Molecular crystals have rich information on molecular motions that may lead to understanding or design of physical and/or chemical properties from the atomic point of view. Quite recently, molecular vibrations in the terahertz (THz) region have been studied both experimentally and theoretically. Such molecular motions can be represented in terms of normal modes that may have contributions, under the harmonic approximation, from both intermolecular and intramolecular nuclear motions. Therefore it is important to know as to how intermolecular forces affect these two types of nuclear motions, in other words, the mixing ration of them in a given normal mode in the THz region. To establish the relationship between the origin of the THz normal modes and the nature of intermolecular forces, we have developed a theoretical method of mode analysis that allows quantitative determination of the ratio of the intramolecular vibrations and the intermolecular vibrations contributed in the optical phonon modes of interest. This ratio can be regarded as an index for molecular individuality in the crystals. Several molecular crystals such as C<sub>60</sub>, anthracene, adenine, α-glycine, and L-alanine have been investigated using our approach and we have identified two forms of the vibrational mixing. One is strong mixing within similar characteristic frequency regions, which can be seen in all five molecular crystals. The other is weak mixing of distinct characteristic frequency regions separated by a large energy gap, which appear in the crystals where both hydrogen bonding and ionic interaction are involved. The strong mixing can be interpreted using classical mechanics. The weak mixing, on the other hand, is non-classical, which we think cannot be reproduced by conventional molecular dynamics type of simulations. The typical feature of this latter category has been illustrated in amino acids (torsional motions). We believe that 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.

Effect of intermolecular interactions on mixing of intermolecular and intramolecular vibrations: Terahertz spectroscopy and solid-state density functional theory

253rd National Meeting (2017)

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