino acids. This finding provides new insights into the nature of molecular optical phonon modes and related physical and/or chemical processes such as energy transfer between internal and external modes with large energy separation. WIREs Comput Mol Sci 2016, 6:386–409. doi: 10.1002/wcms.1256 This article is categorized under: Structure and Mechanism > Molecular Structures Electronic Structure Theory > Density Functional Theory Theoretical and Physical Chemistry > Spectroscopy