Intermolecular vibrations of a crystalline molecular complex

Abstract

Comment on: Structures, stabilities, and intermolecular vibrational frequencies of small ammonia complexes by molecular mechanics for clusters analysis J. Chem. Phys. 95, 3861 (1991); 10.1063/1.460792Structures, stabilities, and intermolecular vibrational frequencies of small ammonia complexes by molecular mechanics for clusters analysisThe nature of intermolecular vibrations in a crystalline molecular complex is examined using a simple model of a crystal with one molecule of a 1:1 complex per unit cell. It is shown that two limiting descriptions can result depending on the coupling strength between molecular components in the complex: (i) The giant molecule limit describes the case of a strong coupling where the intermolecular motions are the motions of the complex as one entity. (ii) The sublattice limit is applicable in the weak coupling case. Here, the motion can be characterized as an intermolecular motion belonging to the sublattice of the individual components. The phonon bands of naphthalene:octafluoronaphthalene crystals are examined within the framework of the simple model. The results predict phonons to be close to the sublattice limit. Our experimental study of Raman phonon spectra using isotopic mixed crystals of the complex is in agreement' with the theoretical predictions and show that the low frequency phonons belong predominantly to the CJOFg sublattice, while the high frequency phonons characterize the CJOHg sublattice motions.

Elusive Forms and Structures of N-Hydroxyphthalimide: The Colorless and Yellow Crystal Forms of N-Hydroxyphthalimide

“…Particularly, the bands observed in the 50−120 cm -1 region can be assigned to the stretching and deformation modes associated with intermolecular hydrogen bonding of the CO and C−O···H units in the NHP crystals. The frequencies below 120 cm -1 in the INS spectra for both NHP forms also belong to external modes, e.g., librations about the principal crystallographic axes, as well as to translational and phonon modes . A tentative thermal motion analysis on the basis of the obtained crystallographic parameters of the colorless NHP I form and the yellow form was performed to calculate the components of the librational and translational tensors for all non-hydrogen atoms of the molecule .…”