Density functional theory applyed to vibrational dynamics of crystals of ionic systems.

Abstract

Salts with low melting points, the so-called ionic liquids, exhibit complex phase diagrams since glass transition or crystallization are seen depending on the rate of change of temperature and pressure. In the past years we have been studying thermodynamics, structure and dynamics of ionic liquids in the Laboratório de Espectroscopia Molecular do Instituto de Química da USP, LEM/IQ-USP. We use a combined experimental and theoretical approach by calorimetry, Raman spectroscopy and x-ray diffraction, and quantum chemistry calculation and classical molecular dynamics simulation of the liquid phase. The quantum chemistry calculations allowed us to assign the vibrational spectra within the high-frequency spectral range of intramolecular normal modes. In this project, we aim to extend the computational studies with ab initio simulations using the functional density theory (DFT) to the calculation of lattice vibrations of crystalline phases of ionic liquids. One motivation for DFT simulations is the usual finding that crystalline phases of ionic liquids exhibit polymorphism, in which vibrational spectroscopy clearly shows that the crystalline phases are related to different conformations of cations and anions. Whereas quantum chemistry calculation of a single ion is enough to relate intramolecular vibrations to a specific molecular conformation, the DFT calculation will allow us to overcome a limitation of previous works carried in our lab, namely, the assignment of lattice vibrations in the low-frequency range of the Raman spectra. The ab initio molecular dynamics will also provide insights at molecular level of solid-solid transitions, protonic conduction in crystals with a network of hidrogen bonds and the initial steps of the mechanism of fusion of ionic liquids. Polymorphism as a function of pressure, as we have been studying in our group using a diamond anvil cell for spectroscopy under high pressure, i.e. within the GPa range, will be emphasized in this project since DFT calculation of solid state is particularly appropriate for spectral assignment or even prediction of crystalline structures under high pressure.