Molecular motion and vibrational dephasing in a model of liquid N₂O₄

Abstract

To study the microdynamics of translational, orientational and vibrational motions in liquid N₂O₄, molecular dynamics simulations, including all degrees of freedom of vibrating N₂O₄ molecules, were carried out. Atom-atom Lennard-Jones potentials with and without Coulomb potentials (LJ + C and LJ, respectively), were adopted in the simulation. When Coulomb interaction between atoms was included, mean-squared force and torque increased, the velocity and angular momentum correlation functions decayed faster to more evident negative parts, and both the translational and orientational diffusion slowed down. Simulated rotational correlation functions for LJ + C agreed reasonably with those derived from Raman spectroscopy, while those for LJ were found to decay too fast. Vibrational correlation functions for the NN stretching v ₃ mode were calculated and the vibrational spectra were obtained from their Fourier transforms. Two simulations for the harmonic and the anharmonic intramolecular potentials, where anharmonicity is taken into account for the NN and NO stretching, suggested that pure intermolecular interaction-induced broadening predominated the bandwidth of this mode, while vibrational anharmonicity coupled to the forces due to the environment did not play any important role. Results of the simulation were compared with the isotropic Raman band shapes in liquid N₂O₄.