Molecular dynamics simulation of liquid N2O4⇌2NO2 by orientation-sensitive pairwise potential. II. Vibrational dephasing of the dissociation mode

Abstract

Vibrational dephasing of the N-N dissociation mode in equilibrium liquid N2O4⇌2NO2 in the extreme limit of pure N2O4 is studied by classical molecular dynamics simulations of liquid NO2. An OSPP+LJ potential between NO2 molecules, which is a sum of an orientation-sensitive pairwise potential (OSPP) between N-N atoms proposed in Paper I [J. Chem. Phys. 115, 10852 (2001)] and Lennard-Jones potentials between N-O and O-O atoms, has been used in the simulation. The vibrational correlation function and Raman spectrum of the N-N stretching ν3 mode of N2O4 are calculated, and are found to be very sensitive to the well depth De and anisotropy factors of OSPP: Aθ (0⩽Aθ⩽1) for rocking angle between the N-N bond and ONO direction, and Aτ (0⩽Aτ⩽1) for the torsional angle. As Aτ increases in equilibrium liquid N2O4⇌2NO2, the equilibrium shifts to dissociation, and the isotropic Raman band shifts to low frequency and broadens. The OSPP potential for De=0.12×10−18 J, Aθ=0.5, and Aτ=0.1 is found to reproduce both the observed liquid phase equilibrium properties and Raman band shapes (peak frequency, bandwidth, and band asymmetry) of the dissociation mode very well. The long persistent positive correlation in the vibrational frequency fluctuation correlation function Gω(t)=〈Δω(0)Δω(t)〉/〈Δω(0)2〉 is mainly ascribed to the slow energy fluctuation in the dissociation mode. It is concluded that the asymmetry of the ν3 Raman band is due to the scattering by molecular pairs excited in the dissociation mode.