Interpretation of Vapor-Liquid Frequency Shift of CH stretching vibration of chloroform and Fluoroform

Abstract

AbstractThe change in the frequency of the CH stretching vibration and in its anharmonicity constant, quadratic, cubic and quartic force constants and the equilibrium CH bond distance, on condensation, were investigated by performing model calculations for liquid chloroform and fluoroform. The interactions between the hydrogen atom and the CH oscillator, represented by a Morse potential, and the halogen atoms of its neighbor molecules were described by the sum of Lennard- Jones and Coulomb potential functions employed in molecular dynamics simulation. The calculations were carried out for different molecular arrangements in the liquid and have shown that mainly the size of the halogen atom, consequently the intermolecular distances, govern the direction of the CH vibrational frequency shift and of the change in the anharmonicity constant on the vapor-liquid transition. The pressure and temperature dependence of the CH stretching vibration was also studied for liquid chloroform. While the calculated pressure dependence is in good agreement with that observed, only the direction of the temperature dependence is in accordance with experimental observation.