Methodical aspects of experimental studies for the relationship of static and dynamic features of crystal structure: application of various atomic scattering factors to study the vibrational characteristics of molecular crystals

Abstract

The X-ray diffraction studies of one of chloroacetic acid polymorphs with two independent molecules in the unit cell made it possible to test different methods for description of the electron density distribution anisotropy (independent atom model, multipole refinement, and "invarioms") when refining the atomic displacement parameters to reveal subtle distinctions of vibrational characteristics of independent molecule. It was shown that features of molecular motion, derived by modeling of matrices of mean-squared displacement amplitudes against multitemperature X-ray diffraction data using the independent atom model and approach of "invarioms," are qualitatively the same, while their relationship with the static characteristics of the crystal structure of chloroacetic acid can be explained using topological analysis of the charge density distribution and ONIOM quantum chemical calculations. The total energy of intermolecular interactions of one of the independent molecules, whose geometry is less favorable in the isolated state, is higher, which is consistent with a lower contribution of the intermolecular vibration mode is its low-frequency vibrations.